Posted by: oikosasa | March 11, 2014

Modelling of human-driven changes in meta-communities

What will happen at the meta-community level with all exposure to human activities in various ecosystems? To answer this, Anne Teyssèdre and Alexandre Robert have simulated a few alternative, presented in the Early View study “Contrasting effects of habitat reduction, conversion and alteration on neutral and non neutral biological communities”

Below is the author’s summary of the paper:

How can we explain the local coexistence of numerous ecologically similar species in the same trophic level communities (like plant, perching bird, or rodent communities), and how will these communities react to the current massive global habitat changes driven by human activities?

While these two questions are necessarily linked, scientists’ current answers seem contradictory. On one hand, Hubbell’s (2001) neutral theory of biogeography and biodiversity (NTB) succeeds to explain – and even predict – many community patterns observed and measured by biologists and biogeographers for several decades, among which the well-known “Arrhenius law”, or power law species-area relationship (SAR). [First proposed by Arrhenius in 1921, this empiric ‘law’ relates the richness at equilibrium (S) of a same trophic level community, in number of species, to the area (A) it occupies, in a power relationship: S = c.Az]

Hubbell’s NTB assumes that the small ecological differences among the species composing a community can be neglected confronted to the large stochasticity (i.e. randomness) of local colonization, reproduction, extirpation and speciation events. It assumes the ecological and demographic equivalence of all species in the community at a local scale, in other terms. But this assumption clearly contradicts many biological and evolutionary data, among which the mere fact of evolution by natural selection. [Hubbel’s neutral model must hence be considered as a useful null hypothesis to confront other community dynamics models, and to explore the correlates of “ecological drift”, like Kimura’ s neutral theory of genetic evolution may be used to explore the correlates of genetic drift.]

To tackle this intriguing issue, we modeled the dynamics of different species communities confronted to different types of habitat changes. More explicitly, we defined a small number of species categories differing in their level of specialization to different habitat types and explored the impact of different simulated habitat changes on a regional community mixing generalist and specialist species (specialization model), compared to that of a community composed of ecologically equivalent species (neutral model), combining stochastic, deterministic and selective processes.

We noteworthy found that (i) both models ruled with habitat reduction predict an approximately power law SAR, in conformity with empirical observations; (ii) with the specialization model, but not with the neutral one, habitat conversion (i.e. land use change) and alteration (e.g. aridification, acidification, eutrophization…) may increase regional species richness until a threshold; (iii) habitat alteration, with the specialization model, leads to the rarefaction of specialist species and the expansion of generalist species, i.e. to the functional homogenization of the community at local and regional scale.

While not predicted by the NTB, these two later patterns are currently observed in many local or regional communities confronted to habitat changes.  We conclude that this kind of model mixing a few stochastic, deterministic and selective processes may be use to explore and anticipate the dynamics and biodiversity patterns of living communities at different geographic scales, in response to different environmental strategies and scenarios.

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