The Notion That Landscapes Are Continually Changing Through Time is Often Called

Climax Communities

Instead, climax communities form a continuum that varies across environmental gradients largely characterized by local variations in climatic and edaphic conditions, local disturbance regimes, and biotic factors, particularly herbivores.

From: Soil Microbiology, Ecology and Biochemistry (Third Edition) , 2007

Succession, Phenomenon of

H.H. Shugart , in Encyclopedia of Biodiversity (Second Edition), 2013

Glossary

Autotrophic successions

These generate energy from internal processes (photosynthesis).

Climax community

According to some theories of succession, the end result of succession in which successional change ends with a community that does not change and that is in equilibrium with the climate.

Disturbance

Major alternations in vegetation due to events such as wildfires, hurricanes, landslides, and human clearing.

Heterotrophic successions

Dependent on already fixed energy, such as the successional of communities associated with the decomposition of dead logs.

Individualistic view of succession

The concept that succession is a consequence of species interacting with one another and their environment.

Primary succession

Succession on newly exposed substrates such as a sandbar or rubble at the foot of a receding glacier.

Progressive succession

Successions in which the dynamic changes are in the directions of increasing species diversity, structural complexity, greater biomass, and increased stability. Retrogressive successions are in the opposite directions.

Secondary succession

Succession on existing substrate (soil) following a disturbance.

Succession

The pattern of change expected in a community over time after a disturbance or after a new substrate has been exposed.

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Succession

J.M. Pandolfi , in Encyclopedia of Ecology, 2008

Expected Community Trends of Succession

The expected community trends of succession are explicit and detailed ( Table 1 ). They rely on an early-successional community that ultimately leads to the 'climax' community, or the most stable state for the successional sequence. This climax community is maintained until an extrinsic disturbance resets the successional clock. The attributes are grouped according to community energetics, community structure, life-history characteristics, cycling of nutrients, selection pressure, and overall homeostasis. These trends follow from the notion that the climax community is a steady state maintained by internal feedback control mechanisms. But they break down if it is not, and there is little evidence so far that they are.

In forests there are a variety of aspects of production ecology which vary over the successional sequence including forest floor depth, chemistry and content of decaying wood, nutrient availability, leaf area, carbon allocation, species composition and their nutritional adaptations, the relative roles of trees and minor vegetation, the relative importance of geochemical, biogeochemical and internal cycles, rate of soil organic matter accumulation, and total ecosystem organic matter. Early-successional forest species generally are shade intolerant, require a mineral seedbed, may be fast-growing, can generally utilize nutrients that occur in newly formed gaps, may be nitrogen-fixing, are tolerant of microhabitats in newly disturbed settings, and are superior competitors in such settings. Late-successional forest species generally are shade tolerant where they are able to grow slowly, may require a more organic seedbed, and are generally poorly adapted to newly disturbed settings where they are poor competitors. Nutrients are generally gained from litter decomposition and mycorrhizae.

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Succession Phenomenon of

H.H. Shugart , in Encyclopedia of Biodiversity, 2001

III.A. Descriptive Mechanistic Models of Succession

When one compares modern descriptive models of succession with the Clementsian model, it is apparent that there is a substantial difference in the spatial scale considered by the two schools. Clements' climax community was considered by him to be a phenomenon that occurred over large areas. This is evident by the fact that Clementsian succession proceeded toward a regionalscale climax. Also, the union of the climax community (vegetation) and associated animals was a "biome" of which there were thought to be only 13 in nontropical North America. Clements believed that when the community in a given location was too small to have all of the species represented, succession might take different courses. However, he also believed that the succession processes he had described (nudation, ecesis, coaction, etc.) would still operate at these smaller scales.

In 1987, Pickett and colleagues produced a table of mechanisms and causes of succession that can be compared directly to earlier writing of Clements (Table I). In this comparison, one sees that many of the processes deemed important by Clements (notably, nudation, migration, ecesis, coaction, and reaction) are also represented by a mechanistic explanation of succession, albeit with different names and differing emphases. The differences are most pronounced with regard to the importance of the reaction (facilitation). Certainly in many primary successions, the changes produced by one set of species appear necessary for the success of the next. For example, the rapidly growing willows (Salix sp.) that stabilize sandbars in rivers seem to be a necessary preamble to the success of subsequent species; the ability of alder (Alnus sp.) to symbiotically fix nitrogen increases the fertility of sites uncovered by receding glaciers; and the organic acids produced by blue-green algae, lichens, and mosses speed the breakdown of granite and the development of a thin soil to support grasses, herbs, and even trees in primary successions on granite outcrops.

Table I. Clementsian Succession Processes Compared with a Mechanistic Explanation of the Causes of Succession ^a

General causes of succession	Contributing processes of Succession	Modifying factors	Clementsian succession analog processes
Site availability	Coarse-scale Disturbance	Size, severity and timing of disturbance, dispersion of species	Nudation and migration
Differential species availability	Dispersal	Landscape configuration, dispersal agents	Migration
	Propagule pool	Time since last disturbance, land use conditions	Migration and ecesis
	Availability of resources	Soil condition, topography, microclimate, site history	Ecesis
Differential species performance	Ecophysiology	Germination requirements, photosynthesis rates, growth rates, population differentiation	Coaction
	Life history	Photosynthesis allocation pattern, timing of reproduction, mode of reproduction
	Competition	Hierarchy of competitive interactions, the presence of competitors, identity of competitors, within-community disturbances, predators, herbivores, resource base
	Herbivory, predation, disease	Climate cycles, predator cycles, plant vigor, plant defenses, community composition, patchiness
	Environmental stress	Climate cycles, site history, prior occupants	Reaction (in part)
	Allelopathy	Soil chemistry, soil structure, microbes, neighboring species	?

a

The first three columns are from a review by Pickett et al. (1987).

However, some species appear to block the success of others and hold sites against species that might otherwise succeed them. In some secondary successions, all the species involved in the succession are present as seeds or other propagules from the initiation. In these cases, the familiar successional sequence of grasses and herbs yielding to shrubs and then to trees may reflect a difference in rate of growth of individuals present from the start. Evolutionarily, it is difficult to explain why a species would evolve to help another take over a site it could otherwise occupy.

Connel and Slatyer (1977) developed a descriptive model of the succession processes based on a mechanistic understanding of succession (Fig. 1). They used the reaction/facilitation issue to frame three models of succession based on mechanisms of interaction (the facilitation model, tolerance model, and inhibition model). In Fig. 1, the facilitation model is most like Clementsian succession as typically interpreted. The three models are different and have different implications for land management and particularly land reclamation. If one had the objective of restoring degraded landscapes, then one might speed the restoration by eliminating established species in the case of the inhibition model, but this would be ill advised in the facilitation model (Fig. 1).

Figure 1. Mechanistic models of ecological succession (reproduced with permission from Connell and Slatyer, 1977).

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The roles of mycorrhizas in successional processes and in selected biomes

Sally E. Smith FAA , David Read FRS , in Mycorrhizal Symbiosis (Third Edition), 2008

Introduction

Studies of the species composition and community structure of assemblages of land plants and their dependent heterotrophs over the last century have enabled the delineation of distinct biomes at the global scale (Odum, 1971). They have also revealed some of the successional dynamics involved in progression from immature and disturbed to mature and stable states in some of these biomes. Over broadly the same time period, commencing with pioneers like Janse (1897) and Gallaud (1905), extensive below-ground surveys have confirmed the presence of the mycorrhizal symbioses in most, but not all, successional stages and in most of the plants that make up the mature biomes. They have also revealed some segregation between plant families, both in the extent to which they are colonized by mycorrhizal fungi and the types of mycorrhiza that they support (Trappe, 1987; Newman and Redell, 1987; Fitter and Moyersoen, 1996; Wang and Qui, 2006).

While it has been relatively easy to achieve the sampling necessary to describe both the species composition of biomes above ground and the nature of mycorrhizal communities below ground, it has proved more demanding to determine relationships between records of the occurrence of mycorrhizas and the possible contributions of the symbioses to the dynamic properties of the biomes in which they occur. Despite these difficulties, some progress has been made. What is emerging is a picture suggesting that the functions of the symbioses go far beyond simple scenarios involving facilitation of mineral nutrient capture by individual plants or of organic C by the associated fungi. It can be hypothesized that while soil and climate have combined to configure the distinctive composition of the autotroph community in each biome, selection will also have favoured mycorrhizal symbioses and mycobionts that are appropriate to that particular set of environmental circumstances. There follows an analysis of the extent to which the emerging evidence supports this hypothesis.

After commencing with a consideration of the roles of mycorrhizas in successional dynamics, attention is turned to arctic-alpine, heathland, boreo-temperate forest and tropical forest biomes, each in turn viewed as a stable climax community occurring along a latitudinal gradient from the poles to equatorial regions. ¹ The extent and nature of mycorrhizal colonization in each case is considered with a view to elucidating the functions that might be important under the conditions prevailing in each system. At this stage, there are more questions than there are answers. It is to be hoped that identification of the questions can at least be of assistance to the next generation of researchers who will continue attempts to test hypotheses of the kind presented above.

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The Future of Agricultural Landscapes, Part II

Julian Brown , ... Saul A. Cunningham , in Advances in Ecological Research, 2021

2.1 Early ecological thought and nature reserve design

Equilibrium in ecology most commonly refers to "…a particular system state at which all the factors or processes leading to change are being resisted or balanced" (Wu and Loucks, 1995). The self-sustaining, climatically and edaphically determined "climax" community was thought to be the equilibrium state in which vegetation naturally existed and would return to if disturbed by humans (Clements, 1916; Weaver et al., 1938). Animal populations and communities were thought to exist at or near an equilibrium number of individuals and species due mostly to density-dependence (populations) and interspecific competition (communities) arising from limited resources (Hairston et al., 1960; Simberloff, 2014; Wiens, 1984; Wu and Loucks, 1995). Species coexistence was only thought possible with minimal niche overlap (Cody, 1981; Diamond, 1978; Hutchinson, 1959).

The human-nature dualism encouraged human-modified parts of landscapes to be thought of as inhospitable areas that must be traversed between habitat patches (Haila, 2002). This was associated with the simplified patch-matrix model proposed by Island Biogeography Theory (IBT) (MacArthur and Wilson, 1967) (Table 1, Fig. 1A).

The design of nature reserves reflected early ecological thinking (Haila, 2002). It was believed that nature reserves would maintain a state of equilibrium or return to that equilibrium following disturbance, through natural processes of self-regulation, provided that humans were excluded (Pickett et al., 1992). Nature reserves were seen as islands in a sea of inhospitable matrix that needed to be large and well connected to preserve resident biota (Diamond, 1975; Gilpin and Diamond, 1980; May, 1975; Terborgh, 1975). While it was recognized that some "successional" or "weedy" species could tolerate human-modified landscapes, they were thought to be of "…negligible interest to conservationists…" by IBT proponents (Diamond et al., 1976), such that management of the (agricultural) matrix was not considered important for conservation.

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Abundance Biomass Comparison Method

R.M. Warwick , in Encyclopedia of Ecology, 2008

Introduction

The 'abundance biomass comparison' (ABC) method is a means of detecting the effects of anthropogenic perturbations on assemblages of organisms that is underpinned by the r- and K-selection theory (see r-Strategist/K-Strategists). Under stable conditions of infrequent disturbance the competitive dominants in the climax community are K-selected or conservative species with a large body size and long life span, and are usually of low abundance so that they are not dominant numerically but are dominant in terms of biomass. Frequently disturbed assemblages are kept at an early successional stage and comprise r-selected or opportunistic species characterized by small body size, short life span and high abundance. The ABC method exploits the fact that when an assemblage is perturbed the conservative species are less favored in comparison with the opportunists, and the distribution of biomass among species behaves differently from the distribution of numbers of individuals among species.

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THE ECOLOGY OF SOIL ORGANISMS

SHERRI J. MORRIS , CHRISTOPHER B. BLACKWOOD , in Soil Microbiology, Ecology and Biochemistry (Third Edition), 2007

CHANGES IN COMMUNITY STRUCTURE THROUGH TIME AND SPACE

Communities change through a number of processes that can operate over very short to very long time scales. "Succession" is the replacement of populations in a habitat through time due to ecological interactions. A "landscape," in ecology, is the particular spatial arrangement of components of the environment that are important in some way to population dynamics of a given species. Landscapes usually include patches of multiple habitats, as well as variability in conditions that affect habitat quality. Unlike some definitions of the term landscape, this definition does not link landscapes to a particular spatial scale. Instead, it recognizes that landscapes are different for different organisms, depending on the spatial scales over which the organisms interact with the environment (Wiens, 1997). Landscapes have an important impact on local and regional community structure. For example, the structure of a metapopulation (e.g., the number of and distance between populations) is embedded within a landscape.

The habitat that is present in the largest proportion in a landscape and that has the greatest connectivity is considered the habitat "matrix," within which other habitat patches are distributed. A habitat matrix can be occupied by a competitively dominant species or by a diversity of species that coexist through the various mechanisms previously discussed. Alternative nonmatrix habitat patches are created in many ways, and many species are adapted to exploit patchily distributed habitats. The dynamics of these habitats are obviously important to community structure. We have to ask, how are the habitats formed, and what proportion of the landscape do they cover?

One type of nonmatrix habitat is created where the competitively dominant species are absent. This habitat is characterized by an abundance of resources due to lack of competition. Fugitive species are adapted to exploit these patches. A process that causes the removal of an otherwise competitively dominant species or group of species is known as a "disturbance." Disturbances also alter distributions of resources or modulators. Many communities are dependent upon disturbances to maintain species diversity and ecosystem function. Light, for example, determines density and diversity of plants within a stand. In a closed-canopy forest, little light hits the ground. Density is generally high in these stands and diversity low. If these areas are subject to disturbances such as tree fall or fire, density of the stand is decreased, light will strike the forest floor, nutrients and water will not be captured as rapidly, and herbaceous layer species will be allowed to establish.

The initial species to appear after a disturbance are r -selected species with dispersal strategies (in space or time) designed to place them in such habitats first. These pioneer species are also capable of making opportunistic use of available resources or have mechanisms to increase rates of nutrient cycling such as N fixation. These species are replaced in time by more competitive species; for example, plant species more tolerant of shade or low soil nutrients. The "climax community" is a stable endpoint of succession, or at least an assemblage in which succession has slowed to the point at which other processes are more important. The initial model of a single climax community has been shown to be inaccurate. Instead, climax communities form a continuum that varies across environmental gradients largely characterized by local variations in climatic and edaphic conditions, local disturbance regimes, and biotic factors, particularly herbivores. Normally the climax community also dominates the landscape and is therefore the matrix community.

Secondary succession follows disturbances that leave soils largely unchanged and plant propagules in the seed bank. The progress of plant succession is often predictable based on climate, soil type, and the presence of seeds in the seed bank. Nutrient cycling is also often altered by disturbances. Enrichment phenomena increase available nutrients, such as by release from litter layers and humic materials through burning. Nutrient availability and rates of nutrient cycles may also be decreased such as when vegetation and humic layers are removed entirely from an area through hurricanes, floods, or intentional management such as plowing. Secondary succession cannot occur following a catastrophic disturbance that removes soil and all biota, such as glacial activity and volcanic eruptions. In this case primary succession, or succession without inputs from a dormant community at the site of the disturbance, occurs at the site. This type of succession often takes hundreds of years to return the community to the predisturbance state. The time required for soil development and recovery of soil populations such as decomposers and mycorrhizal symbionts can often delay recovery of plant communities (Allen et al., 1992).

Disturbances are an inherent part of community structure in a large number of systems. Most disturbances are caused by events that are repeated at some rate and spatial scale. The constant creation of disturbed patches and gradual return to a climax community creates a "shifting mosaic" of different habitats at different stages of succession (Wu and Loucks, 1995). The proportion of the landscape that is in the climax community should equal some steady-state value determined by the rate and spatial scale of the disturbance events and the rate of return to the climax community through succession. This allows fugitive species to depend entirely on the presence of minor habitats with relatively quick turnover rates. Some matrix communities are dependent on widespread repeated disturbance; in this case the matrix community is not the same as the climax community. Such species have evolved mechanisms to persist or regerminate following disturbance. Mangrove forests are dependent on hurricanes to remove colonizing species that without disturbance have the potential to outcompete mangroves. Many grasslands are maintained by fires, as they are responsible for removing tree seedlings that can lead to the establishment of deciduous forests. Unfortunately, catastrophic events such as fires in areas with high debris loads and in areas unaccustomed to fire, such as the tropical rain forest, result in a great deal of damage because species have not evolved mechanisms to tolerate such disturbances. These events have become more common due to human intervention, as have chronic disturbances such as acid deposition and excessive nutrient loading to which no communities are accustomed.

There are many habitats that are qualitatively different from the matrix habitat and are created through some process other than disturbance. Often an entirely different suite of organisms is adapted to exploit these habitats. Examples include the riparian zone near a river and the river itself. In soil, the rhizosphere, fecal matter, and decomposing plant tissue are important examples of this type of habitat (Blackwood and Paul, 2003). The latter two examples represent habitats defined by a limited pool of resources. Microbial succession in these habitats is driven by a constant change in environmental conditions as resources are used up and the environment is restructured. Some of these habitats, including all the soil habitats in the previous example, are created by events that, like disturbances, have a particular rate of occurrence and spatial scale, followed by community succession. Therefore, they also fit into our model of the landscape as a shifting mosaic of habitats.

The shifting mosaic picture of the landscape is based on a dynamic equilibrium model. However, the particular characteristics of a patch edge, the surrounding habitat patches, and ease of dispersal across other elements of the landscape may all be important determinants of metapopulation and local patch population dynamics. Climate change, human activity, and other novel events can result in nonequilibrium dynamics. Under these conditions, populations are kept from reaching carrying capacity or the stable equilibrium predicted by logistic models.

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Impact of bioenergy for the diminution of an ascending global variability and change in the climate

Poonam Kumari , ... Prasann Kumar , in Microbiome Under Changing Climate, 2022

21.11 Ecological succession due to global variability

The change in the composition of species at a specific area is known as ecological succession. The structure and the composition of the species change according to the environmental condition of that area. These changes also occur with the physical environment. This change in the community then maintains the equilibrium with the environment, known as the climax community (Ovington, 1962). Due to the succession in a specific area, the population of some species increases rapidly, but the population of other species declines. So, the changes that occur due to the succession in that area are known as sere. So, this leads to the increase in the number of some species and the increase in biomass. The process of succession starts from where there is no living organism like bare rocks and lava that are called primary succession (Pickett & Cadenasso, 2002). After the primary succession, it changes into secondary succession. On the bare rock or another primary succession area, the biotic community grows very slowly like the rock changes into the soil after thousands of years. The secondary succession starts where some soil is present in some burnt land, deforested land, or forests. The secondary succession is much faster than the primary ones (Post et al., 2000; Reichstein et al., 2007; Seidl et al., 2016).

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Benthic living: sublittoral and deep seabed

Frances Dipper , in Elements of Marine Ecology (Fifth Edition), 2022

7.2.4 Limitations to classifications

Whatever classification system is devised and used, it will never be perfect. Although some parts of the sea floor present sharp discontinuities, for instance, an abrupt change from rock to sand, alterations of substratum from one place to another are mostly gradual. Furthermore, every species has its own particular distribution which is never identical with that of any other. Consequently, boundaries between communities are usually indefinite with intergrading along transitional zones. On almost every part of the seabed the inhabitants comprise a climax community for that particular area, stable in composition within natural, short-term fluctuations. Wherever environment changes with locality, there are corresponding adjustments in the make-up of the assemblage of species.

The concept of a community is essentially an abstraction from studies of overlapping distributions of many species along various ecological gradients. The ease with which communities may be characterized by particular conspicuous species (sometimes rather misleadingly described as 'dominants') is evident in marine benthos around the British Isles, but is less apparent at low latitudes where the composition of communities generally shows a greater diversity. This diversification may be a feature of more mature communities, which have evolved in stable conditions over a long period, permitting the survival of species specialized for narrow ecological niches. The communities of the north-east Atlantic may be regarded as relatively immature, having evolved in fluctuating conditions since the extremes of the Pleistocene period. This favours the evolution of polymorphic populations which survive by virtue of their wide variability, with 'dominant' species occupying broad ecological niches.

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Community Structure

Walter H. Adey , Karen Loveland , in Dynamic Aquaria (Third Edition), 2007

FEATURES OF COMMUNITIES

Under a given set of environmental conditions, the development of the structural elements of the community from a "bare" surface to one of seagrass, kelp, coral reef, etc., changes the local environment. This allows new organisms that require the cover, the substrate, the food organisms, and more or less light, to enter a community that they would be excluded from based only on substrate. Thus, a developing community alters its own environment and slowly drifts to a different structure. This process, called succession, finally reaches a substable state, the climax community, which may take many months or many years to attain. Normally, disturbing factors such as storms, ice, floating logs, or larger predatory animals continually "knock" the community back to the pioneer or intermediate stages of succession. However, such disturbances usually happen in a patchy way and give rise to heterogeneity in community structure. A wild community is dynamic in its composition; the aquarist should not expect a model to be greatly different.

A now-classic example of these dynamic processes, succession, and disturbance, was described for the rocky intertidal shore of northwestern North America by Dayton (1971). In that region and many other temperate and boreal rocky intertidal zones, mussels (e.g. Mytilis spp.) can become the primary structure-creating member of a climax community covering enormous areas. However, logs or ice driven by waves and the feeding of seastars (e.g. Pisaster) and gulls on the mussels constantly remove large patches of these attached bivalves, allowing several species of barnacles to colonize. As a general principle, great disturbance reduces the number of species. However, a moderate level of disturbance, physical or biotic, results in the highest species numbers (Figure 12.10).

FIGURE 12.10. Control of algal species diversity by disturbance created by the grazing of littorinid snails.

After Ehrlich and Roughgarden (1987, from Lubchenco, 1978). Copyright © 1987

A feature of community and trophic structure not yet discussed is the tendency of more complex communities to be characterized by guilds. A guild is a group of species populations that occupies the same or very similar niches. This feature has been most studied in birds and insects but certainly characterizes marine communities such as coral reefs. An example of a bird guild and the resultant resource partitioning is shown in Figure 12.11. This becomes particularly critical in model systems where scaling does not allow the inclusion of all members of the guild and niche overlap allows the utilization of one or a few members to satisfy the need for the niche without overburdening the limited size of the resource in the model.

FIGURE 12.11. Fruit sizes eaten by different fruit-eating pigeons in the South Pacific.

After Ehrlich and Roughgarden (1987, from Diamond, 1975). Copyright © 1987

Within the biomes discussed above, many communities can be delimited. Particularly within terrestrial ecology, considerable dispute has been engendered over the past several decades as to whether communities and their boundaries exist. The interested reader is referred to Ehrlich and Roughgarden (1987) and Blackburn and Gaston (2003) for a discussion of these issues. Regardless of the situation in the terrestrial environment, marine, freshwater, and wetland communities do tend to have sharp boundaries. In part this is because water surface, light, and substrate, which is often wave controlled, are the primary environmental community determiners. These parameters usually have considerably sharper boundaries than the temperature and precipitation boundaries of the terrestrial environment, though there is some parallel where topography and rock type are critical controlling factors. In very extensive sand or mud-bottom biomes, the same difficulty in sharply delimiting communities is encountered.

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