CHAPTER OVERVIEW 18: Ecological Succession Learning Objectives Introduce succession by exploring the history of its theory Describe role of disturbance and expected outcomes regarding diversity Summarize important models of successional change Go beyond plants by investigating the roles of consumers in succession 18.1: Introduction 18.2: What are the Effects of Disturbance? 18.3: Patterns of Diversity Following Disturbance 18.4: What causes successional change? 18.5: The Role of Consumers and Alternative Stable States Summary Succession, or the process of change in the species composition of a community over time, was one of the first theories developed in ecology. Succession is brought about by environmental disturbances, and generally speaking, disturbance increases biodiversity. However, the level of disturbance significantly influences species diversity and may be maximized at intermediate levels. The patterns of successional change in biodiversity are scale dependent and must be quantified appropriately using alpha, beta, and gamma diversities. There are three common models used to explain the dynamics of community development through succession. In these models pioneer species facilitate, inhibit, or can be tolerated by later successional organisms. Succession is commonly framed within plant communities. However, animal communities affect succession through phenomena like the redirection of successional trajectories and the development of alternative stable states. 18: Ecological Succession is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
18.1: Introduction Ecological succession is the process of change in the species composition of an ecological community over time. It is a process by which an ecological community undergoes more or less orderly and predictable changes following a disturbance or the initial colonization of a new habitat. Succession may be initiated either by formation of new, unoccupied habitat, such as from a lava flow or a severe landslide, or by some form of disturbance of a community, such as from a fire, severe windthrow, or logging. Succession that begins in new habitats, uninfluenced by pre-existing communities is called primary succession, whereas succession that follows disruption of a pre-existing community is called secondary succession. 18.1.1: History of succession Succession was among the first theories advanced in the ecological literature. Ecological succession was first documented in the Indiana Dunes of Northwest Indiana and remains an important ecological topic of study (Smith & Mark, 2009). Between 1899-1910, Henry Chandler Cowles, at the University of Chicago, developed a more formal concept of succession. Inspired by studies of Danish dunes by Eugen Warming, Cowles studied vegetation development on sand dunes on the shores of Lake Michigan (the Indiana Dunes). He recognized that vegetation on dunes of different ages might be interpreted as different stages of a general trend of vegetation development on dunes (an approach to the study of vegetation change later termed space-fortime substitution, or chronosequence studies). From about 1900 to 1960, however, understanding of succession was dominated by the theories of Frederic Clements, a contemporary of Cowles, who held that seres were highly predictable and deterministic series of successional plant communities that converged on a climatically determined stable climax community regardless of starting conditions. Clements explicitly analogized the successional development of ecological communities with ontogenetic development of individual organisms, and his model is often referred to as the pseudo-organismic theory of community ecology. Clements and his followers developed a complex taxonomy of communities and successional pathways. Henry Gleason offered a contrasting framework as early as the 1920s. The Gleasonian model was more complex and much less deterministic than the Clementsian. It differs most fundamentally from the Clementsian view in suggesting a much greater role of chance factors and in denying the existence of coherent, sharply bounded community types. Gleason argued that species distributions responded individualistically to environmental factors, and communities were best regarded as artifacts of the juxtaposition of species distributions. Gleason's ideas, first published in 1926, were largely ignored until the late 1950s. Two quotes illustrate the contrasting views of Clements and Gleason. Clements wrote in 1916: The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. As an organism the formation arises, grows, matures, and dies. Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development. -- Frederic Clements (1916) while Gleason, in his 1926 paper, said: An association is not an organism, scarcely even a vegetational unit, but merely a coincidence. -- Henry Gleason (1926) Gleason's ideas were, in fact, more consistent with Cowles' original thinking about succession. About Clements' distinction between primary succession and secondary succession, Cowles wrote (1911): This classification seems not to be of fundamental value, since it separates such closely related phenomena as those of erosion and deposition, and it places together such unlike things as human agencies and the subsidence of land.
-- Henry Cowles Succession theory was developed primarily by botanists. The study of succession applied to whole ecosystems initiated in the writings of Ramon Margalef, while Eugene Odum's publication of The Strategy of Ecosystem Development is considered its formal starting point (Bazzaz, 1996). Animal life also exhibits changes with changing communities. In the lichen stage fauna is sparse. It comprises a few mites, ants and spiders living in cracks and crevices. The fauna undergoes a qualitative increase during the herb grass stage. The animals found during this stage include nematodes, insects larvae, ants, spiders, mites, etc. The animal population increases and diversifies with the development of the forest climax community. The fauna consists of invertebrates like slugs, snails, worms, millipedes, centipedes, ants, bugs; and vertebrates such as squirrels, foxes, mice, moles, snakes, various birds, salamanders and frogs. A more rigorous, data-driven testing of successional models and community theory generally began with the work of Robert Whittaker and John Curtis in the 1950s and 1960s. Succession theory has since become less monolithic and more complex. J. Connell and R. Slatyer attempted a codification of successional processes by mechanism. Among British and North American ecologists, the notion of a stable climax vegetation has been largely abandoned, and successional processes have come to be seen as much less deterministic, with important roles for historical contingency and for alternate pathways in the actual development of communities. Debates continue as to the general predictability of successional dynamics and the relative importance of equilibrial vs. non-equilibrial processes. Former Harvard professor Fakhri A. Bazzaz introduced the notion of scale into the discussion, as he considered that at local or small area scale the processes are stochastic and patchy, but taking bigger regional areas into consideration, certain tendencies can not be denied (1996). View this video for a short summary of ecological succession before you continue to explore some of the key concepts involved in this chapter. Ecological Succession: Change is Go CrashCourse
Watch on 18.1.2: Sources Bazzaz, F. A. (1996). Plants in changing environments. Cambridge University Press. Clements, F. E. (1916). Plant succession: An analysis of the development of vegetation. Carnegie Institution of Washington. Cowles, H. C. (1911). The causes of vegetational cycles. Annals of the Association of American Geographers, 1(1), 3-20. https://doi.org/10.2307/2560846 Gleason, H. A. (1926). The individualistic concept of the plant association. Bulletin of the Torrey Botanical Club, 53(1), 7-26. https://doi.org/10.2307/2479933 Smith, S., & Mark, S. (2009). The historical roots of The Nature Conservancy in the Northwest Indiana/Chicagoland region: From science to preservation. The South Shore Journal, 3. Archived from the original on January 1, 2016. Retrieved November 22, 2015, from https://www.iun.edu/~southsj/
Thoreau, H. D. (n.d.). The succession of forest trees, and wild apples. Archive.org. Retrieved April 12, 2014, from https://archive.org/ 18.1.3: Contributors and Attributions Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources: https://en.wikipedia.org/wiki/Ecological_succession 18.1: Introduction is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
18.2: What are the Effects of Disturbance? Changes in community structure and composition over time are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state. In primary succession, newly exposed or newly formed land is colonized by living things; in secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain. Thus, disturbance can initiate successional change. Species that are well adapted for exploiting disturbance sites are referred to as pioneers or early successional species. These shadeintolerant species are able to photosynthesize at high rates, produce a lot of offspring, and grow and mature quickly. Their fast growth is usually balanced by short life spans. Furthermore, although these species often dominate immediately following a disturbance, they are unable to compete with shade-tolerant species later on and are replaced by these species through succession. However these shifts may not reflect the progressive entry to the community of the taller long-lived forms, but instead, the gradual emergence and dominance of species that may have been present, but inconspicuous directly after the disturbance (Nobel, n.d.). Disturbances have also been shown to be important facilitators of non-native plant invasions (Lembrechts, 2016). While plants must deal directly with disturbances because of their lack of mobility, many animals are mobile and thus are not as immediately affected by disturbance. For example, some animals could successfully evade the initial destruction of a forest fire, but can later return to the burned area and thrive on new growth on the forest floor. Disturbed communities (such as a forest after a fire) often support a wider variety of plants compared to pre-disturbance vegetation. The plants in turn support a variety of wildlife, temporarily increasing biological diversity in the forest (Pringle, 1979). 18.2.1: Intermediate Disturbance Hypothesis The intermediate disturbance hypothesis (IDH) suggests that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem (Dial & Roughgarden, 1988). At high levels of disturbance, due to frequent forest fires or human impacts like deforestation, all species are at risk of going extinct. According to intermediate disturbance hypothesis theory, at intermediate levels of disturbance, diversity is thus maximized because species that thrive at both early and late successional stages can coexist. Intermediate disturbance hypothesisis a nonequilibrium model used to describe the relationship between disturbance and species diversity. The intermediate disturbance hypothesis is based on the following premises: 1. ecological disturbances have major effects on species richness within the area of disturbance, 2. interspecific competition results from one species driving a competitor to extinction and becoming dominant in the ecosystem, and 3. moderate ecological scale disturbances prevent interspecific competition (Wilkinson, 1999; Kricher, 2011; Catford et al., 2012).
: I describes how, at low levels of ecological disturbance species richness decreases as competitive exclusion
increases; II shows that at intermediate levels of disturbance, diversity is maximized because species that thrive at both early and
late successional stages can coexist; III shows that at high levels of disturbance species richness is decreased due to an increase in
species movement. "Intermediate Disturbance Hypothesis Graph" by Sciencerelatedusername is licensed under CC BY-SA 4.0.
Disturbances act to disrupt stable ecosystems and clear species' habitat. As a result, disturbances lead to species movement into the newly cleared area (secondary succession) (Wilkinson, 1999). Once an area is cleared there is a progressive increase in species richness and competition between species takes place. Once the conditions that create a disturbance are gone, and competition between species in the formerly disturbed area increases, species richness decreases as competitive exclusion increases (Vandermeer et al., 1996).
"Gause's Law", also known as competitive exclusion, explains how species that compete for the same resources cannot coexist in the same niche (Kricher, 2011). Each species handles change from a disturbance differently; therefore, intermediate disturbance hypothesis can be described as both "broad in description and rich in detail" (Wilkinson, 1999). The broad intermediate disturbance hypothesis model can be broken down into smaller divisions which include spatial within-patch scales, spatial between-patch scales, and purely temporal models. Each subdivision within this theory generates similar explanations for the coexistence of species with habitat disturbance. Joseph H. Connell proposed that relatively low disturbance leads to decreased diversity and high disturbance causes an increase in species movement (1978). These proposed relationships lead to the hypothesis that intermediate disturbance levels would be the optimal amount of disorder within an ecosystem.
Another way of thinking about the intermediate disturbance hypothesis requires that we consider the types of organisms that could specialize in areas with different levels of disturbance. K-selected species generally demonstrate more competitive traits. Their primary investment of resources is directed towards growth, causing them to dominate stable ecosystems over a long period of time. In contrast, r-selected species colonize open areas quickly and can dominate landscapes that have been recently cleared by disturbance (Catford et al., 2012). These characteristics attribute to the species that thrive in habitats with higher and lower amounts of disturbance. Based on the contradictory characteristics of both of these examples, areas of occasional disturbance allow both r and K species to flourish in the same area. If K-selected and r-selected species can live in the same region, species richness can reach its maximum.
Several alternative hypotheses to the intermediate disturbance hypothesis have been proposed (Hall et al., 2012). One alternative hypothesis states that the species diversity in a disturbance-mediated coexistence between species is maximized by the presence of a disturbance regime resembling the historic processes. This is because species generally adapt to the level of disturbance in their ecosystem through evolution (whether disturbance is of high, intermediate or low level). In addition, many species (e.g. ruderal plants and fire-adapted species) even depend on a specific disturbance in ecosystems where it often occurs.
18.2.2: References Catford, J. A., Daehler, C. C., Murphy, H. T., Sheppard, A. W., Hardesty, B. D., Westcott, D. A., Rejmánek, M., Bellingham, P. J., et al. (2012). The intermediate disturbance hypothesis and plant invasions: Implications for species richness and management. Perspectives in Plant Ecology, Evolution and Systematics, 14(3), 231-241. https://doi.org/10.1016/j.ppees.2011.12.002 Connell, J. H. (1978). Diversity in tropical rain forests and coral reefs. Science, 199(4335), 1302-1310. https://doi.org/10.1126/science.199.4335.1302 Dial, R., & Roughgarden, J. (1988). Theory of marine communities: The intermediate disturbance hypothesis. Ecology, 79(4), 1412-1424. https://doi.org/10.1890/0012-9658(19...OMCTI]2.0.CO;2 Hall, A. R., Miller, A. D., Leggett, H. C., Roxburgh, S. H., Buckling, A., & Shea, K. (2012). Diversity-disturbance relationships: Frequency and intensity interact. Biology Letters, 8(5), 768-771. https://doi.org/10.1098/rsbl.2012.0282 Kricher, J. C. (2011). Tropical ecology. Princeton University Press. Lembrechts, J. J., Pauchard, A., Lenoir, J., Nuñez, M. A., Geron, C., Ven, A., Bravo-Monasterio, P., Teneb, E., Nijs, I., & Milbau, A. (2016). Disturbance is the key to plant invasions in cold environments. Proceedings of the National Academy of Sciences, 113(49), 14061-14066. https://doi.org/10.1073/pnas.1608980113 Nobel, I. R. (n.d.). The use of vital attributes to predict successional changes in plant communities subject to recurrent disturbances.
Pringle, L. (1979). Natural fire: Its ecology in forests (pp. 27-29). William Morrow and Company. Vandermeer, J., Boucher, D., Perfecto, I., & de la Cerda, I. G. (1996). A theory of disturbance and species diversity: Evidence from Nicaragua after Hurricane Joan. Biotropica, 28(4), 600-613. https://doi.org/10.2307/2389100 Wilkinson, D. M. (1999). The disturbing history of intermediate disturbance. Oikos, 84(1), 145-147. https://doi.org/10.2307/3546878 18.2.3: Contributors and Attributions Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources: Connie Rye (East Mississippi Community College), Robert Wise (University of Wisconsin, Oshkosh), Vladimir Jurukovski (Suffolk County Community College), Jean DeSaix (University of North Carolina at Chapel Hill), Jung Choi (Georgia Institute of Technology), Yael Avissar (Rhode Island College) among other contributing authors. Original content by OpenStax (CC BY 4.0; Download for free at http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87). https://en.wikipedia.org/wiki/Interm...nce_hypothesis https://en.wikipedia.org/wiki/Disturbance_(ecology) 18.2: What are the Effects of Disturbance? is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
18.3: Patterns of Diversity Following Disturbance In general, communities in early succession will be dominated by fast-growing, well-dispersed species (opportunist, fugitive, or rselected life-histories). As succession proceeds, these species will tend to be replaced by more competitive (K-selected) species (See Life History Strategies). Trends in ecosystem and community properties in succession have been suggested, but few appear to be general. For example, species diversity almost necessarily increases during early succession as new species arrive, but may decline in later succession as competition eliminates opportunistic species and leads to dominance by locally superior competitors.
Ecological succession was formerly seen as having a stable end-stage called the climax, sometimes referred to as the 'potential vegetation' of a site, and shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of non-equilibrium ideas of ecosystems dynamics (See Intermediate Disturbance Hypothesis). Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state. Additions to available species pools through range expansions and introductions can also continually reshape communities.
Several graphs line up along a timeline labeled forest succession over time in six stages. A line graph shows time on the x-axis with a series of peaks showing the shift over time from bare rock to mosses and grasses to grasses and perennials to woody plants and pioneers to fast growing trees and finally a climax forest made up of some of all previous stages. A diagram below shows disturbance labeling symbols for fire, humans, water, and biohazards. Arrows point from the 6 stages to simple illustrations of them. An additional line graph on the bottom shows a gentle logarithmic increase in biodiversity, biomass, and soil layer over time as the land becomes a climax forest.
: Trajectory of forest secondary succession following an intense disturbance, resulting in nearly no original biomass,
to a mature forest state. Biodiversity, biomass, and soil depth all increase with time, and community composition changes. Image
by Lucas Martin Frey is licensed under CC BY 3.0.
18.3.1: Patterns of Diversity in Primary vs. Secondary Succession Primary succession begins on rock formations, such as volcanoes or mountains, or in a place with no organisms or soil (eg, an abandoned parking lot or railroad). In primary succession pioneer species like lichen, algae and fungi as well as abiotic factors like wind and water start to develop soil and initiate other important mechanisms for greater diversity to flourish. These pioneer species are then replaced by plants better adapted to less harsh conditions, these plants include vascular plants like grasses and some shrubs that are able to live in thin soils that are often mineral-based. Water and nutrient levels increase with the amount of succession exhibited (Fujiyoshi et al., 2005). The early stages of primary succession are dominated by species with small propagules (seed and spores) which can be dispersed long distances. The early colonizers--often algae, fungi, and lichens--stabilize the substrate. Nitrogen supplies are limited in new soils, and nitrogen-fixing species tend to play an important role early in primary succession (Korablev & Neshataeva, 2016). Successional dynamics following severe disturbance or removal of a pre-existing community are called secondary succession. Dynamics in secondary succession are strongly influenced by pre-disturbance conditions, including soil development, seed banks, remaining organic matter, and residual living organisms. Because of residual fertility and pre-existing organisms, community change in early stages of secondary succession can be relatively rapid. Secondary succession is much more commonly observed and studied than primary succession. Particularly common types of secondary succession include responses to natural disturbances such as fire, flood, and severe winds, and to human-caused disturbances such as logging and agriculture. Unlike in primary succession, the species that dominate secondary succession, are usually present from the start of the process, often in the soil seed bank. In some systems the successional pathways are fairly consistent, and thus, are easy to predict. In others, there are many possible pathways, potentially leading to alternative stable states. For example, nitrogen-fixing legumes alter successional trajectories (Chapin et al., 2002).
18.3.2: Patterns of Diversity at Different Scales While we will discuss ways to measure biodiversity more in a later chapter (see section 22.2 on Diversity Indices), it's helpful to have some vocabulary to describe patterns of biodiversity when talking about succession and disturbance.
How is biodiversity measured? Part 1, Alpha, Beta, and Gamma Diversity Alpha diversity (-diversity) is the mean species diversity in a site at a local scale. At it's simplest, it is the number of species in a given location. More complex metrics of diversity take into account not only how many species there are, but how even their abundances are. Examples of places with extremely high alpha diversity include tropical rainforests. Beta diversity (-diversity) is the ratio between regional and local species diversity and connects alpha and gamma diversity. It describes the rate at which species composition changes across a region. For example, if every wetland in a region was inhabited by a similar suite of plant species, then the region would have low beta diversity; in contrast, if several wetlands in a region had plants communities that were distinct and had little overlap with one another, the region would have high beta diversity. Beta diversity is calculated as gamma diversity divided by alpha diversity. Beta diversity as a measure of species turnover overemphasizes the role of rare species. The difference in species composition between two sites or communities likely reflects the presence and absence of some rare species in the assemblages. Examples of places with high beta-diversity include matrices of recently disturbed and undisturbed land. Gamma diversity (-diversity) is the total species diversity in a landscape. Here, questions of scale become very important to appropriately distinguish between alpha and gamma diversity. These terms were introduced by R. H. Whittaker (1960; 1974). Whittaker's idea was that the total species diversity in a landscape (gamma diversity) is determined by two different things, the mean species diversity in sites at a more local scale (alpha diversity) and the differentiation among those sites (beta diversity). This idea can be mathematically notated in this way: =/. The simplest calculations of alpha and beta diversity involve reworking this equation algebraically, with the result being that alpha, beta, and gamma diversity scale together. Disturbance and following successional change can increase biodiversity at the alpha, beta, and gamma scales, although the patterns of diversity increase are scale dependent. For instance, a recently disturbed area might have higher alpha diversity than an undisturbed area but this is a result of different patches within the area where existing biomass either was or was not removed, allowing for colonization by novel species. Thus, it is not always clear whether the increase in diversity should be attributed to alpha or gamma diversity. However, the presence of patches in different successional stages also increases beta diversity (and gamma diversity) by supporting cohorts of species with different life history traits and characteristics in the same landscape, whether or not the straightforward number of species present in any given patch is higher than any other patch. Simply comparing alpha diversity in each patch might not demonstrate the impact disturbance has on patterns of biodiversity, whereas calculating beta and gamma diversity provide a more accurate picture.
: Calculating Alpha, Beta, and Gamma diversity
: This figure shows biodiversity for nine mountain peaks across three Ecoregions. Each symbol represents a
different species; some species have populations on only one peak, while others are found on two or more peaks.
1. Calculate alpha, beta, and gamma diversity for each mountain/Ecoregion.
2. Does the "most diverse" mountain or Ecoregion change depending on which metrics we use?
: Calculations for each alpha, beta, and gamma measurements of the three ecoregions are shown.
Ecoregion 1 has the highest highest average species richness (alpha diversity) across its three peaks. If we were only to look at average alpha diversity, we might consider it the most diverse region. However, Ecoregion 3 has much higher turnover (beta diversity) and higher species diversity at the landscape scale. Furthermore, all of the species represented in Ecoregions 1 and 2 are also represented in Ecoregion 3.
18.3.3: References Chapin, F. S., Matson, P. A., & Mooney, H. A. (2002). Principles of terrestrial ecosystem ecology (pp. 281-304). Springer. Fujiyoshi, M., Takahashi, Y., & Shinano, T. (2005). Effects of arbuscular mycorrhizal fungi and soil developmental stages on herbaceous plants growing in the early stage of primary succession on Mount Fuji. Ecological Research, 21(2), 278-284. Korablev, A. P., & Neshataeva, V. Y. (2016). Primary plant successions of forest belt vegetation on the Tolbachinskii Dol Volcanic Plateau (Kamchatka). Izvestiya Akademii Nauk. Seriya Biologicheskaya, 2016(4), 366-376. Whittaker, R. H. (1960). Vegetation of the Siskiyou Mountains, Oregon and California. Ecological Monographs, 30, 279-338. https://doi.org/10.2307/1943563 Whittaker, R. H. (1972). Evolution and measurement of species diversity. Taxon, 21, 213-251. https://doi.org/10.2307/1218190
18.3.4: Contributors and Attributions Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources:
18.3: Patterns of Diversity Following Disturbance is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
18.4: What causes successional change? The trajectory of successional change can be influenced by site conditions, by the type of events initiating succession, by the interactions of the species present, and by more stochastic factors such as availability of propagules or weather conditions at the time of disturbance. Some of these factors contribute to predictability of succession dynamics; others add more probabilistic elements. Autogenic succession can be brought by changes in the soil caused by the organisms there. These changes include accumulation of organic matter in litter or humic layer, alteration of soil nutrients, or change in the pH of soil due to the plants growing there. The structure of the plants themselves can also alter the community. For example, when larger species like trees mature, they produce shade on to the developing forest floor that tends to exclude light-requiring species. Shade-tolerant species will invade the area. Allogenic succession is caused by external environmental influences and not by the vegetation. For example, soil changes due to erosion, leaching or the deposition of silt and clays can alter the nutrient content and water relationships in the ecosystems. Animals also play an important role in allogenic changes as they are pollinators, seed dispersers and herbivores. They can also increase nutrient content of the soil in certain areas, or shift soil about (as termites, ants, and moles do) creating patches in the habitat. This may create regeneration sites that favor certain species. Climatic factors may be very important, but on a much longer time-scale than any other. Changes in temperature and rainfall patterns will promote changes in communities. As the climate warmed at the end of each ice age, great successional changes took place. The tundra vegetation and bare glacial till deposits underwent succession to mixed deciduous forest. The greenhouse effect resulting in increase in temperature is likely to bring profound Allogenic changes in the next century. Geological and climatic catastrophes such as volcanic eruptions, earthquakes, avalanches, meteors, floods, fires, and high wind also bring allogenic changes. 18.4.1: Models of Successional Change Joseph Connell and Ralph Slatyer further developed the understanding of successional mechanisms in their 1977 paper and proposed that there were 3 main modes of successional development. These sequences could be understood in the context of the specific life-history theories of the individual species within an ecological community. Facilitation model The facilitation model is based on the assumption that only particular species with qualities ideal for "early succession" can colonize the newly exposed landforms after an ecological disturbance. These "colonizing" qualities include: highly effective methods of dispersal, the ability to remain dormant for long periods of time, and a rapid growth rate. However, the pioneer species are often subsequently less successful once an area has been heavily populated by surrounding species due to increased shade, litter or concentrated roots in the soil, etc. (Connell & Slatyer, 1977). Thus, the presence of early successional species often changes the environment so that the habitat is less hospitable for the original species' own ecological demands and facilitates the invasion of later-successional species. The facilitation model suggests that the presence of an initial species aids and increases the probability of the growth of a second species. For example, the presence of alder plants aids the growth of willow and poplar seedlings in an Alaskan floodplain (Walker & Chapin, 1986). Alder roots contain nitrogen-fixing bacteria, which greatly increase the amount of inorganic nitrogen present in soils. This increased availability of nitrogen aids the growth of both willow and poplar seedlings in areas without other competition. Eventually, however, willow and poplar grow more rapidly than alder, leading to a reduction in the abundance of the pioneer species, and eventually, spruce becomes a later-succession species, due to its increased ability (over alder) to grow in shaded areas. Another case of facilitation comes from the colonization of lakeshore sand dunes (Connell & Slatyer, 1977; Walker et al., 1986). Adjacent pioneer plants colonize the otherwise moving sands and alter the environmental constraints of the sandy environment to better suit other plant species, which can then allow for soil binding to take place (National Biological Information Infrastructure, 2011). The giant saguaro cactus, in this respect, can only survive in the shade of other plants (or in some cases rocks) - pioneer species facilitate their existence by providing shade. (The argument has also been made that this type of interaction is exemplary of the tolerance model; see below) (Connell & Slatyer, 1977).
Bare rock is frequently replaced by species A, species A by species B, and species B by species C. All these species are equally
likely to be replaced by bare rock. Species C is unique in that it is almost always replaced by itself, only rarely by bare rock, and
Tolerance model In this case, new pioneer species neither inhibit nor facilitate the growth and success of other species. The sequences of succession are thus entirely dependent on life-history characteristics such as the specific amount of energy a species allocates to growth (Connell & Slatyer, 1977). The climax community is composed of the most "tolerant" species that can co-exist with other species in a more densely populated area. Eventually, dominant species replace or reduce pioneer species abundance through competition. The tolerance model is completely dependent upon life history characteristics. Each species has an equally likely chance to establish itself in the early stages of succession and their establishment results in no environmental changes or impacts on other species (Moorcroft, 2011). Eventually, early species, typically dominated by r-selected species, which prioritize fast rates of reproduction, are out-competed by K-selected species (species that become more dominant when there is competition for limited resources). For example, we can examine succession in the Loess Plateau in China, where there is initial dominance of the Artemisia scoparia (virgate wormwood), the pioneer species. Over time, however, the Bothriochloa ischaemum (yellow bluestem) becomes the dominant species and the abundance of A. scoparia greatly declines. This is due to the rapid rate of reproduction of the A. scoparia, resulting in the species' early abundance, and the dominant competition from the K-selected B. ischaemum, resulting in that species' later abundance (Wang, 2002). A characteristic that is often associated with the tolerance model and well documented in forest succession is survival in conditions of shade. As an uninhabited area becomes populated by different plant species, shade increases - which makes less light available for the next generation. Species that are better adapted to shady conditions will then become dominant.
: Any species is equally likely to replace any other, and equally susceptible to disturbance.
Inhibition model Earlier successional species actually inhibit growth of later successional species and reduce growth of colonizing species already present (Connell & Slatyer, 1977). Example: Pioneer species might modify the environment through rapid growth and make the area increasingly shady (essentially increasing competition for light). The environment is thus less hospitable to other potential colonizing species. The only possibility for new growth/colonization in this successional sequence arises when a disturbance leads to dominating species being destroyed, damaged, or removed. This frees up resources and allows for the invasion of other species that were not previously present. In this model, one species inhibits the presence of another, either through direct means, such as predation (by eating the other species or attacking them), or indirect means, such as competition for resources. Sometimes in inhibition models, the time of establishment of a species determines which species becomes dominant. This phenomenon is referred to as the priority effect and suggests that the species that became established earlier are more likely to become the dominant species. One example of the inhibition model, and the priority effect, occurs in South Australia. In areas where bryozoans are established first, tunicates and sponges cannot grow. The inhibition model has also been observed at work in forest ecosystems; in these systems the early arrivers hold a monopoly on the land, keeping other species out. Closed shrub canopies have been known to prevent tree growth and access to land for periods of up to 45 years - in an experimental study on inhibition it was found that areas occupied by large areas of Lantana (Lantana camara) sprawling shrubs excluded and inhibited the growth of tree species (Connell & Slatyer, 1977; Ricklefs, 2008).
: Each species is equally likely to colonize bare rock, and all species are equally susceptible to disturbance. Each
species holds its site and inhibits occupancy by all others, so replacement occurs only by disturbance. Source: Donovan and
References Connell, J. H., & Slatyer, R. O. (1977). Mechanisms of succession in natural communities and their role in community stability and organization. The American Naturalist, 111(982), 1119-1144.
Moorcroft, P. (2011). Terrestrial succession. Organismic and Evolutionary Biology, 55. Maxwell Dworkin, Cambridge.
Ricklefs, R. E. (2008). The economy of nature (6th ed.). W. H. Freeman and Co.
Walker, L. R., & Chapin, F. S. III. (1986). Physiological controls over seedling growth in primary succession on an Alaskan floodplain. Ecology, 67(6), 1508-1523.
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Contributors and Attributions Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources: 18.4: What causes successional change? is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
18.5: The Role of Consumers and Alternative Stable States 18.5.1: Succession in Consumers Animal communities also undergo change throughout successional time. In the lichen stage fauna is sparse. It comprises a few mites, ants and spiders living in cracks and crevices. The fauna undergoes a qualitative increase during the herb grass stage. The animals found during this stage include nematodes, insects larvae, ants, spiders, mites, etc. The animal population increases and diversifies with the development of the forest climax community. The fauna consists of invertebrates like slugs, snails, worms, millipedes, centipedes, ants, bugs; and vertebrates such as squirrels, foxes, mice, moles, snakes, various birds, salamanders and frogs. The pioneering fauna will colonize an area only after flora and fungi have inhabited the area. Soil fauna, ranging from microscopic protists to larger invertebrates, have a role in soil formation and nutrient cycling. Bacteria and fungi are the most important groups in the breakdown of organic detritus left by primary producing plants such as skeletal soil, moss and algae. Soil invertebrates enhance fungal activity by breaking down detritus. As soil develops, earthworms and ants alter soil characteristics. Worm burrows aerate soil and ant hills alter sediment particle size dispersal, altering soil character profoundly. Though vertebrates in general would not be considered pioneer species, there are exceptions. Natterjack toads are specialists in open, sparsely vegetated habitats which may be at an early seral stage (Faucher et al., 2017). Wide-ranging generalists visit early succession stage habitats, but are not obligate species of those habitats because they use a mosaic of different habitats. Vertebrates can effect early seral stages. Herbivores may alter plant growth. Fossorial mammals could alter soil and plant community development. In a profound example, a seabird colony transfers considerable nitrogen into infertile soils, thereby altering plant growth. A keystone species may facilitate the introduction of pioneer species by creating new niches. For example, beavers may flood an area, allowing new species to immigrate (Wallwork, 1970). Succession of micro-organisms including fungi and bacteria occurring within a microhabitat is known as microsuccession or serule. Like in plants, microbial succession can occur in newly available habitats (primary succession) such as surfaces of plant leaves, recently exposed rock surfaces (i.e., glacial till) or animal infant guts, and also on disturbed communities (secondary succession) like those growing in recently dead trees, decaying fruits, or animal droppings (Ortiz-Álvarez et al., 2018; Martin et al., 2021). 18.5.2: Alternative Stable States One of the main ways consumers can affect succession is through the redirection of successional trajectories. By preferentially consuming some species, consumers can change the direction of succession, resulting in a different community type than would typically be predicted. Expected successional trajectories are predicted based on the observation of community changes through time, or through space-for-time substitutions where stands of different ages are compared, with the expectation that the youngest stands will grow to look like the oldest as time passes. However, given substantial perturbations, communities may end up going down an entirely different path than we would predict. Sometimes, they will even get "stuck" in this state, and never return to look as we would expect. This becomes especially problematic when they place they get "stuck" is low-diversity or dominated by invasive species, and makes it very hard to restore healthy ecosystems. We can visualize a community about to undergo succession as a ball about to roll down a hill. The trajectory of succession is predictable, and gravity will pull the ball to the bottom of the hill. Disturbance can provide energy to push the ball back up the hill, but it should consistently return to the same stable end point. However, the ball may exist in a landscape with many low points, where the ball would have to roll far up a hill to change positions. Thus, there may be several alternative stable states where the community will be resistant to change. Catalysts, such as invasive species or ecosystem engineering animals can push the ball over a peak and into a different valley, resulting in a different endpoint to our successional trajectory. At this point, even a disturbance may not provide enough energy to push the ball all the way back up the hill and over a peak into the original successional trajectory. The new endpoint is stable, and hard to restore from.
: a) the starting point of early succession, which should proceed "downhill" along a predictable trajectory. b) the stable
climax community at the end of a successional trajectory. c) some catalyst has pushed our community out of its original state and
into an alternative stable community state.
: Moose range expansions and implications for succession
As the cover of forested land increases and the climate warms, moose (Alces alces americana) are expanding their range on the northern and southern edges, even as far south as Connecticut (Wildlife of Connecticut , n.d.). Moose often spend time in early-successional forests because of the high-availability of young tree and shrub growth to browse on. Moose not only browse the tips of trees, reducing their ability to grow vertically, but they can actually break trees in half as they try to reach the palatable tips of branches and leaves. As a result, moose may be redirecting the trajectory of early-successional forests, especially in the southernmost edges of their range, in Connecticut, by preferentially browsing certain species of trees while avoiding others (Olmsted, 2021). A typical successional trajectory for a mixed-hardwood forest in Connecticut would look something like this:
However, when moose browsing is added to the equation, we would expect one of two possible outcomes.
1. If moose browse all species equally and they are sufficiently abundant, they may actually delay succession, keeping the forest in a more-open, shorter state (the ball takes longer to roll down the hill).
: Possible outcome of typical successional trajectory when moose are added to the scenario, where the moose delay the succession development rate.
2. If moose browse preferentially, avoiding species they dislike (such as Spruce), they may redirect the trajectory of the forest. The mature forest which eventually takes hold will be dominated by these browse-resistant tree species, and the community composition will look very different than the trajectory in the absence of moose.
: Alternate possible outcome of typical successional trajectory when moose are added to the scenario, where the moose redirect the species diversity.
Both of these outcomes occur where moose are present, but which outcome we see is contingent on the abundances of preferred and non-preferred tree species. In the first case, if moose are controlled or removed in the third time point, the closed forest should eventually look like the original successional trajectory. However, in the second case, even if moose are no longer present in the third time point (this is likely, because moose frequent open, younger forests), the forest will still be dominated by browse-tolerant unpalatable tree species. It will no longer resemble the expected successional trajectory, and it will be unlikely to revert without major disturbance. The ability of moose to redirect succession in this way makes them an example of an ecosystem engineer. In the second case, by keeping forests open and in an early successional stage for longer, moose provide habitat and maintain food resources for many species of bird and other small animals.
References Faucher, L., Hénocq, L., Vanappelghem, C., Roundel, S., Tocqueville, R., Galina, S., Godé, C., Jaquiéry, J., & Arnaud, J. F. (2017). When new human-modified habitats favor the expansion of an amphibian pioneer species: Evolutionary history of the natterjack toad (Bufo calamita) in a coal basin. Molecular Ecology, 26(17), 4434-4451. https://doi.org/10.1111/mec.14229 Martin, P. L., King, W., Bell, T. H., & Peter, K. (2021). The decay and fungal succession of apples with bitter rot across a vegetation diversity gradient. Phytobiomes Journal. https://doi.org/10.1094/PBIOMES-06-21-0039-R Olmsted, C. F., Betras, T. L., Pasquini, S. C., DeStephano, S., Faison, E. K., & Carson, W. P. (2021). Characteristics of stembreaking by moose (Alces alces, Cervidae): A case-study and worldwide review. The Journal of the Torrey Botanical Society, 147(4), 304-315. Ortiz-Álvarez, R., Fierer, N., de los Ríos, A., Casamayor, E. O., & Barberán, A. (2018). Consistent changes in the taxonomic structure and functional attributes of bacterial communities during primary succession. The ISME Journal, 12(7), 1658-1667. https://doi.org/10.1038/s41396-018-0076-2 Wallwork, J. A. (1970). Ecology of soil animals. McGraw-Hill. Wildlife of Connecticut. (n.d.). Moose. Retrieved April 11, 2025, from http://wildlifeofct.com/moose.html Contributors and Attributions Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources: Original text for Alternative Stable States and Box 1, ball diagrams, by Castilleja Olmsted. Forest illustrations for Box 1 by Tiffany Betras, reproduced and modified here with permission. https://en.wikipedia.org/wiki/Ecological_succession https://en.wikipedia.org/wiki/Pioneer_species 18.5: The Role of Consumers and Alternative Stable States is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.