ABSTRACT
The term “niche” refers to the range of conditions, resources – and indeed all biotic and abiotic factors – that permit populations of a species to persist (deterministically) in a given habitat without immigration. In effect, the niche is a mapping of population dynamics onto an abstract environment space (e.g., with axes of temperature, pH, food availability, predator density, etc.; Hutchinson 1958, Maguire 1973, Holt and Gaines 1992), emphasizing in particular the limits outside of which a species faces extinction. Formally, if environmental conditions in a given habitat are such that the low-density intrinsic rate of growth r (instantaneous per capita birth rate – per capita death rate) is negative, then conditions by definition are outside the niche, and
introductions of the species should fail. By contrast, introductions into a habitat with r > 0 should tend to increase. So the niche of a species in effect partitions the world into areas where it can persist, and areas where it faces extinction. (For a species with discrete generations, sources and sinks can be defined in terms of the average fitness at low density, with unity being the threshold.)
To a first approximation, the geographical distribution of a species should be determined by its niche (Pulliam 2000), as should its habitat distribution at a more local, landscape scale. Understanding niches is of great practical importance, for instance in predicting how changes in climate might lead to shifts in distribution, and changes in land use can lead to altered patterns of abundance on a landscape. But all such predictions – and the scientific literature is replete with them – rest on the assumption that species’ niches remain unchanged, even as the world changes. Such evolutionary conservatism, or the lack of change in the niche in a heterogeneous world, is called “niche conservatism.”
