Environment of Earth

February 23, 2016

Concepts of diversity and stability


A very perplexing ecological problem is unravelling the nature of relationship between diversity and stability of ecosystem. The problem here is that neither the meaning of stability is entirely clear, nor what aspects of diversity are being consided. Are only the number of species in the community and its relation to stability, or the relationship of evenness to stability, or some combination of both is to be considered? Most of the theoretical and empirical studies for long focused only on species diversity, i.e., the number of species present. Only recently works have focussed on the degree of functional diversity represented by species diversity. Advocates of conserving biological diversity have always invoked the diversity-stability hypothesis to justify concern about the loss of individual species. However, if other aspects of diversity also play important roles in the structure and function of ecosystems, a focus on the number of species alone may hinder proper appreciation of the role that evenness plays in the ability of ecosystems to respond to changes in energy and nutrient inputs.
There are at least three ways in which ecosystem stability might be defined:

1. Constancy- The ability of a community to resist changes in species composition and abundances in response to any disturbance.
This is not a particularly useful concept of stability for conservationists because few, if any, ecosystems could be described as truly constant. Even the ecosystems having powerful mechanisms for reacting to environmental fluctuations do so through internal changes that as quickly as possible bring it back to a stable state. But these surely involve responses and changes. These may more appropriately be regarded as examples of resilience than of constancy.
2. Resilience- The ability of a community to return to its pre-disturbance characteristics after changes induced by a disturbance.
Resilience corresponds to stability in the way it is studied in mathematical models. If deviations from an equilibrium are reduced with time, system is stable or if these are amplified with time, system is unstable. This approach still has little applicability to actual ecosystems. It measures a system’s tendency to return to a single stable point, but many ecological sytems appear to have multiple stable points. If disturbance remains below a particular threshold, ecosystem will return to its predisturbance configuration. If it exceeds that threshold, it may move to a new configuration. Furthermore, most ecological systems change not only in response to disturbance but also in response to natural, successional change. There is little evidence that ecological communities ever represent an equilibrium configuration from which it would make sense to study perturbations. Common between the constancy and resilience is focus of both on species persistence and abundance as measures of stability. For example, Selmants et al.[1] show that with decline in species diversity of serpentine grasslands in California, their susceptibility to invasion by exotic species increases. Put differently, diverse grasslands were more resilient than those with lesser diversity.
3. Dynamic stability- A system’s ability to determine its future states largely by its own current state with little or no reference to outside influences.
In many ways, this approach corresponds with our intuitive notions of stability and seems to make sense of the relationship between diversity and stability. It recalls saying of Commoner [2]: “The more complex the ecosystem, the more successfully it can resist a stress.” A dynamically stable system is relatively immune to disturbance like a rapidly spinning gyroscope is dynamically stable because the gyroscopic forces generated by it resist external forces that would alter is plane of rotation. This approach reflects the hope that stable systems would be able to maintain themselves without intervention.
A biological system with high diversity is more likely to be dynamically stable than one that has low diversity. The reason is very important role played by biotic interactions in ecosystem dynamics. This has increasingly been appreciated through many studies. In diverse communities, biotic interactions may often play a larger and very important role in the success of a species than its interactions with the physical environment. To the extent that changes in the system are driven by biotic interactions, it is dynamically stable, since characteristics of the system itself are determining its future state.
However, this formulation of the diversity-stability hypothesis is also not free from problems. How to identify systems whose future state depends primarily on their own internal characteristics?[3] Without a method to identify a dynamically stable system, even testing the truth of this approach to diversity-stability hypothesis is not possible. It seems to verge on circularity. The larger (more diverse) the system considered, fewer things are left out of it. Fewer the things left out, smaller the possible outside influences on the system. Smaller the possible outside influences, greater the degree of dynamic stability. Thus, dynamic stability is (almost) a necessary consequence of diversity simply because diverse systems include many components.[4] Moreover, the argument as presented says nothing about the types of diversity present, e.g., a diverse community assembled from non-native species would be as good as one composed solely of natives.

Ives and Carpenter [5] have suggested a different approach to understanding community stability. Their approach may be quite useful, because it points out firstly, that systems move to a region different from the one from which they were perturbed[6] and secondly, that things other than diversity (like the frequency and character of perturbation) may also affect the stability of ecosystems. A new concept in relation to stability is ‘Biological integrity’ that refers to a system’s wholeness, including presence of all appropriate elements and occurrence of all processes at appropriate rates.[7] But this approach too poses problems.
1. What are ‘appropriate elements’?
2. What are ‘appropriate rates of processes?’
By definition, naturally evolved assemblages possess biological integrity but random assemblages do not. It, therefore, provides justification for management of ecosystems focusing on native species rather than introduced ones. This seems like the logical fallacy of affirming the consequent. However, species composition of lakes exposed to nutrient enrichment or acidification responds more quickly and recovers more slowly than processes like primary production, respiration, and nutrient cycling. Shifts in biotic composition don’t necessarily lead to changes in process rates. These observations mean a focus on integrity rather than diversity makes sense but it makes more sense to conclude that species changes are a more sensitive indicator of what is going on than the process changes. Loss of native species from a system is truly a warning of process changes that may have consequences much larger than are suspected.

From the point of view of conservation, there are still many  problems.

1. Can it be psossible that constancy or resilience based approaches to diversity-stability hypothesis probably are not true and may not provide a solid conceptual basis for arguing that conservation of biological diversity is an important goal.
2. Is it possible that a less specific version defining stability as a dynamic property related to the degree that the components of a system determine their own future state, provides a plausible basis for the hypothesis. Unfortunately, this version of the hypothesis verges on circularity and is almost immune to empirical investigation. It may also be pointed out that a system that is “stable” with respect to some perturbations like hurricanes, drought, or other extreme weather events may not be stable to others like invasion by exotic plants or animals, extinctions of component species, or other biotic changes. From the point of view of practical conservation applications, the diversity-stability hypothesis seems to provides merely a useful heuristic.
There seems something more useful for practical applicability in the idea of biological integrity. Easily observable changes in species composition and community structure may act as pointer to underlying changes in ecosystem processes more quickly than attempts to directly measure these processes. Diverse systems provide more indicators of change in these underlying processes and if the systems are managed so that they are protected then the underlying processes will remain intact too. Chapin et al. [8] summarized that:

1. High species richness maximizes resource acquisition at each trophic level and the retention of resources in the ecosystem. 2. High species diversity reduces the risk of large changes in ecosystem processes in response to directional or stochastic variation in the environment. 3. High species diversity reduces the probability of large changes in ecosystem processes in response to invasions of pathogens and other species. 4. Landscape heterogeneity most strongly influences those processes or organisms that depend on multiple patch types and are controlled by a flow of organisms, water, air, or disturbance among patches.

Wang and Loreau [9] developed the last point more formally and suggested that when thinking about a meta-community or meta-ecosystem it is useful to decompose the variability in response across the entire system into components analogous to those used in partitioning species diversity i.e.
1. Variation within the individual components of a meta-community or meta-ecosystem. 2. Variation among different components of a meta-community or meta-ecosystem. 3. Variation across the entire system, the sum (or product) of alpha and beta variation.

Thinking about ecosystem functioning at various scales, as Wang and Loreau suggest, leads to recognition that the experimental focus on diversity and variation in functioning leaves out a vital component for those trying to manage ecosystems that includes a variety of different habitats. Stability at the whole-system level may depend as much or more on retaining those distinct components as it does on stability within any one of them.

Bibliography
1. Paul C Selmants, Erika S Zavaleta, Jae R Pasari, and Daniel L Hernandez. Realistic plant species losses reduce invasion resistance in a California serpentine grassland. Journal of Ecology,
100(3):723-731, 2012.
2. B Commoner. The Closing Circle. Alfred Knopf, New York, NY, 1972. 3. R MacArthur. Fluctuations of animal populations and a measure of community stability. Ecology, 35:533-536, 1955.
4. B G Norton. Why Preserve Natural Variety? Princeton University Press, Princeton, NJ, 1987.
5. Anthony R Ives and Stephen R Carpenter. Stability and Diversity of Ecosystems. Science, 317(5834):58-62, 2007.
6. Laurie J Raymundo, Andrew R Halford, Aileen P Maypa, and Alexander M Kerr. Functionally diverse reef-fish communities ameliorate coral disease. Proceedings of the National Academy of Sciences,
106(40):17067-17070, 2009.
7. Noah A Rosenberg, Jonathan K Pritchard, James L Weber, Howard M Cann, Kenneth K Kidd, Lev A Zhivotovsky, and Marcus W Feldman. Genetic structure of human populations. Science, 298(5602):2381-2385, 2002.             8. F S Chapin III, O E Sala, I C Burke, J P Grime, D U Hooper, W K Lauenroth, A Lombard, H A Mooney, A R Mosier, S Naeem, S W Pacala, J Roy, W L Steffen, and D Tilman. Ecosystem consequences of changing biodiversity: experimental evidence and a research agenda for the future. BioScience, 48:45-52, 1998.
9. Shaopeng Wang and Michel Loreau. Ecosystem stability in space: , and variability. Ecology Letters, 17(8):891-901, 2014.

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