Resilience and resistance
[Note by author: this article is in a very early stage of development; please do not cite it yet; MV, 12 march 2009]]. [Coastal and marine ecosystems are constantly changing and can respond to environmental disturbances both slowly or quickly. Resistance is related to the capacity of an ecosystem to remain relatively unchanged when confronted by a disturbance, while resilience has been defined in different ways: it can be a measure for the speed at which an ecosystem returns to its former state following a (minor) disturbance. The idea is that a system with a short return time is more resilient than one with a long return time. Such resilience measured as (1 / the return time to a stable equilibrium) has also been called engineering resilience. It has however a long history of use among ecologists (Pimm 1982, DeAngelis 1992, Vos et al. 2005). It is also used in a way that more closely resembles the definition of resistance. Ecological resilience ... (cite Gunderson 2000 to avoid confusion)
Biodiversity allows ecosystems to adapt to changing conditions. Humans, however, have acted to increase the rate of change and consequently, it will be a great challenge for the marine environment to adapt rapidly enough in the future. These changes have been induced through pollution, fishing, sediment deposition and alteration of the global climate. Without genetic diversity, natural selection cannot occur and natural selection is limited, then adaptation is impossible. It is evident that the preservation of biodiversity and, more specifically, genetic diversity is of paramount importance for successful adaptation to our rapidly changing environments.
Biodiversity may not act as a protect ecosystems from major abiotic disturbances
for Biodiversity: cite: Folke et al. 2004
Resilience through re-colonization
To understand resilience of ecosystems it is essential to understand what drives succession within these ecosystems. Succession determines how, and how fast, communities return back to their original state, or perhaps enter a new state. many aspects of succession can be understood in terms of trade-offs between the ability to be either a good early (re-)colonizer, or a good competitor (r-species versus a K-species). Succession involves a gradual replacement of species that differ in these traits and that differ in the degree they tolerate, facilitate or inhibit certain environmental conditions and other species. ... see (Rossi et al. 2009)
We could thus also call a system resilient when it is organized in such a way that succession leads to a recovery of the original state.
Resilience and nutrient inputs
DeAngelis et al. 1989 showed in their analysis of a food chain model that resilience, measured as (1/return time), increased smoothly with nutrient enrichment. The idea is that the system can more quickly return to its original state when the nutrient input is high. However, this result depends on the exact shape of the functional response, the way predators consume their prey as a function of prey density. DeAngelis et al. had used a highly stabilizing Type III (a sigmoid-shaped) functional response. Such a functional response is realistic for learning predators and those that switch between different prey types. Vos et al. 2005 found that resilience (1/return time) would first increase, but then decrease again under nutrient enrichment, when a Type II functional response was used in the model. Many species of consumers throughout the animal kingdom consumer their resources with such a functional response, where intake rate first increases, and then gradually levels off with resource density. Food chains with such consumers would be much less resilient under a high pressure of nutrient enrichment.
Resistance to changes in abiotic and biotic factors
Community composition and ecosystem function may change very little under environmental change when the organisms can acclimate to such change or tolerate it for some time (when the change is only temporary). However, all organisms have bounds to what they can temporarily or permanently tolerate, and when change exceeds some of these limits, the community compostion and ecosystem functioning is likely to change.
It is unlikely that communities can be resistant to continuous gradual change, such as global warming. Acclimation and phenotypic plasticity do not suffice to maintain the system as it is. Genetic adaptation could allow community members to track such abiotic environmental change, but it is more likely that the area where the community is functioning will be invaded by species that function well at higher temperatures. The original species will thus have to deal with new competitors and predators, in addition to the changed abiotic factor. To some extent the original community can track the preferred temperature range, by moving spatially to greater depths or to alternative gepgraphic areas. But these new areas are likely to differe in other ecological aspects such as water pressure, light climate and perhaps speeds of water flow etc.
Adaptation and the consequences of mortality at different trophic levels
External disturbance interacts with internal mechanisms that shape community structure. To understand how an increased mortality of top-predators will affect the entire food chain, it is essential to understand how processes of mutual adaptation within food chains already give shape to existing patterns such as trophic structure (how biomass in ecosystems is partitioned between trophic levels (such as algae, herbivores, carnivores and top-predators)
... Abundances at different trophic levels (such as algae, herbivores, carnivores and top-predators) and their responses to increased mortality (as under environmental change) depend critically on different mechanisms of adaptation within food chains and on the importance of density dependence at each of these trophic levels. However, different types of adaptation to living in a food chain context (balancing the need to acquire resources with the need to avoid predation) can often have very similar consequences. For example, micro-evolution or behaviour, species replacement and induced defenses at a middle trophic level may all have similar effects on trophic level abundances in disturbed food chains (Abrams and Vos 2003).
Adaptation assisted by Man
Protecting sources, not sinks when creating Marine Protected Areas. Protecting sources of populations at all stages of succession, to preserve 'ecological memory' to the fullest possible extent. This includes protecting not only 'high quality' habitats that harbour healthy mature communities, but also 'low quality' and disturbed habitats that are required for those species that contribute to early recovery of perturbed areas (see Rossi et al. 2009).
- Pimm, S.L. 1982. Food Webs. The University of Chicago Press.
- DeAngelis, D.L. 1992. Dynamics of Nutrient Cycling and Food Webs. Chapman and Hall, London.
- Vos, M., Kooi, B.W., DeAngelis, D.L. & Mooij, W.M. Inducible defenses in food webs. In: Dynamic Food Webs. Multispecies Assemblages, Ecosystem Development and Environmental Change. Eds. P.C. de Ruiter, V. Wolters & J.C. Moore. Academic Press. Pp. 114-127.
- Gunderson, L.H. 2000. Ecological Resilience - in Theory and Application. Annual Review of Ecolog and Systematics 31:425-439.
- Folke, C., Carpenter, S., Walker, B., Scheffer, M., Elmqvist, T., Gunderson, L. & Holling, C.S. 2004. Regime Shifts, Resilience, and Biodiversity in Ecosystem Management. Annual Review of Ecolog and Systematics 35:557-581.
- ).Rossi, F., Vos, M. & Middelburg, J.J. 2009. Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities. Oikos, Early View, (January issue).
- DeAngelis, D.L., Bartell, S.M. & Brenkert, A.L. 1989. Effects of nutrient recycling and food chain length on resilience. American Naturalist 134: 778-805.
- Vos, M., Kooi, B.W., DeAngelis, D.L. & Mooij, W.M. 2005. Inducible defenses in food webs. In: Dynamic Food Webs. Multispecies Assemblages, Ecosystem Development and Environmental Change. Eds. P.C. de Ruiter, V. Wolters & J.C. Moore. Academic Press. Pp. 114-127.
- Abrams, P.A & Vos, M. 2003. Adaptation, density dependence and the responses of trophic level abundances to mortality. Evolutionary Ecology Research 5:1113-1132.
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