|Community Structure and Stability|
An ecological community is the assemblage of species found in a given time and place. The species composition of different ecosystems and the ways in which they maintain equilibrium and react to disturbances are manifestations of the community’s stability.
The populations that form a community interact through the processes of competition, predation, parasitism, and mutualism. The structures of communities are determined, in part, by the nature and strength of these biotic factors.
Abiotic factors (physical factors such as temperature, rainfall, and soil fertility) are the other set of important influences determining community structure.
An ecological community together with its physical environment is called an ecosystem. No ecosystem can be properly understood without a careful study of the biotic and abiotic factors that shape it.
The most common way to characterize a community functionally is by describing the flow of energy through it. Based on the dynamics of energy flow, organisms can be classified into three groups: those that obtain energy through photosynthesis (called producers), those that obtain energy by consuming other organisms (consumers), and those that decompose dead organisms (decomposers).
The pathway through which energy travels from producer through one or more consumers and finally to decomposer is called a food chain. Each link in a food chain is called a trophic level. Interconnected food chains in a community constitute a food web.
Very few communities are so simple that they can be readily described by a food web. Most communities are compartmentalized: A given set of producers tends to be consumed by a limited number of consumers,which in turn are preyed upon by a smaller number of predators, and so on.
Alternatively, consumers may obtain energy by specializing on one part of their prey (for example, some birds may eat only seeds of plants) but utilize a wide range of prey species. Compartmentalization is an important feature of community structure; it influences the formation, organization, and persistence of a community.
Dominant and Keystone Species
Some species, called dominant species, can exert powerful control over the abundance of other species because of the dominant species’ large size, extended life span, or ability to monopolize energy or other resources. Communities are named according to their dominant species: for example, oak-hickory forest, redwood forest, sagebrush desert, and tall-grass prairie.
|Dominant and Keystone Species|
Keystone species may also be the dominate species, but they may also appear insignificant to the community until they are gone. For example, cord grass (Spartina) is the dominant plant in many tidal estuaries, and it is also a keystone species because so many members of the community depend on it for food and shelter.
The species that make up a community are seldom distributed uniformly across the landscape; rather, some degree of patchiness is characteristic of virtually all species. There has been conflicting evidence as to the nature of this patchiness.
Moving across an environmental gradient (for example, from wet to dry conditions or from low to high elevations), there is a corresponding change in species and community composition. Some studies have suggested that changes in species composition usually occur along relatively sharp boundaries and that these boundaries mark the borders between adjacent communities.
Other studies have indicated that species tend to respond individually to environmental gradients and that community boundaries are not sharply defined; rather, most communities broadly intergrade into one another, forming what is often called an ecotone.
Degrees of Species Interaction
These conflicting results have fueled a continuing debate as to the underlying nature of communities. Some communities seem to behave in a coordinated manner.
For example, if a prairie is consumed by fire, it regenerates in a predictable sequence, ultimately returning to the same structure and composition it had before the fire. This process, called ecological succession, is to be expected if the species in a community have evolved together with one another.
In this case, the community is behaving like an organism, maintaining its structure and function in the face of environmental disturbances and fluctuations (as long as the disturbances and fluctuations are not too extreme). The existence of relatively sharp boundaries between adjacent communities supports this explanation of the nature of the community.
In other communities, it appears that the response to environmental fluctuation or disturbance is determined by the evolved adaptations of the species available. There is no coordinated community response but rather a coincidental assembly of community structure over time.
Some sets of species interact together so strongly that they enter a community together, but there is no evidence of an evolved community tendency to resist or accommodate environmental change. In this case, the community is formed primarily of species that happen to share similar environmental requirements.
Competition and Predation
Disagreement as to the underlying nature of communities usually reflects disagreement about the relative importance of the underlying mechanisms that determine community structure. Interspecific competition has long been invoked as the primary agent structuring communities.
Competition is certainly important in some communities, but there is insufficient evidence to indicate how widespread and important it is in determining community structure.
Much of the difficulty occurs because ecologists must infer the existence of past competition from present patterns in communities. It appears that competition has been important in many vertebrate communities and in communities dominated by sessile organisms, such as plants.
It does not appear to have been important in structuring communities of plant-eating insects. Furthermore, the effects of competition typically affect individuals that use identical resources, so that only a small percentage of species in a community may be experiencing significant competition at any time.
The effects of predation on community structure depend on the nature of the predation. Keystone predators usually exert their influence by preying on species that are competitively dominant, thus giving less competitive species a chance.
Predators that do not specialize on one or a few species may also have a major effect on community structure, if they attack prey in proportion to their abundance. This frequency-dependent predation prevents any prey species from achieving dominance.
If a predator is too efficient, it can drive its prey to extinction, which may cause a selective predator to become extinct as well. Predation appears to be most important in determining community structure in environments that are predictable or unchanging.
Disasters and Catastrophes
Chance events can also influence the structure of a community. No environment is completely uniform. Seasonal or longer-term environmental fluctuations affect community structure by limiting opportunities for colonization, by causing direct mortality, or by hindering or exacerbating the effects of competition and predation.
Furthermore, all communities experience at least occasional disturbance: unpredictable, seemingly random environmental changes that may be quite severe.
It is useful in this regard to distinguish between regular disturbances and rarer, less frequent catastrophic events. For example, fire occurs so often in tall-grass prairies that most of the plant species have become fire-adapted—they have become efficient at acquiring nutrients left in the ash and at sprouting or germinating quickly after a fire.
In contrast, the 1980 eruption of Mount St. Helens, a volcanic peak in Washington State, was so violent and so unexpected that no members of the nearby community were adequately adapted to such conditions.
Natural disturbances occur at a variety of scales. Small-scale disturbances may simply create small openings in a community. In forests, for example, wind, lightning, and fungi cause single mature trees to die and fall, creating gaps that are typically colonized by species requiring such openings.
Large disturbances are qualitatively different from small disturbances in that large portions of a community may be destroyed, including some of the ability to recover from the disturbance.
For example, following a large, intense forest fire, some tree species may not return for decades or centuries because their seeds were consumed by the fire, and colonizers must travel a long distance.
Early ecologists almost always saw disturbances as destructive and disruptive for communities. Under this assumption, most mathematical models portrayed communities as generally being in some stable state; if a disturbance occurred, the community inevitably returned to the same (or some alternative) equilibrium.
It later became clear, however, that natural disturbance is a part of almost all natural communities. Ecologists now recognize that few communities exhibit an equilibrium; instead, communities are dynamic, always responding to the last disturbance.
Long-Term Community Dynamics
The evidence suggests that three conclusions can be drawn about the long-term dynamics of communities. First, it can no longer be assumed that all communities remain at equilibrium until changed by outside forces.
Disturbances are so common, at so many different scales and frequencies, that the community must be viewed as an entity that is constantly changing as its constituent species read just to disturbance and to one another.
Second, communities respond in different ways to disturbance. A community may exhibit resistance, not markedly changing when disturbance occurs, until it reaches a threshold and suddenly and rapidly shifts to a new state. Alternatively, a community may exhibit resilience by quickly returning to its former state after a disturbance.
Resilience may occur over a wide range of conditions and scales of disturbance in a dynamically robust system. On the other hand, a community that exhibits resilience only within a narrow range of conditions is said to be dynamically fragile.
Finally, there is no simple way to predict the stability of a community. At the end of the 1970’s, many ecologists predicted that complex communities would be more stable than simple communities. It appeared that stability was conferred by more intricate food webs, greater structural complexity, and greater species richness.
On the basis of numerous field studies and theoretical models, many ecologists now conclude that no such relationship exists. Both very complex communities, such as tropical rain forests, and very simple communities, such as Arctic tundra, may be very fragile.