G.E. Hutchinson’s niche concept (as formalized in his 1978 monograph) is an abstract mapping of one aspect of population dynamics (in particular a measure of absolute fitness) onto an environmental space. For continuously growing populations, the metric is traditionally assumed to be the intrinsic growth rate, r, at low densities (with units 1/time). But at small absolute numbers, extinctions can occur for populations with a positive intrinsic rate of growth because of demographic stochasticity. Another fitness metric, R0 (the expected number of offspring produced per individual over their lifetime) is pertinent to quantifying extinction risk as a function of environmental variables. This metric naturally arises in branching process models for studying the ecological and evolutionary dynamics of small populations.
Results/Conclusions
We show that choice of fitness metric can substantially influence the qualitative shapes of niche response surfaces; the talk will illustrate how environments which are optimal with respect to r may be strongly suboptimal as assessed using R0. We illustrat how demographic stochasticity can be relevant for both absolute and relative fitness in small populations, such as at range margins, in sink habitats, and in rapidly degrading environments. The conclusions of these theoretical studies suggest modifications in traditional niche concepts; these modifications are relevant to many issues, such as refining species distribution models, and predicting when one might observe evolutionary rescue in rapidly changing environments – or not.