6 Terrestrial and Aquatic Biomes
6Terrestrial and Aquatic Biomes Growing grapes for wine during the hot, dry summer. At the Chateau Corcelles in southern France, the climate is ideal for growing the grapes that are used for wine. The World of Wine The fascinating history of winemaking dates back thousands of years. Archaeologists have found signs of winemaking in many cultures around the Mediterranean Sea, including those of the ancient Egyptians, Romans, and Greeks. Indeed, the entire Mediterranean region has a long tradition of cultivating wine grapes, and the production of wine has played an important role in the economic development of many societies and in religious rituals. European explorers spread winemaking to other parts of the world. For example, in the sixteenth century, Spanish explorers brought grape vines to Chile, Argentina, and California. Grape vines accompanied the Dutch to South Africa in the seventeenth century, and the British to Australia in the nineteenth century.
Although grapes can grow in many parts of the world, specific growing conditions are required to produce grapes for the best wines. The ideal climate is a combination of hot, dry summers and wet, mild winters. The hot and dry summer climate allows the grapes to develop the right balance of sugar and acidity that provides the complex flavors of a fine wine. The dry summers also prevent various plant diseases that flourish under moist conditions. Domesticated grapes have deep roots, so they are well adapted to dry summer landscapes. Because below-freezing temperatures can harm the vines, the presence of mild, wet winters is equally important. While climate is critical, the flavor of a fine wine is also influenced by the pH, fertility, and mineral content of the soils in which the vines grow. The composition of a soil affects how well the grape vines grow and gives the grapes a distinctive flavor that characterizes the wine made from them. In short, distinctive tasting wines from around the world are the result of unique combinations of climate and soil. “Superior wine-making regions not only have similar climates, but their landscapes also contain similar looking plants, despite being separated by thousands of kilometers.” Given the conditions required to make a great wine, it is perhaps not surprising that most of the major wine-producing locations around the world have the same climate—hot, dry summers followed by cool, moist winters. This is the climate of the countries surrounding most of the Mediterranean Sea. It is also the climate of most regions where wine grapes have been introduced, including Chile, Argentina, California, South Africa, and the southwestern coast of Australia. Interestingly, these regions all lie on the west side of continents and are located at latitudes between 30° and 50° in the Northern and Southern Hemispheres. Superior winemaking regions not only have similar climates, but their landscapes also contain similar-looking
plants, despite being separated by thousands of kilometers. For example, while each winemaking region contains a large number of unique plant species, the plants are similar in their growth form. Whether in France, California, Chile, or South Africa, the plant communities are dominated by drought-adapted grasses, wildflowers, and shrubs. In this chapter, we will explore how particular climates that are found in different locations around the world are associated with very similar-looking plants and how scientists use these patterns to categorize terrestrial ecosystems. We will also examine why scientists categorize aquatic ecosystems in a different way, based on differences in salinity, flow, and depth. SOURCES: Retallack, G. J., and S. F. Burns. 2016. The effects of soil on the taste of wine. Geological Society of America Today 26:4-9. http://www.geosociety.org/gsatoday/archive/26/5/article/i1052-5173-26-5-4.htm. A brief history of wine. 2007. New York Times, November 5. http://www.nytimes.com/2007/11/05/timestopics/topics-winehistory.html.
Learning Objectives
After reading this chapter, you should be able to:
6.1 Explain how terrestrial biomes are categorized by their major plant growth forms.
6.2 Describe the nine categories of terrestrial biomes.
6.3 Describe the many aquatic biomes, which are categorized by their flow, depth, and salinity. As we saw in Chapter 5, climate patterns around the globe are determined by a range of factors, including air currents, water currents, Coriolis forces, and local geographic features. Together, these factors are responsible for the climates that occur in different regions of the world. Different climates
provide unique seasonal temperature and precipitation conditions, and these unique conditions favor different types of plants.
#### 6.1 Terrestrial Biomes Are Categorized by Their Major Plant Growth Forms
6.1 Terrestrial biomes are categorized by their major plant growth forms Successful survival strategies vary with climate. In the world’s deserts, for example, we find plants that are well adapted to scarce water availability. In North American deserts, many species of cacti have thick, waxy outer layers covered with hairs and spines to help reduce water loss. In Africa, we find a group of plants called euphorbs that are not closely related to the cacti of North America yet have many similar features (Figure 6.1). Though the two groups of desert-adapted plants are descended from unrelated ancestors, they look similar because they have evolved under similar selective forces, a phenomenon known as convergent evolution. Convergent evolution can be observed in many organisms. For example, sharks and dolphins are not closely related to each other—one is a fish and the other is a mammal—yet both have evolved fins, powerful tails, and streamlined bodies. To perform well in an aquatic environment, natural selection has favored this set of traits because it allows both groups of animals to swim rapidly.
Figure 6.1 Convergent evolution. Similar conditions in the deserts of the world have selected for similar water-conserving life forms in two groups of unrelated plants: (a) an organ pipe cactus (Stenocereus thurberi) in Organ Pipe Cactus National Monument, Arizona, and (b) a euphorb (Euphorbia virosa) in Namibia, Africa. Convergent evolution A phenomenon in which two species descended from unrelated ancestors look similar because they have evolved under similar selective forces. Convergent evolution explains why we can recognize an association
between the forms of organisms and the environments in which they live. Trees found in tropical rainforests have the same general appearance no matter where they are located on Earth or their evolutionary lineage. The same can be said of shrubs inhabiting seasonally dry environments; they tend to have small, deciduous leaves and often have stems with spines to discourage herbivores from eating them. Geographic regions that contain communities composed of organisms with similar adaptations are called biomes. Because of convergent evolution, we can categorize terrestrial ecosystems by dominant plant forms that are associated with distinct patterns of seasonal temperatures and precipitation. In aquatic ecosystems, the major producers are often not plants but algae. As a result, aquatic biomes are not easily characterized by the dominant growth forms of the producers. Instead, aquatic biomes are characterized by distinct patterns of depth, flow, and salinity. Biome A geographic region that contains communities composed of organisms with similar adaptations. Biomes provide convenient reference points for comparing ecological processes around the globe, which makes the biome concept a useful tool that enables ecologists to understand the structure and functioning of large ecological systems. As in all classification systems, exceptions occur. Boundaries between biomes can be unclear and not all plant growth forms correspond to climate in the same way. Australian eucalyptus trees, for example, form forests under climatic conditions that support only scrubland or grassland on other continents. Finally, plant communities reflect factors other than temperature and rainfall. Topography, soils, fire, seasonal variations in climate, and herbivory all affect which species can live in different plant communities. The overview of the major terrestrial biomes in this chapter emphasizes the distinguishing features of the physical environment and how these features are reflected in the form of the dominant plants. As a final note, although ecologists use plant forms to categorize biomes, there is generally a good association between the plant forms in a biome and the animal forms that live there. For example, deserts contain plants and animals that are adapted to dry
conditions. We will use a classification system that recognizes nine major terrestrial biomes, which are listed in Figure 6.2. If we consider all combinations of average annual temperatures and average annual precipitation, as shown in the figure, we see that most places on Earth are located inside a triangular area with corners representing warm moist, warm dry, and cool dry climates. Cold regions with high rainfall are rare because water does not evaporate rapidly at low temperatures and because the atmosphere in cold regions contains very little water vapor.
Figure 6.2 Terrestrial biomes. Distinct plant forms exist under different combinations of average annual precipitation and average annual temperature. The nine biomes fall within three temperature ranges that we refer to often throughout this book. Boreal forest and tundra biomes have average annual temperatures that are generally below 5 °C. Temperate biomes—temperate rainforest, temperate seasonal forest, woodland/shrubland, and temperate grassland/cold desert—are warmer, with average annual temperatures generally between 5 °C and 20 °C. Finally, tropical biomes—tropical rainforest, tropical seasonal forest/savanna, and subtropical desert—are the warmest biomes, with average annual temperatures greater than 20 °C. The
global distribution of these biomes is illustrated in Figure 6.3. As we will see, the average annual precipitation within each of these temperature categories can vary widely.
Figure 6.3 The global distribution of biomes. The nine terrestrial biomes represent locations with similar average annual temperatures and precipitations and similar plant growth forms. Also shown are polar ice caps, which lack plants and therefore are not part of the biome classification system.
Climate Diagrams
To visualize the patterns of temperature and precipitation that are associated with particular biomes, scientists use climate diagrams, which are graphs that plot the average monthly temperature and precipitation of a specific location on Earth. Figure 6.4 provides two sample climate diagrams. As you can see, the shaded area on the x-axis indicates the months in which the average temperature exceeds 0 °C. These months are warm enough to allow plant growth and therefore represent the growing season of the biome. Climate diagrams can also indicate whether plant growth is more limited by temperature or by precipitation. For every 10 °C increase in temperature, plants require an additional 2 cm of monthly precipitation to meet the increased water needs. Climate diagrams adjust their temperature and precipitation axes such that every 10 °C increase in average monthly temperature corresponds to a 2 cm increase in monthly precipitation. This means that in any month in which the precipitation line goes below the
temperature line, plant growth is constrained by a lack of sufficient precipitation. In contrast, any month in which the temperature line goes below the precipitation line, plant growth is constrained by a lack of sufficient temperature. Since climate is the primary force determining the plant forms of different biomes, locations around the world that are from a particular biome have similar climate diagrams. With this background, in the next section, we will take a closer look at the nine terrestrial biomes and their associated climate diagrams.
Figure 6.4 Climate diagrams. By plotting the average monthly temperature and precipitation values over time for a particular location on Earth, we can determine how climates vary throughout the year and the length of the growing season. (a) In this hypothetical climate diagram, there is a seven-month growing season and plant growth is limited by temperature throughout the year. (b) In this example, there is a five-month growing season and plant growth is limited by precipitation. Climate diagram A graph that plots the average monthly temperature and precipitation of a specific location on Earth. Growing season The months in a location that are warm enough to allow plant growth.
Analyzing Ecology
Mean, Median, and Mode Climate diagrams are a useful way of conveying a good deal of
information about the average monthly changes in temperature and precipitation. While the climate of a particular location can vary from year to year, the climate diagrams display the typical conditions based on several years of collecting data. Using these data, one can then determine the mean temperature and precipitation for a particular month. The mean, or average, is calculated by summing all the data and dividing by the total number of data points. The mean value gives you a sense of where the middle value lies in a set of data. However, this assumes that the data have a symmetrical distribution such that half of the values fall above the mean and half of the values fall below the mean. In some sets of data, the values are not symmetrically distributed around a middle value. In such cases, a better estimate of the middle value is the median. The median is found by placing the data in order, from lowest to highest, and finding the number that occurs in the middle. If there is an even number of values, then there are two numbers in the middle and the median is found by taking the average of these two middle numbers. For example, consider the values 95, 93, 90, 85, 81, 75, 63, 42, 21: the mean = (95 + 93 + 90 + 85 + 81 + 75 + 63 + 42 + 21) ÷ 9 = 71.7 In contrast, the median = 81. For a set of data that contains an even number of values, the median is calculated as the mean of the middle two values. For example, consider the following values: 95, 93, 90, 85, 81, 79, 75, 63, 42, 21 In this case, there is an even number of values and two numbers, 79 and 81, are the middle values. As a result, the median is the average of these two numbers = 80. Sometimes scientists are not as interested in the mean or median from a set of data, but rather want to know which values occur most frequently. In this case, they determine how often each value occurs; the mode is the value that occurs most frequently. For example, consider the
following values: 95, 93, 90, 85, 81, 81, 75, 63, 42, 21 In this list, 81 appears more frequently than the other values, so the mode is 81. Generally, the mode is useful only for large samples, where sampling of each possible value is reasonably good. YOUR TURN For the following set of data, determine the mean, median, and mode: 12, 13, 15, 18, 17, 19, 18, 17, 12, 14, 10, 17, 19, 16, 17 Why is the mean of these values different from the median and the mode?
Concept Check
1. Why do unrelated plants often assume the same growth form in different parts of the world? 2. How do we break down biomes into three broad temperature categories? 3. What information about a biome can you obtain from a climate diagram?
#### 6.2 There Are Nine Categories of Terrestrial Biomes
6.2 There are nine categories of terrestrial biomes Terrestrial biomes are traditionally placed into nine categories. In this section, we will take a tour of the biomes. We start with tundras and boreal forests, which have average annual temperatures of less than 5 °C. We will then examine the biomes in temperate regions, with average temperatures between 5 °C and 20 °C. Finally, we will explore the biomes of tropical regions, which have average annual temperatures of more than 20 °C. As we will see, seasonal patterns and amounts of precipitation can differ a great deal within a given temperature range, producing different types of biomes.
The tundra, shown in Figure 6.5, is the coldest biome and is characterized by a treeless expanse above permanently frozen soil, or permafrost. The soils thaw to a depth of 0.5 to 1 m during the brief summer growing season. Annual precipitation is generally less, and often much less, than 600 mm, but in low-lying areas where permafrost prevents drainage, soils may remain saturated with water throughout most of the growing season. Tundra soils contain few nutrients. They also tend to be acidic because of their high content of organic matter, which is the result of cold conditions dramatically slowing the decomposition of organic matter. In this nutrient-poor environment, plants hold their foliage for years. Most plants are dwarf, prostrate woody shrubs, which grow low to the ground to gain protection under the winter blanket of snow and ice, since anything protruding above the surface of the snow is sheared off by blowing ice crystals. For most of the year, the tundra is an exceedingly harsh environment, but during summer days with 24 hours of sunlight, there is a rush of biological activity.
Figure 6.5 Tundra biome. The Denali National Park in Alaska is an example of a tundra biome, which is characterized by a lack of trees and by soil that is permanently frozen. Tundra The coldest biome, characterized by a treeless expanse above permanently frozen soil. The tundra is found in the Arctic regions of Russia, Canada, Scandinavia, and Alaska, and in the Antarctic regions along the edge of Antarctica and nearby islands. At high elevations within temperate latitudes, even within the
tropics, one finds vegetation resembling that of Arctic tundra, including some of the same species or their close relatives. These areas of alpine tundra above the tree line occur widely in the Rocky Mountains of North America, the Alps of Europe, and, especially, on the Plateau of Tibet in central Asia. In spite of their similarities, alpine and Arctic tundra have important differences. Areas of alpine tundra generally have warmer and longer growing seasons, higher precipitation, less severe winters, greater productivity, better-drained soils, and higher species diversity than Arctic tundra. However, harsh winter conditions ultimately prevent the growth of trees in both Arctic tundra and alpine tundra.
Boreal Forests
Stretching in a broad belt centered at about 50° N in North America and about 60° N in Europe and Asia lies the boreal forest. As shown in Figure 6.6, the boreal forest, sometimes called taiga, is a biome densely populated by evergreen needle-leaved trees, with a short growing season and severe winters. The average annual temperature is generally below 5 °C and annual precipitation generally ranges between 40 and 1,000 mm. Since evaporation is low, soils are moist throughout most of the growing season. The vegetation consists of dense, seemingly endless stands of 10- to 20-m tall evergreen needle-leaved trees, mostly spruces and firs. Because of the low temperatures, plant litter decomposes very slowly and accumulates at the soil surface, forming one of the largest reservoirs of organic carbon on Earth. The needle litter produces high levels of organic acids, so the soils are acidic, strongly podsolized (as discussed in Chapter 5), and generally of low fertility. Growing seasons rarely exceed 100 days, and are often half that long. The vegetation is extremely frost-tolerant; temperatures may reach −60 °C during the winter. Since few species can survive in such harsh conditions, species diversity is very low. The boreal forest is not well suited for agriculture, but it serves as a source of timber products that include lumber and paper.
Figure 6.6 Boreal forest biome. Boreal forests, such as this one in the Boundary Waters
Canoe Area Wilderness of the Superior National Forest, Minnesota, typically have cold temperatures and are dominated by evergreen trees, including spruces and firs. Boreal forest A biome densely populated by evergreen needle-leaved trees, with a short growing season and severe winters. Also known as Taiga.
Temperate Rainforests
As we begin to move closer to the equator, we find the four temperate biomes: temperate rainforest, temperate seasonal forest, woodland/shrubland, and temperate grassland/cold desert. The temperate rainforest biome, shown in Figure 6.7, is known for mild temperatures and abundant precipitation and is dominated by evergreen forests. These conditions are due to nearby warm ocean currents. This biome is most extensive near the Pacific coast in northwestern North America and in southern Chile, New Zealand, and Tasmania. The mild, rainy winters and foggy summers create conditions that support evergreen forests. In North America, these forests are dominated toward the south by coast redwood (Sequoia sempervirens) and toward the north by Douglas fir. These trees are typically 60 to 70 m tall and may grow to over 100 m, making them very attractive for harvesting as lumber. The fossil record shows that these plant communities are very old and they are remnants of forests that were vastly more extensive 70 million years ago. In contrast to tropical rainforests, temperate rainforests typically support few species.
Figure 6.7 Temperate rainforest biome. Temperate rainforests exist along the coasts of several continents, including this forest of giant Sitka spruce trees (Picea sitchensis) in British Columbia, Canada. They have mild temperatures and high amounts of precipitation.
Temperate rainforest A biome known for mild temperatures and abundant precipitation, dominated by evergreen forests.
Temperate Seasonal Forests
The temperate seasonal forest biome, shown in Figure 6.8, occurs under moderate temperature and precipitation conditions, and is dominated by deciduous trees. Winter temperatures can drop below freezing in this biome. The environmental conditions in this biome fluctuate much more than they do in the temperate rainforests because they do not benefit from the moderating effects of nearby warm ocean waters. In North America, the dominant plant growth form is deciduous trees, including maple, beech, and oak, which lose their leaves each fall. In North America, this biome stretches across the eastern United States and southeastern Canada; it is also widely distributed in Europe and eastern Asia. This biome is not common in the Southern Hemisphere, where the larger ratio of ocean surface to land moderates winter temperatures at many high altitudes and prevents frost. Temperate seasonal forest A biome with moderate temperature and precipitation conditions, dominated by deciduous trees.
Figure 6.8 Temperate seasonal forest biome. Temperate seasonal forests have warm summers, cold winters, and a moderate amount of precipitation that favors the growth of deciduous trees. Pictured here is the Bialowieza Forest in Poland. In the Northern Hemisphere, the length of the growing season in this biome varies from 130 days at higher latitudes to 180 days at lower latitudes. Precipitation usually exceeds evaporation and transpiration; consequently, water tends to move downward through soils and to drain from the landscape as groundwater and as surface streams and rivers. Soils are often podsolized,
tend to be slightly acidic and moderately leached, and contain abundant organic matter. The vegetation often includes a layer of smaller tree species and shrubs beneath the dominant trees, as well as herbaceous plants on the forest floor. Many of these herbaceous plants complete their growth and flower in early spring before the trees have fully leafed out. Warmer and drier parts of the temperate seasonal forest biome, especially where soils are sandy and nutrient poor, tend to develop needle-leaved forests dominated by pines. This includes the pine forests of the coastal plains of the Atlantic and Gulf coasts of the United States; pine forests also exist at higher elevations in the western United States. Because of the warm climate in the southeastern United States, decomposition is rapid. The rapid decomposition rates and sandy soils lead to the low availability of nutrients. The low nutrients and relatively dry conditions, in turn, favor the evergreen, needleleaved trees, which resist desiccation and give up nutrients slowly because they retain their needles for several years. Since soils in this biome tend to be dry, fires are frequent in the pine forests, although most species are able to resist fire damage. The temperate seasonal forest was one of the first biomes that European settlers in North America used for agriculture.
Woodlands/Shrublands
The woodland/shrubland biome, shown in Figure 6.9, is characterized by hot, dry summers and mild, wet winters, a combination that favors the growth of drought-tolerant grasses and shrubs. Because this type of climate is found around much of the Mediterranean Sea, it is often referred to as a Mediterranean climate regardless of where it is actually found. The woodland/shrubland biome has many different regional names, including chaparral in southern California, matorral in southern South America, fynbos in southern Africa, and maquis in the area surrounding the Mediterranean Sea.
Figure 6.9 Woodland/shrubland biome. This biome is characterized by hot, dry summers and mild, wet winters, a combination that favors the growth of drought-tolerant grasses and shrubs. An example of this biome can be found in Cape Town, South Africa. The climate diagram is from Paso Robles, California. Woodland/shrubland A biome characterized by hot, dry summers and mild, wet winters, a combination that favors the growth of drought-tolerant grasses and shrubs.
As you can see in the climate diagram, although there is a 12-month growing season, plant growth is limited by dry conditions in the summer and by cold temperatures in the winter. This biome supports thick evergreen shrubby vegetation 1 to 3 m in height, with deep roots and drought-resistant foliage. The small, durable leaves of typical Mediterranean-climate plants have earned the label of sclerophyllous (“hard-leaved”) vegetation. Fires are frequent in the woodland/shrubland biome, and most plants have either fireresistant seeds or root crowns that resprout soon after a fire. Traditional human use of this biome has been for grazing animals and growing deeprooted crops such as wine grapes, as we discussed at the beginning of the chapter. Sclerophyllous Vegetation that has small, durable leaves.
Temperate Grasslands/Cold Deserts
The temperate grassland/cold desert biome, shown in Figure 6.10, is characterized by hot, dry summers and cold, harsh winters, and is dominated by grasses, nonwoody flowering plants, and drought-adapted shrubs. Plant growth is constrained by a lack of precipitation in the summer and by cold temperatures in the winter. The biome is also known by a variety of different names around the world, including prairies in North America, pampas in South America, and steppes in eastern Europe and central Asia.
Figure 6.10 Temperate grassland/cold desert biome. Grasslands such as this one at the Theodore Roosevelt National Park in North Dakota are characterized by hot, dry summers and very cold winters. Where moisture is more abundant, the dominant vegetation is grass. Where moisture is less abundant, in areas known as cold deserts, the dominant vegetation is composed of widely scattered shrubs. Temperate grassland/cold desert A biome characterized by hot, dry summers and cold, harsh winters and dominated by grasses, nonwoody flowering plants, and drought-adapted shrubs.
As the biome name suggests, the dominant plant forms in temperate grasslands are grasses and nonwoody flowering plants that are well adapted to the frequent fires. Precipitation varies widely across this biome. For example, on the eastern edge of North American prairies, annual precipitation can be 1,000 mm. In such areas, grasses can grow to more than 2 m high and are referred to as tallgrass prairies. There is even enough moisture in such areas to support the growth of trees, but the frequent fires prevent the trees from becoming a dominant component of this biome. As one moves west, annual precipitation declines to 500 mm or less. In these areas, grasses generally do not grow taller than 0.5 m and are referred to as shortgrass prairies. Because precipitation is infrequent, organic detritus does not decompose rapidly, and this makes the soils rich in organic matter. In addition, the weak acidity of the soils means that they are not heavily leached, and they tend to be rich in nutrients. Even farther west in North America, annual precipitation drops below 250 mm and the temperate grasslands grade into cold deserts, also known as temperate deserts. In the United States, the cold desert extends across most of the Great Basin, which sits in the rain shadow of the Sierra Nevada and Cascade Mountains. In the northern part of the region, the dominant plant is sagebrush, whereas toward the south and on somewhat moister soils, widely spaced juniper and piñon trees predominate, forming open woodlands with trees less than 10 m in stature and sparse coverings of grass. In these cold deserts, evaporation and transpiration exceed precipitation during most of the year, resulting in dry soils. Fires are infrequent in cold deserts because the habitat produces so little plant material to burn. However, because of the low productivity of the plant community, grazing can exert strong pressure on the vegetation and may even favor the persistence of shrubs, which are not good forage. Indeed, many dry grasslands in the western United States and elsewhere in the world have been converted to deserts by overgrazing.
Tropical Rainforests
Our final group of biomes is found in areas of tropical temperatures and includes tropical rainforests, tropical seasonal forests/savannas, and subtropical deserts. Tropical rainforests, shown in Figure 6.11, generally fall within 20° N and 20° S of the equator, are warm and rainy, and are characterized by multiple layers of lush vegetation. Tropical rainforests have
a continuous canopy of 30 to 40 m trees with emerging trees that occasionally reach 55 m. Shorter trees and shrubs form a layer known as the understory below the canopy. The understory also contains an abundance of epiphytes and vines. Species diversity is higher in tropical rainforests than anywhere else on Earth. This biome occurs in much of Central America, the Amazon Basin, the Congo in southern West Africa, the eastern side of Madagascar, Southeast Asia, and the northeast coast of Australia. In many of these locations, however, much of the rainforest has been destroyed to harvest lumber and to make room for agriculture.
Figure 6.11 Tropical rainforest biome. Tropical rainforests, such as this site in Borneo, have very warm temperatures and very high amounts of precipitation. As a result, this biome has multiple layers of lush vegetation. Tropical rainforest A warm and rainy biome, characterized by multiple layers of lush vegetation.
Climates that support tropical rainforests are always warm and receive at least 2,000 mm of precipitation throughout the year, with rarely less than 100 mm during any single month. The tropical rainforest climate often exhibits two peaks of rainfall centered on the equinoxes, corresponding to the periods when the intertropical convergence zone passes over the equator (as discussed in Chapter 5). Rainforest soils are typically old and deeply weathered from the high amounts of precipitation. Because they are relatively devoid of organic matter and clay, they take on the reddish color of aluminum and iron oxides and they retain nutrients poorly. This is the process of laterization that we also discussed in Chapter 5. Despite the poor ability of these soils to hold nutrients, the biological productivity of tropical rainforests per unit area exceeds that of any other terrestrial biome. Moreover, the standing biomass exceeds that of all other biomes except temperate rainforests. This tremendous growth is possible because the continuously high temperatures and abundant moisture cause organic matter to decompose quickly, and vegetation immediately takes up the released nutrients. While rapid nutrient cycling supports the high productivity of the rainforest, it also makes the rainforest ecosystem extremely vulnerable to disturbance. When tropical rainforests are cut and burned, many of the nutrients are carted off in logs or go up in smoke. The vulnerable soils erode rapidly and fill the streams with silt. In many cases, the environment degrades rapidly and the landscape becomes unproductive.
Tropical Seasonal Forests/Savannas
Tropical seasonal forests, shown in Figure 6.12, are located mostly beyond 10° N and 10° S of the equator. These regions experience warm temperatures and, as the intertropical convergence zone moves during the year, pronounced wet and dry seasons. Because tropical seasonal forests have a preponderance of deciduous trees that shed their leaves during the dry season, this biome is sometimes referred to as a tropical deciduous forest. In areas where the dry season is longer and more severe, the vegetation becomes shorter and develops thorns to protect leaves from grazing animals. With even longer dry periods, the vegetation grades from dry forest into thorn forest and finally into savannas, which are open landscapes containing grasses and occasional trees, including acacia and baobab trees.
Figure 6.12 Tropical seasonal forest/savanna biome. Tropical seasonal forests and savannas have warm temperatures like the tropical rainforests, but also distinct wet and dry seasons due to the movement of the intertropical convergence zone. As a result, this biome has trees that shed their leaves during the dry season. An example of this biome can be found in the Masai Mara National Reserve of Kenya. Tropical seasonal forest A biome with warm temperatures and pronounced wet and dry seasons, dominated by
deciduous trees that shed their leaves during the dry season. The tropical seasonal forest/savanna biome occurs in Central America, the Atlantic coast of South America, sub-Saharan Africa, southern Asia, and northwestern Australia. Fire and grazing play important roles in maintaining the character of the savanna biome. Under these conditions, grasses can persist better than other forms of vegetation. When grazing and fire are prevented within a savanna habitat, dry forest often begins to develop. As in more humid tropical environments, the soils tend to hold nutrients poorly but the warm temperatures favor rapid decomposition. Rapid decomposition provides a rapid recycling of nutrients into the soil, which trees can quickly take up and use for growth and reproduction. Such a rapid cycling of nutrients also makes this biome an attractive place for agriculture, including raising cattle. On the Pacific coast of Central America and on the Atlantic coast of South America, for example, over 99 percent of this biome has been converted to agriculture.
Subtropical Deserts
Subtropical deserts, shown in Figure 6.13, are characterized by hot temperatures, scarce rainfall, long growing seasons, and sparse vegetation. Also known as hot deserts, subtropical deserts develop at 20° to 30° north and south of the equator, in areas associated with the dry, descending air of Hadley cells that we discussed in Chapter 5. Subtropical deserts include the Mojave Desert in North America, the Sahara Desert in Africa, the Arabian Desert in Asia, and the Great Victoria Desert in Australia.
Figure 6.13 Subtropical desert biome. Subtropical deserts, such as this site in the Atacama Desert of Chile, have hot temperatures and scarce rainfall. This favors drought-resistant plants such as cactus, creosote bush, mesquite, and euphorbs. Subtropical desert A biome characterized by hot temperatures, scarce rainfall, long growing seasons, and sparse vegetation.
Because of the low rainfall, the soils of subtropical deserts are shallow, virtually devoid of organic matter, and neutral in pH. Whereas sagebrush dominates the cold deserts of the Great Basin, creosote bush (Larrea tridentata) dominates the subtropical deserts of the Americas. Moister sites support a profusion of succulent cacti, shrubs, and small trees, such as mesquite and paloverde (Cercidium microphyllum). Most subtropical deserts receive summer rainfall. After summer rains, many herbaceous plants sprout from dormant seeds, grow quickly, and reproduce before the soils dry out again. Few plants in subtropical deserts are frost-tolerant. Species diversity is usually much higher than in temperate arid lands.
Concept Check
1. Why is the boreal forest biome found on several different continents, including North America, Europe, and Asia? 2. What types of terrestrial plants are found in each of the four biomes situated at temperate latitudes? 3. Why do tropical rainforests experience two peaks of rainfall?
#### 6.3 Aquatic Biomes Are Categorized by Their Flow, Depth, and Salinity
6.3 Aquatic biomes are categorized by their flow, depth, and salinity As we discussed earlier in this chapter, ecologists categorize aquatic biomes using a range of physical factors, including water depth, water flow, and salinity. The major types of aquatic biomes include streams and rivers, lakes and ponds, freshwater wetlands, salt marshes, mangrove swamps, intertidal zones, coral reefs, and the open ocean.
Streams and Rivers
Because streams and rivers are characterized by flowing fresh water, they are often referred to as lotic systems. Although there is no exact specification to determine classification differences between a stream and a river, in general, streams, also called creeks, are narrow channels of fast-flowing fresh water, whereas rivers are wide channels of slow-flowing fresh water (Figure 6.14). As streams flow down from their headwaters, they join together with other streams and eventually grow large enough to be considered a river. Streams and some rivers are usually bordered by a riparian zone, a band of terrestrial vegetation influenced by seasonal flooding and elevated water tables.
Figure 6.14 Streams and rivers. Streams and rivers are characterized by flowing fresh water. This example is on the Vefsna River in Norway. Lotic Characterized by flowing fresh water. Stream A narrow channel of fast-flowing fresh water. Also known as Creek. River A wide channel of slow-flowing fresh water. Riparian zone A band of terrestrial vegetation alongside rivers and streams that is influenced by seasonal flooding and elevated water tables.
Downstream, water flows more slowly and becomes warmer and richer in nutrients. Under these conditions, ecosystems generally become more complex and more productive. In general, streams support fewer species than other aquatic biomes. Small streams are often shaded and nutrient poor, which limits the productivity of algae and other photosynthetic organisms. Much of the organic content of stream ecosystems depends on allochthonous inputs of organic matter, such as leaves, that come from outside the ecosystem. In large rivers, a higher proportion of the organic inputs are autochthonous, meaning that they are produced from inside the ecosystem by algae and aquatic plants. As rivers progress from their source, they typically become wider, slower-moving, more heavily laden with nutrients, and more exposed to direct sunlight. They also accumulate sediments that are washed in from the land and carried downstream. High turbidity caused by suspended sediments in the lower reaches of silt-laden rivers can block light and reduce production. Allochthonous Inputs of organic matter, such as leaves, that come from outside of an ecosystem. Autochthonous Inputs of organic matter that are produced by algae and aquatic plants inside an ecosystem. Lotic systems are extremely sensitive to modification of their water flow by dams. In the United States, tens of thousands of dams—built to control flooding, to provide water for irrigation, or to generate electricity—interrupt stream flow. Dams also alter water temperature and rates of sedimentation. Typically, water behind dams becomes warmer, and the original stream bottoms become filled with silt that destroys habitat for fish and other aquatic organisms. Water released downstream from large dams often has low concentrations of dissolved oxygen. Using dams for flood control changes the natural seasonal cycles of flooding that are necessary for maintaining many kinds of riparian habitats on floodplains. Dams also disrupt the natural movement of aquatic organisms upstream and downstream, fragmenting river systems and isolating populations.
Ponds and Lakes
Ponds and lakes are aquatic biomes characterized by nonflowing fresh water
with at least some area of water that is too deep for plants to rise above the water’s surface (Figure 6.15a). Although there is no clear-cut distinction between ponds and lakes, ponds are smaller. Many lakes and ponds were formed as glaciers retreated, gouging out basins and leaving behind glacial deposits containing blocks of ice that eventually melted. The Great Lakes of North America formed in glacial basins, overlain until 10,000 years ago by thick ice. Lakes are also formed in geologically active regions, such as the Great Rift Valley of Africa, where vertical shifting of blocks of the Earth’s crust created basins in which water accumulates. Broad river valleys, such as those of the Mississippi and Amazon rivers, have oxbow lakes, which are broad bends of what was once the river, cut off by shifts in the main channel.
Figure 6.15 Ponds and lakes. Ponds and lakes are characterized by nonflowing fresh water
with areas of water that are too deep for emergent vegetation. (a) Red Rock Lake, Colorado. (b) Lakes contain a variety of zones. The littoral zone exists around the edge of the lake and contains rooted, emergent plants. The limnetic zone consists of the open water in the middle of the lake, where the dominant photosynthetic organisms are floating algae. Below the limnetic zone is the profundal zone, which is too deep for sunlight to penetrate enough to permit photosynthesis. The layer of sediments at the bottom of the lake is the benthic zone. Pond An aquatic biome that is smaller than a lake and is characterized by nonflowing fresh water with some area of water that is too deep for plants to rise above the water’s surface. Lake An aquatic biome that is larger than a pond and is characterized by nonflowing fresh water with some area of water that is too deep for plants to rise above the water’s surface. As shown in Figure 6.15b, lakes can be subdivided into several ecological zones, each with distinct physical conditions. The littoral zone is the shallow area around the edge of a lake or pond containing rooted vegetation, such as water lilies and pickerelweed. The open water beyond the littoral zone is the limnetic (or pelagic) zone, where the dominant photosynthetic organisms are floating algae, or phytoplankton. Very deep lakes also have a profundal zone that does not receive sunlight because of its depth. The absence of photosynthesis, as well as the presence of bacteria that decompose the detritus at the bottom of the lake, causes the profundal zone to have very low concentrations of oxygen. The sediments at the bottoms of lakes and ponds constitute the benthic zone, which provides habitat for burrowing animals and microorganisms. Littoral zone The shallow area around the edge of a lake or pond containing rooted vegetation. Limnetic zone The open water beyond the littoral zone, where the dominant photosynthetic organisms are floating algae. Also known as Pelagic zone. Benthic zone The area consisting of the sediments at the bottoms of lakes, ponds, and oceans. Profundal zone The area in a lake that is too deep to receive sunlight. Circulation in Ponds and Lakes While lakes and ponds can be separated into four zones based on the
proximity to the shore and the amount of light penetration, the water depths can also be classified by temperature. In most lakes and ponds in temperate and polar regions, the temperature of the water forms layers. The surface water, known as the epilimnion, can have a different temperature than the deeper water, known as the hypolimnion. Between these two temperature regions is the thermocline, which is a middle depth of water that experiences a rapid change in temperature over a relatively short distance in depth. The thermocline serves as a barrier to a mixing between the epilimnion and hypolimnion. Epilimnion The surface layer of the water in a lake or pond. Hypolimnion The deeper layer of water in a lake or pond. Thermocline A middle depth of water in a lake or pond that experiences a rapid change in temperature over a relatively short distance in depth. Most production in a lake occurs in the epilimnion, where sunlight is most intense. Oxygen produced by photosynthesis and oxygen entering the lake at the interface of the water and atmosphere keep the epilimnion well aerated and thus suitable for animal life. Throughout a growing season, however, plants and algae often deplete the supply of dissolved mineral nutrients in the epilimnion and this curtails their growth. In the hypolimnion, which can include the lower limnetic zone and the profundal zone, bacteria continue to decompose organic material, but the reduced intensity of light causes a reduction in photosynthesis. The result is that oxygen is used up faster than it is produced, and this leads to anaerobic conditions. Oxygen is in particularly short supply deep in productive lakes that generate abundant organic matter in the epilimnion. Lakes in the temperate zone experience changing temperatures across the seasons. These temperature changes drive changes in water density, which, in turn, causes the shallow and deep water to mix. Figure 6.16 shows this process. As you may recall from Chapter 2, water becomes more dense as it cools to 4 °C and then less dense as it cools below 4 °C. During the winter in cold climates, the coldest lake water (0 °C) lies at the surface just beneath the ice, while the slightly warmer, denser water (4 °C) sinks to the bottom of the lake.
In early spring, the sun gradually warms the lake. As the surface temperature increases toward 4 °C, the sun-warmed water sinks into the cooler layers immediately below and the water begins to mix. At the same time, winds drive surface currents that can cause deep water to rise in a manner similar to upwelling currents in the oceans. The vertical mixing of the lake water that occurs in early spring and is assisted by winds that drive the surface currents is known as the spring turnover. The spring turnover brings nutrients from sediments on the bottom to the surface and oxygen from the surface to the depths. This mixing results in the rapid growth of phytoplankton, the algae the float throughout the water column and serve as a major food source for herbivores.
Figure 6.16 Circulation and turnover in temperate lakes. (a) In the spring, seasonal winds cause the lake water to mix, which brings nutrients from the sediments to the surface water and oxygen from the surface water down to the deeper water. (b) During the summer, the surface water warms faster than the deep water, so the lake experiences thermal stratification. The zone at which water temperature changes rapidly with depth is known as the thermocline. (c) In autumn, the surface water cools, stratification breaks down, and autumn winds cause the surface water and deep waters to mix once again. (d) In winter, the surface waters are exposed to freezing temperatures, so ice forms at the surface. Because 4 °C water is the most dense, the bottom of the lake does not freeze. Spring turnover The vertical mixing of lake water that occurs in early spring, assisted by winds that drive the surface currents.
In late spring and early summer, surface layers of water gain heat faster than deeper layers. At this point, the thermocline is created. Once the thermocline is well established, the surface and deep waters no longer mix because the warmer, less dense surface water floats on the cooler, denser water below, a condition known as stratification. During the fall, the temperature of the surface layers of the lake drops. As this water becomes denser than the underlying water, it begins to sink. The vertical mixing that occurs in the fall and is assisted by winds that drive surface currents is called fall turnover. Similar to the spring turnover, fall turnover brings oxygen to deep waters and nutrients to the surface. The infusion of nutrients into surface waters in the fall may cause a second phytoplankton bloom. This mixing persists into late fall, until the temperature at the lake surface drops below 4 °C and winter stratification ensues. Stratification The condition of a lake or pond when the warmer, less dense surface water floats on the cooler, denser water below. Fall turnover The vertical mixing of lake water that occurs in fall, assisted by winds that drive the surface currents. The spring and fall turnover are typical of lakes that exist in temperate climates because they experience cold winters and warm summers. The seasonality of vertical mixing is much less dramatic in lakes that are not exposed to such dramatic temperature changes. In warmer climates, water temperatures do not fall below 4 °C. As a result, such lakes do not stratify in the winter, and many have only one mixing event each year following summer stratification.
Freshwater Wetlands
Freshwater wetlands are aquatic biomes that contain standing fresh water, or soils saturated with fresh water for at least part of the year, and are shallow enough to have emergent vegetation throughout all depths. Most of the plants that grow in wetlands can tolerate low oxygen concentrations in the soil; many are specialized for these anoxic conditions and grow nowhere else. Freshwater wetland An aquatic biome that contains standing fresh water, or soils saturated with fresh water for
at least part of the year, and which is shallow enough to have emergent vegetation throughout all depths. Freshwater wetlands include swamps, marshes, and bogs (Figure 6.17). Swamps contain emergent trees. Some of the best-known swamps are the Okefenokee Swamp in Georgia and Florida and the Great Dismal Swamp in Virginia and North Carolina. Marshes contain emergent nonwoody vegetation such as cattails. Some of the largest marshes in the world include the Everglades in Florida and the Pantanal of Brazil, Bolivia, and Paraguay. In contrast to swamps and marshes, bogs are characterized by acidic waters and contain a variety of plants, including sphagnum mosses and stunted trees that are specially adapted to these conditions. Some of the largest bogs are found in Canada, northern Europe, and Russia.
Figure 6.17 Freshwater wetlands. This biome includes a variety of aquatic habitats. (a) Swamps contain emergent trees, such as this bald cypress swamp (Taxodium distichum) in Reelfoot Lake State Park, Tennessee. (b) Marshes contain emergent nonwoody vegetation that includes cattails, such as in this location near Fairfax, Virginia. (c) Bogs are characterized by acidic waters and plants that are well adapted to these conditions, such as this bog in northern Wisconsin. Freshwater wetlands provide important habitats for a wide variety of animals, notably waterfowl and the larval stages of many species of fish and invertebrates that are characteristic of open waters. Wetland sediments
immobilize potentially toxic or polluting substances dissolved in water and therefore function as a water purification system.
Salt Marshes/Estuaries
Salt marshes are a saltwater biome that contains nonwoody emergent vegetation. Salt marshes are found along the coasts of continents in temperate climates, often within estuaries, which are areas along the coast where the mouths of freshwater rivers mix with the salt water from oceans (Figure 6.18). Estuaries are unique because of their mix of fresh and salt water. In addition, they contain an abundant supply of nutrients and sediments carried downstream by rivers. The rapid exchange of nutrients between the sediments and the surface in the shallow waters of an estuary supports extremely high growth of plants and algae. Because estuaries tend to be areas of sediment deposition, they are often edged by extensive saltwater marshes at temperate latitudes and by mangrove swamps in the tropics. With a combination of high nutrient levels and freedom from water stress, tidal marshes are among the most productive habitats on earth. They contribute organic matter to estuarine ecosystems, which, in turn, support large populations of oysters, crabs, fish, and the animals that feed on them.
Figure 6.18 Salt marsh. Salt marshes occur in salt water and estuaries and contain nonwoody emergent vegetation. An example of a salt marsh can be found in Plum Island Sound off the coast of Massachusetts. Salt marsh A saltwater biome that contains nonwoody emergent vegetation. Estuary An area along the coast where the mouths of freshwater rivers mix with the salt water from oceans.
Mangrove Swamps
Mangrove swamps are a biome that exists in salt water along tropical and subtropical coasts and contains salt-tolerant trees with roots submerged in water. This biome can also occur in estuaries where fresh water and salt water mix (Figure 6.19). By living along the coasts, these salt-tolerant trees play important roles in preventing the erosion of coastal shorelines from constant incoming waves. The swamps also provide critical habitats to many species of fish and shellfish.
Figure 6.19 Mangrove swamps. Mangrove swamps, including this location off the coast of Australia, are saltwater biomes that contain salt-tolerant trees along tropical and subtropical coastlines. Mangrove swamp
A biome that occurs along tropical and subtropical coasts and contains salt-tolerant trees with roots submerged in water.
Intertidal Zones
The intertidal zone is a biome consisting of the narrow band of coastline between the levels of high tide and low tide. As the tides come in and go out, the intertidal zone experiences widely fluctuating temperatures and salt concentrations. The species living in this biome—including crabs, barnacles, sponges, mussels, and algae—must therefore possess adaptations that allow them to tolerate such harsh conditions. Intertidal zones can occur along steep rocky coastlines, as one might find in Maine, or gently sloping mudflats, as one might find in Cape Cod Bay in Massachusetts (Figure 6.20).
Figure 6.20 Intertidal zone. Intertidal biomes are the coastal regions around the world that exist between the high tide and the low tide of the oceans. (a) Rocky coasts produce rocky intertidal habitats, such as this one along the Alaskan coast. (b) Muddy coasts produce mudflat habitats around the world, including this site at Los Llanos, Venezuela. Intertidal zone A biome consisting of the narrow band of coastline between the levels of high tide and low tide.
Coral Reefs
Coral reefs are a marine biome found in warm, shallow waters that remain above 20 °C year-round. Coral reefs often surround volcanic islands, where they are fed by nutrients eroding from the rich volcanic soil and by deepwater currents forced upward by the profile of the island. Coral reef A marine biome found in warm, shallow waters that remain 20 °C year-round. Corals are tiny animals—related to hydra and other cnidarians—that live in a mutualistic relationship with algae. An individual coral is a hollow tube that secretes a hard exoskeleton made of calcium carbonate. It also has tentacles that sweep food particles of detritus and plankton into the tube. As it digests these particles, the coral produces CO2 that can be used by their symbiotic algae in photosynthesis. Some of the sugars and other organic compounds the algae produce leak into the coral tissues and further support coral growth. Although an individual coral is tiny, corals live in huge colonies. As an individual coral dies, the soft tissues decompose but the hard outer skeletons remain behind. Over time, these skeletons accumulate to form massive coral reefs. The complex structure that corals build provides a wide variety of substrates and hiding places for algae and animals. This helps to make coral reefs among the most diverse biomes on Earth (Figure 6.21). As you may recall from our discussion of coral reefs in Chapter 2, rising sea surface temperatures in the tropics are causing the departure of the algal symbionts of corals over large areas—a phenomenon known as coral bleaching. Because the algal symbionts are critical to the survival of the coral, the stability of these biomes is now at risk.
Figure 6.21 Coral reefs. The hard exoskeletons of millions of tiny corals form massive coral reefs in the ocean, which serve as home to an incredible diversity of organisms. Coral reefs can be found in shallow, warm ocean waters, such as this reef off the coast of the Maldives.
The Open Ocean
The open ocean is characterized as the part of the ocean that is away from the shoreline and coral reefs. Open oceans cover the largest portion of the surface of Earth. Beneath the surface lies an immensely complex realm with large variations in temperature, salinity, light, pressure, and currents. Ecologists recognize a number of zones in the open ocean, shown in Figure 6.22. Beyond the range of the lowest tidal level, the neritic zone extends to depths of about 200 m, which corresponds to the edge of the continental shelf. Because strong waves move nutrients to the sunlit surface layers from sediments below, the neritic zone is generally a region of high productivity. Beyond the
neritic zone, the seafloor drops rapidly to the great depths of the oceanic zone. Here, nutrients are sparse, and production is strictly limited. Finally, the benthic zone consists of the seafloor underlying the neritic and oceanic zones.
Figure 6.22 Open ocean. The open ocean is represented by water that is offshore and away from coral reefs. This biome can be broken up into several zones. Neritic zone The ocean zone beyond the range of the lowest tidal level, and which extends to depths of about 200 m. Oceanic zone The ocean zone beyond the neritic zone. The neritic and the oceanic zones may be subdivided vertically into a photic zone and an aphotic zone. The photic zone is the area of the neritic and oceanic zones that contains sufficient light for photosynthesis by algae. The aphotic zone is the area of the neritic and oceanic zones where the water is so deep that sunlight cannot penetrate. However, as we saw in Chapter 1, bacteria in the aphotic zone use chemosynthesis to convert inorganic carbon into simple sugars. Other organisms in the aphotic zone depend on the organic material that falls from the photic zone. One of the fascinating adaptations of many organisms in the aphotic zone is the ability to generate their own source of light, known as bioluminescence, to help them find and
consume prey. A number of jellyfish, crustaceans, squid, and fish species have independently evolved this ability. Photic zone The area of the neritic and oceanic zones that contains sufficient light for photosynthesis by algae. Aphotic zone The area of the neritic and oceanic zones where the water is so deep that sunlight cannot penetrate. In this chapter, we have explored how differences in climates determine the types of dominant plant forms that can persist in different parts of the world, forming the basis of categorizing terrestrial biomes. In contrast, the aquatic biomes are categorized by differences in water flow, depth, and salinity. In all cases, there is a close association between the environmental conditions and the species that have evolved adaptations to live under these conditions. Of course, adaptations reflect not only the physical factors in the environment but also the many interactions with other organisms. In the next chapter, we examine the process of evolutionary adaptation and see how it has created the tremendous diversity of life on Earth.
Concept Check
1. How do headwater streams and larger rivers differ in their major source of organic material? 2. Why does productivity in the ocean differ between the photic and aphotic zones? 3. What are the five types of saltwater biomes?
Concepts
Climate change. The changing climates around the world are predicted to alter the distribution of many organisms, including plants in the genus Banksia in the shrublands of southwestern Australia. These changes are also affecting human agriculture, including the vineyards that are planted in the shrubland biomes around the world. Throughout this chapter, we have seen that climate largely determines the location of the terrestrial biomes. Climatic conditions, combined with species interactions, set the edges of biome boundaries. Given our understanding of how biome boundaries form, what would happen to the biomes, and the species living in them, if the climate changed? Records show us that during the past 130 years, temperatures of the surface of Earth have increased an average of 1 °C. In fact, according to NASA, 15 of the 16 warmest years recorded since 1880 have occurred since 2001. This small average increase in Earth’s temperature conceals the fact that some regions have become 1 °C to 2 °C cooler during this time, while others have become up to 4 °C warmer. Scientists predict even greater increases in temperature and large changes
in precipitation patterns for the twenty-first century. If climate determines the location of biomes and the climate is changing, it seems reasonable to predict that the boundaries of biomes are also going to change. In some cases, scientists believe that shifting biome boundaries might occur relatively easily. Where no barriers prevent movement, plant and animal populations will be able to shift north or south over several decades without much difficulty. However, when movement is blocked, for example, by mountain ranges or large highways, the plants and animals may not be able to survive in their current locations given the changing conditions. Consider the woodland/shrubland biome on the southwestern coast of Australia. This small biome is located on a relatively small area of coastal land, with an ocean to the south and west, and a desert to the north and east. Scientists predict that this biome will become hotter and drier during the current century. If this prediction is correct, organisms that cannot tolerate the increase in temperature will have nowhere hospitable to go since the neighboring desert biome is already too dry for them to survive. Scientists who examined one group of plants from the genus Banksia, which is composed of 100 species, concluded that over the next 70 years, 66 percent of these species will decline in abundance and 25 percent will go extinct. Climate changes also affect human agriculture. Recall from the beginning of this chapter that most of the world’s wine is produced in the shrubland/woodland biome. However, changes in climate have already affected the cultivation of grapes used in winemaking. In France, for example, over the past 30 years warmer growing seasons have caused grapes to ripen 16 days earlier. This warmer
climate alters the sugar content and acidity of the grapes— two components that must be in balance to make a fine- tasting wine. This problem is so serious for winemakers and wine consumers that in 2009 French wine growers called upon world leaders to take immediate action to try to reverse global climate change. On the other hand, locations at slightly higher latitudes that had temperatures historically too cool for growing quality wine grapes are now reporting increased summer temperatures, which have allowed them to produce some of the best wine grapes in years. In England, for example, the amount of land dedicated to growing grapes for wine has increased by 148 percent during the past decade. In fact, researchers in 2016 forecasted an additional increase of 2.2 °C and a 6 percent increase in precipitation by 2100, which will favor the production of wine grapes even more. This is very good news for winemakers in England, but devastating for the French who have a long history of making some of the world’s finest wines. SOURCES: Chaudhuri, S. 2016. Climate change uncorks British wine production. Wall Street Journal, December 1. http://www.wsj.com/articles/britains-wine-production-could-be- boosted-by-climate-change-1480619748. Iverson, J. T. 2009. How global warming could change the winemaking map. Time Magazine, December 3. Fitzpatrick, M. C., A. D. Grove, N. J. Sanders, and R. R. Dunn. 2008. Climate change, plant migration, and range collapse in a global biodiversity hotspot: The Banksia (Proteaceae) of Western Australia. Global Change Biology 14:1337–1352.
Summary of Learning Objectives
6.1 Terrestrial biomes are categorized by their major plant growth forms. Ecologists use the dominant plant forms to categorize ecosystems into terrestrial biomes because many plants have evolved convergent forms in response to similar climate conditions. Climate and dominant plant forms are similar within biomes. Key Terms: Convergent evolution, Biome, Climate diagram, Growing season
6.2 There are nine categories of terrestrial biomes. The coldest biomes are the tundra and boreal forests. In temperate regions, we can find temperate rainforests, temperate seasonal forests, woodlands/shrublands, and temperate grasslands/cold deserts. In tropical latitudes, the biomes can be categorized as tropical rainforests, tropical seasonal forests/savannas, and subtropical deserts. Key Terms: Tundra, Boreal forest, Temperate rainforest, Temperate seasonal forest, Woodland/shrubland, Sclerophyllous, Temperate grassland/cold desert, Tropical rainforest, Tropical seasonal forest, Subtropical desert
6.3 Aquatic biomes are categorized by their flow, depth, and salinity. Freshwater biomes include streams and rivers, ponds and lakes, and freshwater wetlands. Saltwater biomes include salt marshes/estuaries, mangrove swamps, intertidal zones, coral reefs, and the open ocean. Key Terms: Lotic, Stream, River, Riparian zone, Allochthonous, Autochthonous, Pond, Lake, Littoral zone, Limnetic zone, Profundal zone, Benthic zone, Epilimnion, Hypolimnion, Thermocline, Spring turnover, Stratification, Fall turnover, Freshwater wetland, Salt marsh, Estuaries, Mangrove swamp,
Intertidal zone, Coral reef, Neritic zone, Photic zone, Aphotic zone
Critical Thinking Questions
1. Compare and contrast the factors used to categorize terrestrial biomes with those used to categorize aquatic biomes. 2. How do atmospheric convection currents discussed in Chapter 5 help to determine the locations of tropical seasonal forests? 3. In considering all the terrestrial biomes, what is the general effect of precipitation on soil fertility? 4. Given your knowledge of terrestrial biomes, why can temperate seasonal forests retain more of their soil fertility than tropical rainforests after they are logged? 5. How do environmental conditions differ among the four temperate biomes? 6. Compare and contrast allochthonous and autochthonous inputs to streams and rivers. 7. What parallels can you draw between the zones in a lake and zones in the ocean? 8. If northern latitudes continue to become warmer over the centuries, what effect might this have on lake circulation during the spring and fall? 9. Compare and contrast swamps, marshes, and bogs. 10. How will increasing global temperatures over the coming centuries likely affect the future distributions of biomes?
#### Graphing the Data: Creating a Climate Diagram
GRAPHING THE DATA Creating a Climate Diagram Scientists have collected climate data for locations all over the world. Using the monthly temperature and precipitation data for Miami, Florida (provided in the table), create a climate diagram. Remember to make each 10 °C increase in temperature correspond to a 20 mm increase in precipitation. Month Temperature (°C) Precipitation (mm) January February March April May June July August September October November December