18 |Biodiversity and Community Stability
Viewpoint: Yes, greater species diversity does lead to greater stability in ecosystems. relationships, species diversity, the relative frequency of different species, generally thought of healthy natural communities in terms of species diversity. It uses a theoretical approach based on simple statistical relationships and some to a positive correlation between species diversity and community stability. Investigating the effect of biodiversity on the stability of ecological communities is complicated by the numerous ways in which models of community interactions.
Disturbances can result in the continual density-independent removal of organisms from a community. The disturbance hypothesis is essentially an alternative to the competition hypothesis; these two mechanisms would seem to be mutually exclusive.
In communities that are not fully saturated with individuals, competition is reduced and coexistence is possible without competitive exclusion. Thus, this hypothesis suggests that communities or portions of communities can in some sense be oversaturated with species in that more species coexist than would be possible if the system were allowed to become truly saturated with individuals.
Disturbance may operate through primary- secondary- or tertiary-level mechanisms see also mechanism 10, Predation. Catastrophic winter cold snaps and subsequent density-independent kills see Table 9. An important variant of this hypothesis, termed the intermediate disturbance hypothesis, recognizes that even though infrequent to moderately frequent disturbances enhance diversity, extremely frequent disturbances can be so decimating that they operate in reverse to reduce diversity.
Thus, diversity may actually peak at intermediate levels of disturbance. By either selective or random removal of individual prey organisms, predators can act as rarefying agents and effectively reduce the level of competition among their prey. Indeed, as described in Chapters 14 and 15, predators can allow the local coexistence of species that are eliminated by competitive exclusion in the absence of the predator. Because many predators prey preferentially upon more abundant prey types, predation is often frequency dependent, which promotes prey diversity.
Clearly, several of these mechanisms may often act together to determine the diversity of a given community, and the relative importance of each mechanism doubtless varies widely from community to community. A multitude of various possible ways in which these mechanisms could interact have been suggested; one is shown in Figure One way in which various mechanisms might interact to determine community diversity.
Although tree species diversity is thus exceedingly high, 1 many of these trees are phenotypically almost identical with broad evergreen leaves and smooth bark indeed, only an expert can distinguish among most species. Many species are quite rare with densities below one tree per hectare.
How can so many similar species, apparently all light limited, coexist at such low densities? Explaining why tropical tree diversity is so high is among the most challenging questions facing ecologists. Numerous hypotheses have been proposed but relevant data on this fascinating phenomenon remain distressingly scant.
General relationships between species diversity and stability in competitive systems.
Because seed predation is intense in the tropics, Janzen argued that seedlings cannot establish themselves in the vicinity of parental trees since the high densities of seeds attract many seed predators see Figure This argument predicts that successful recruitment of seeds to seedlings will occur in a ring around but at some distance from the parental tree.
Inside and outside this ring, other tree species can establish themselves. The variety of seed protection tactics such as toxic matrices has forced many seed predators to specialize on the seeds of particular species.
Heavy seed predation coupled with species-specific seed predators holds down densities of various tree species and creates a mosaic of conditions for seedling establishment. Hubbell examines data relevant to this hypothesis and concludes that factors other than seed predation limit the abundance of tropical tree species and prevent single-species dominance. The number of ways in which plants can differ is decidedly limited, especially in the wet tropics where variation in soil moisture is relatively slight.
One mechanism that could help to maintain high plant species diversity involves differentiation in the use of various materials, such as nitrogen, phosphorus, potassium, calcium, various rare earth metals, and so on.
According to this argument, each tree species has its own particular set of requirements; the soil underneath each species becomes depleted of those particular resources, making it unsuitable for seedlings of the same species. Eventually, after the tree falls and is decomposed, these materials reenter the nutrient pool and that species grows again. Thus, like the seed predation hypothesis, this hypothesis predicts a "shadow" around a parent tree where seedlings of that species will be rare or non-existent.
In this mechanism, species A is envisioned as being competitively superior to species B, while species B in turn excludes species C, whereas species C wins in competition with species A. Under such a circular hierarchy of competitive ability, the identity of the species occurring at a particular spot will be continually changing from C to B to A and then back to C again, repeating the cycle.
Does greater species diversity lead to greater stability in ecosystems?
Circular networks with many more species could exist. Such non-transitive competitive interactions could help to maintain the high diversity of tropical trees. Frequent disturbances by fires, floods, and storms might interrupt the process of competitive exclusion locally and allow maintenance of high diversity Connell A variant on this hypothesis involving epiphytes as agents of disturbance many more epiphytes occur in the tropics than in the temperate zones has been developed by Strongwho suggests that tree falls due to epiphyte loads are frequent in the tropics, continually opening up patches in the forest and fostering local secondary succession.
The preceding hypotheses barely begin to address the question of why hardly any temperate forests support more than a dozen species of trees. If there are more species-specific seed predators in the tropics, why?
Ecology/Community succession and stability - Wikibooks, open books for an open world
Why should nutrient differentiation be more pronounced in the tropics? Why doesn't the circular network mechanism foster higher diversity in temperate forests? Are disturbances more frequent in the tropics and if so, why? Ultimately, latitudinal variation in any of the previously proposed mechanisms will have to be related to underlying variation in physical variables such as climate.
A tropical dry forest in Costa Rica was subjected to fairly detailed scrutiny by Hubbellwho mapped Prior to Hubbell's work, the traditional generalization had been that tropical trees tend to occur at low densities, more or less uniformly distributed in space evenly spread out. However, in this Costa Rican dry forest, most tree species, especially rarer ones, were either clumped or randomly dispersed.
Hubbell stresses that his results strongly imply that relatively simple physical and biotic factors must govern seed dispersal.
Thus, there seems to be more to high tropical tree species diversity than tree spacing constraints. Hubbell suggests that periodic disturbances are crucial and proposes a random walk-to-extinction model, which generates patterns of relative abundances similar to those observed along a "disturbance" gradient.
In support of this argument, rare species of trees on the study plot had very low reproductive success -- they were not replacing themselves locally, although they may have been invading from near by. Types of Stability In ecology, the term "stability" has often been used loosely and left vague and undefined.
Many different kinds of stability exist, which can actually vary inversely with one another. Various sorts of mathematical models generate equilibria, such as carrying capacity in single-species models or equilibrium population densities in multi-species models see Chapters 9, 12, and The notion of such fixed constant equilibria is no doubt an illusion, as equilibria in real systems presumably wander about state space indeed, some may never be in equilibrium. The simplest equilibrium structure is the point equilibrium.
Point equilibria may be either attractors or repellers. Population trajectories move away from the vicinity of point repellers, but converge on attractors. Associated with point attractors are domains of attraction, bordered regions of state space from within which systems will return to the point attractor.
Examples of various stable equilibrium points can be seen in the Lotka-Volterra competition equations Chapter Local stability is distinguished from global stability finite versus infinite domains of attraction, respectively.
In the absence of any perturbations, as in a perfectly unchanging world, any system would persist at its equilibrium state. Obviously, the real world is not unchanging; therefore, a fundamental question of interest is, "How do systems respond to various sorts of perturbations?
Two distinct kinds of perturbations can be recognized: Most natural perturbations are of the latter sort, which are difficult to understand because they alter both equilibria and domain of attractions. The former sort of direct perturbations are understood much better than the latter.
Numerous different concepts of stability have been applied to populations and communities. Persistence through time, measured by how long a population lasts before going extinct, is of obvious biological relevance.
Various other concepts of stability are represented graphically in Figure Consider the plots in this figure to represent two-dimensional slices through an n-dimensional hypervolume representing population densities relative abundances of all the species in an n-species system.
All four panels should be considered together, as they represent several fundamental, but different, kinds of stability two systems can differ in relative degree of stability depending upon which particular type of stability is under consideration. In each of the four plots, system A is more stable than system B. This kind of stability is known as constancy or its inverse variability.
In some situations, multiple domains of attraction and multiple alternative stable states may exist, with perturbations causing the system to oscillate between them. A more restricted mathematical definition of resilience is Lyapunov stability, which is restricted to recovery from small perturbations rate of return is measured by the degree of negativity of the dominant eigenvalue of a Jacobian matrix see below.
Graphical representations of some concepts of stability. In each graph, system A is more stable than system B. Frequency of occurrence of states of the systems is indicated by the intensity of stippling. Degree to which the system changes following perturbation. In some situations, multiple alternative states may exist. Rate of return to equilibrium following perturbation. Systems return to initial states following disturbances but only from within certain delimited regions.
More complex stability structures, such as cyclic stability, also exist. The simplest of these is neutral stability, wherein the system changes cyclically with the particular trajectory depending solely on initial conditions as in the Lotka-Volterra predator-prey equations Chapter Another family with cyclic stability is limit cycles, which exhibit more interesting and constrained behavior: Limit cycles can be represented as ellipsoidal trajectories, indicating the possible states of the system in such a case, an unstable equilibrium point lies within the ellipse.
The system converges to a particular state from a variety of initial states secondary succession is an example. Quasi-periodic behavior is also approximately cyclic.
Chaotic attractors, also known as fractal and strange attractors, are still more complex in that the system never returns to the same state, but is nevertheless constrained to oscillate within a certain finite hyperspace. Note that this system stays on either of two disks arranged at angles to each other in 3-dimensional space, winding out on one disk until it gets too close to the outside edge when the system then "inserts" to a new trajectory deep within the other disk such behavior is known as "folding".
Finally, another type of stability is trajectory stability Figure Two dimensions of the Lorenz strange attractor. A third dimension y is not shown. Community Stability Numerous concepts of stability can be and have been applied to communities, including constancy, variability, predictability, persistence, resistance, resilience, and others Figures The stability metric most commonly studied is resilience, the rate at which a system returns to equilibrium following perturbation.
When attention is restricted to resilience of the community to small perturbations, this measure of stability is known as Lyapunov stability, which has a clearly defined and tractable mathematical foundation and consequently has received a great deal of theoretical attention.
This equation was formulated independently by Lotka and Volterrabut interestingly each had a rather different method of derivation Real and Levin ; Haydon and Lloyd Lotka assumed that full details of growth rates and interactions between species in communities might not be known.
He considered the right-hand side of equation 1 to be only the first two terms of a Taylor series of the full but unknown per capita growth rate functions expanded around an equilibrium point about which the community might be centered. Such a formulation has the advantage that it possesses great structural generality, and any such equations, however complex, can be approximated by equation 1.
A disadvantage is that parameters have no easy ecological interpretation, and will change depending on the position of the equilibrium around which original underlying equations are expanded. Volterra assumed that full details of growth rates and species interactions were known, and fully and globally described by equation 1. As such, parameters are easily interpreted: The difference between these two formulations is admittedly subtle, but has important consequences for their interpretation.
For example, Volterra's formulation assumes that dynamics are homogeneous and non-spatial, but similarly simple interspecific interactions can lead to locally complex non-equilibrium behavior and patterning when modeled in an explicitly spatial arena. According to Sousa disturbances cause patchiness in natural systems.
Disturbance renew limiting resources and allow species to coexist. These patches allow for migration between them increasing diversity which ultimately prevents local species from extinction. In areas of local species the absence of disturbance results in a decline in species diversity. These recurrent patchy disturbances are characteristics of most natural systems. Examples include fires in terrestrial plant communities, hurricanes and wind throws in forests, storms over coral reefs, burrowing activity of badgers in prairie grasslands, drift logs which batter the marine intertidal, damage caused by grazing elephants in East Africia, and many more.
Disturbances, such as fire, often initiate succession. It should be noted that succession can be managed.
For example, the most important disturbance of Mediterranean forests might possibly be forest fires. Mediterranean forests are a key factor in bio-geochemical cycles, in protection of soil from erosion, regulating water runoff in watersheds, and stabilizing slopes. Therefore, in the recent decade, research and political efforts have been geared towards fire-fighting and returning scorched areas to their pre-fire conditions . Fire is an example of a disturbance. The connection between community stability and disturbance is one that has been questioned many times.
The intermediate disturbance hypothesis suggests that in a community, the highest diversity is seen in communities with intermediate disturbance.
And since an increase in diversity equals an increase stability, this hypothesis shows that communities with an intermediate amount of disturbance tend to be the most stable.
The Intermediate Disturbance Hypothesis takes into part both benthic and pelagic abilities. The benthic success includes adult survival and reproduction, and the pelagic success involves larval settlement. The amount of coexistence that a dominant species can tolerate determines how many species can be supported. This hypothesis also states that further settlement by other species is maintained at equilibrium by the balance between settlement, productivity, and mortality but is independent of adult body size.
Disturbance increases free space, but settlement decreases it. The three factors that determine the fate of an organism in a habitat are equilibrium free space, disturbance, and settlement. Sufficient settlement with little disturbances yields a habitat that has a stable population.
This may require years of observation. Another problem in determining community stability is in areas that have undergone disturbance but return to equilibrium too quickly. This happens in areas affected by abiotic factors that are under so much stress that they never reach their carrying capacity. If there are alternative stable states, it is sometimes difficult to determine the stability of a population.
Ecology/Community succession and stability
As a result the community will move towards another equilibrium never retuning to it original state. Charles Elton[ edit ] Charles Elton was one of the most influential ecologists in history. He spent most of his time at Oxford. One of his most notable works was the book Animal Ecology This was the first book to give the basic principles of modern animal ecology.
Throughout his career he conducted studies on many different species. He published many books and articles many of which were in the Journal of Animal Ecology which he founded and edited for nearly 20 years.