Concepts of diversity and stability

Part of the problem here is that it's not entirely clear what we mean by stability, nor what aspect of diversity we are considering.

• Are we concerned only with the number of species in the community and its relation to stability, or are we concerned with how evenness relates to stability, or are we concerned with some combination of both? Work I am aware of that considers the problem focuses only on species diversity, i.e., the number of species present, and only recently has begun to consider the the degree of functional diversity represented.
  • In one sense this may be legitimate. After all, part of the reason conservationists have invoked the diversity-stability hypothesis is to justify concern about the loss of individual species.

  • We may also be missing something important. If other aspects of diversity play an important role in the structure and function of ecosystems, a focus on the number of species alone may blind us to 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 stability might be defined.
  1. Constancy - The ability of a community to resist changes in composition and abundance in response to disturbance. Not a particularly useful concept of stability for conservationists because
     • Few, if any, ecosystems could be described as constant.

     • Even those that have powerful mechanisms for reacting to environmental fluctuations do so through internal changes that return the system as quickly as possible to a stable state. But these involve responses and changes. It seems better to regard them as examples of resiliency than of constancy.

  2. Resiliency - The ability of a community to return to its pre-disturbance characteristics after changes induced by a disturbance. Resiliency corresponds to stability the way it is studied in mathematical models. Are deviations from an equilibrium reduced with time (stable) or amplified with time (unstable)? Still, it 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, it 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.

     Constancy and resiliency have this in common: both focus on species persistence and abundance as measures of stability.

  3. Dynamic stability - A system is dynamically stable if its future states are determined largely by its own current state, with little or no reference to outside influences. In many ways this seems to correspond with our intuitive notions of stability and to make sense of the relationship between diversity and stability.
     • Recall the quote from Commoner: ``The more complex the ecosystem, the more successfully it can resist a stress.''

     • A system that is dynamically stable is one that is relatively immune to disturbance. A rapidly spinning gyroscope is dynamically stable because the gyroscopic forces that it generates resist external forces that would alter is plane of rotation.

     • It reflects our hope that stable systems should be able to maintain themselves without human intervention.

     • A diverse biological system is more likely to be dynamically stable than one that is not diverse because in diverse communities biotic interactions may often play a larger role in a species' success 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.

     There are some problems with this formulation of the diversity-stability hypothesis, however.

     • Although the last thing I said sounds plausible enough, it constituted only a little more than proof by vigorous assertion. How can we identify systems whose future state depends primarily on their own internal characteristics? Are there any?⁹

     • If we don't know how to identify systems that are dynamically stable, we can't even test the diversity-stability hypothesis to see if its true.

     • It verges on circularity. The larger (more diverse) the system considered, the fewer are the things left out of it. The fewer the things left out, the smaller the possible outside influences on the system. The smaller the possible outside influences, the greater the degree of dynamic stability. Thus, dynamic stability is (almost) a necessary consequence of diversity, simply because diverse systems include many components.¹⁰

     • Moreover, the argument as presented says nothing about the types of diversity present, e.g., a diverse community assembled from non-native species would fill the bill just as easily as one composed solely of natives.

• Ives and Carpenter [9] suggest a different approach to understanding community stability (Figure 1).
  • Alternative stable states

  • Non-point attractors

  • Pulse perturbations

  • Press perturbations

  • Extinctions

  • Invasions

  Figure 1: Types of stability, perturbations, and community responses. From Ives and Carpenter [9].

  

  Their approach strikes me as quite useful, first because it emphasizes that systems move to a region different from the one from which they were perturbed¹¹ and second because it reminds us that things other than diversity, like the frequency and character of perturbation, may affect the stability of ecosystems.

• Biological integrity - ``Biological integrity refers to a system's wholeness, including presence of all appropriate elements and occurrence of all processes at appropriate rates.''¹²
  1. What are ``appropriate elements''?

  2. What are ``appropriate rates'' of processes?

  ``By definition naturally evolved assemblages possess integrity but random assemblages do not.'' Therefore, provides justification for management focusing on native species rather than introduced ones.

  This seems like the logical fallacy of affirming the consequent, but

  • 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.

  • Angermeier and Karr suggest that these observations means a focus on integrity rather than diversity makes sense. To me it makes more sense to conclude that species changes are a more sensitive indicator of what's going on than process changes.

  • Loss of native species from a system is truly ``a canary in the mine,'' a warning of process changes that may have consequences much larger than we suspect.