Conservation Implications of Niche Conservatism and Evolution in Heterogeneous Environments

Species may, in principle, respond to environmental change in several different ways (Pease et al. 1989; Holt 1990). Some species may track environmental states to which they are already well adapted and so shift in abundance and distribution. Other species may not evolve at all and so become extinct. Some species may evolve adaptively in ways that facilitate their persistence in changed environments. Yet other species may evolve in ways that hamper their long-term viability. A fundamental goal of the discipline of evolutionary conservation biology is to understand the factors that govern the relative likelihood of each of these outcomes. Recognizing the importance of directional environmental change in driving extinctions in once-common species raises a profound puzzle. On the one hand, as ecologists we know that extinction risk emerges because directional environmental changes lead to lowered population abundances and/or restricted distributions; in effect, species are pushed outside their niches. On the other, as evolutionists we know that species often have abundant genetic variation, and so can adapt to novel circumstances. Conservation problems arise precisely because species do not adapt sufficiently to the new environments created by anthropogenic activity. In other words, conservation problems reflect a seeming failure of evolution by natural selection to adapt species to environmental change. Such failures are examples of “niche conservatism”. The history of life reveals examples of both niche conservatism (phylogenetic lineages that retain much the same ecological niche over substantial spans of evolutionary history) and niche evolution (Bradshaw 1991; Holt and Gaines 1992). Before proceeding any further, we should be clear about the meaning of “niche” (Schoener 1989). For a species with continuous, overlapping generations the intrinsic growth rate r is its expected per capita birth rate minus its expected per capita death rate, at low densities. Succinctly, if a habitat results in r > 0 for a given species it has conditions that are within that species’ niche. By contrast, if r < 0, the habitat has conditions outside the niche. (For discrete generations, if the environment is such that the per capita growth ratio per generation R0 > 1, the habitat lies within the niche, but if R0 < 1, it is outside.) In effect, the niche of a species is an abstract mapping of the most fundamental attribute of that species’ population dynamics – its persistence versus its extinction – onto environmental states. A population of a species should persist