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Stability . One of the most important features of any ecological system is its inherent stability
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Stability • One of the most important features of any ecological system is its inherent stability • Understanding the stability properties of any system, and the characteristics of structure and dynamics which enhance or jeopardize that stability, is fundamental to our comprehension of natural systems
Stability • Stability is a complex phenomenon influenced simultaneously by many population or community processes (which may interact) • Many studies relate stability to a specific characteristic of a population or community (and frequently in isolation)
Stability • Consequently, we will review some of the different ‘unitary’ relationships which have been established in an attempt to give an overview of the larger picture
Stability: verbiage • MacArthur (1955) defined stability as ‘the ability of both populations and communities to withstand environmental perturbation, to accommodate change,’
Stability: verbiage • Orians (1975) identified a number of different elements which may be recognized within the overall concept of stability, which may be grouped into 3 types: constancy, resilience, and inertia (see Table 9.1)
Stability: verbiage • Attributes of such stability functions are Orian’spersistence, elasticity and amplitude (Table 9.1)
Stability: verbiage • Elasticity is a measure of the speed with which a system returns to its former state following some perturbation (aka resilience in web theory) • The amplitude of a system defines that domain over which it is stable • E.g. a system with a high amplitude can be considerably displaced and still return
Stability: verbiage • When a system can return to its previous state following any perturbation, however large, it is considered ‘globally stable’ • If it can only return after a minor perturbation, then it is considered ‘locally stable’
Stability: verbiage • Persistence is the survival time of the system or of some defined element within it • E.g. if one population has a greater time to extinction than another, it may be considered more stable
Stability: verbiage • There are also the concepts of cyclical or trajectory stability • Cyclical: the property of a system to oscillate around some central point (limit cycles) • Trajectory: the property of a system to move towards some final end point or zone despite different starting points
Stability • While populations may have inherent stability, it is unclear if there are emergent properties of the community which may, or may not, add more stability to the system
Stability • Several authors have reviewed the stability of a species (or even a pair of species), which we will not review (too extensive)
Stability • Evidence to date suggests the relationship between individuals and the community is two-way each contributing to the other (but may be greater than sum of its parts) • The strength of an interaction between species will depend upon the type (e.g. predatory, competitive…)
Stability • Consequently, it has been shown that observed communities of a given size are consistently more stable randomly constructed systems with the same number of species
Stability • One of the most frequently cited examples of this is the stability of trophic structure reported by Heatwole and Levins (1972) in their analysis of the organization and structure of arthropod communities established on mangrove islets in FL following defaunation (Simberloff and Wilson 1969)
Stability • If you recall the results appeared to show remarkable consistency in trophic structure (in terms of number of species per trophic level) of the subsequent colonizing communities relative to the previous ones (although there was a great deal of taxonomic turnover)
Stability • Of course, many other studies have not provided such ‘clean’ support for the idea of such stability (recall Simberloff’s own subsequent analysis)
Stability • It is not clear that Heatwole and Levins study really even shows stability • Communities may simply go back to the similar structure as a result of the physical parameters of the habitat itself • Furthermore, is it really resilience if the community has been completely destroyed rather than just perturbed?
Stability • The fact that secondary succession in communities generated after some perturbation eventually returns to its previous climax state (albeit using different pathways at times) may be adduced as evidence for resilience and the constancy of climax communities
Stability • Another good example comes from analyzing the faunal assemblages of West Malaysia before and after selective logging • Logging was relatively modest (18 trees/ha) • Immediate effects were clear (dramatic declines of birds and mammals) and a large shift in species composition
Stability • There was considerable turnover with complete loss of some mammalian species (30 of 45 lost) and colonization by several new species (5) that had not been recorded • Even of the 15 sp that remained their relative dominance changed • Bird communities showed similar dramatic changes
Stability • After 5-6 years the logged areas began to show evidence of recovery • Trophic structure of the mammal fauna had returned towards its original state, although the species composition and relative abundances had changed
Stability • This study suggested that the communities showed little inertia (low stability in the presence of a small perturbation), low constancy (some trophic, little species), but reasonable resilience
Stability: Population Interaction • Since communities are influenced by population stability, let’s review the general consequences of different types of interaction for population stability
Stability: Population Interaction • We earlier established that predation and interspecific competition generally have a destabilizing effect • There are aspects of these relationships which may tend to stabilize the prey population (through density dependent processes), other aspects of the relationship clearly contribute to the destabilization of the relationship
Stability: Population Interaction • Few interactions are symmetrical in their effects • E.g. top-down relationships where those at the bottom are being more strongly influenced than those at the top; whereas in ‘donor-controlled’ interactions the prey are relatively independent of predator pressures
Stability: Population Interaction • As a general rule, top-down interactions are far more likely to be destabilizing for both populations • Wider interactions, such as those of diffuse competition and competitive mutualism, may help improve the dynamical stability of interaction populations (predator-prey or competitors)
Stability: Population Interaction • We also know that complexity or heterogeneity of the environment may further facilitate stable interactions
Stability & Diversity • The fact that the destabilizing effects of primary interactions such as predation or interspecific competition may be lessened within the context or a more complex matrix of interaction has sometimes led to the idea that ‘dynamic stability’ is a result of their own complexity
Stability & Diversity • Basic observations have led to the idea that habitats comprised of greater complexity (e.g. late succesion forests, tropical forests) also have greater diversity in comparison
Stability & Diversity • Consider the destabilizing effects of competition may be reduced by indirect interaction within a multi-species array – the effects of a third competitor or predator • The injurious effects of predation upon the dynamics of the prey may be ameliorated if the predator has alternative prey species to switch to if the initial prey declines too much
Stability & Diversity • So, is stability the result of the observed diversity or is the diversity a consequence of the inherent stability? • Early works (May) noted a tendency for model communities to become less stable with increasing diversity • Hence, these communities can only exist in predictable environments
Stability & Diversity • By contrast, in an unpredictable environment, there is need for the region over which any one population remains stable to be extensive, with the implication that the system must be relatively simple to persist
Stability: Population Interaction • May concluded that the frequently observed correlation of high diversity with high apparent stability was not in itself a causal relationship, but rather a reflection of two independent consequences of a common cause: that environmental stability permitted the development of high diversity while at the same time ensuring the continued stability of a high-diversity system
Stability: Population Interaction • Random assembly is perhaps an unreasonable assumption; however, additional research has supported the idea that adding diversity to a system (community) generally decreases stability • However, experimental evidence (Tilman) of grassland assemblages suggest otherwise
Stability: Population Interaction • Tilman’s research on grassland communities
Stability: Population Interaction • ‘May’s Paradox’ addresses simple constancy: the more complex a system the greater the potential to accommodate change through minor adjustments elsewhere in the system • This is really ‘redirecting’ the community relationships to absorb the perturbation (is really resilience)
Stability: Population Interaction • Thus May’s conclusions are intuitive: the more complex a system becomes the less likely it may be to maintain constancy of species composition • Conversely, the more complex it is flowing perturbation, because of its capacity to absorb such perturbation
