Populations and communities as fluid on a landscape under nonstationary environmental change
Recognition that temporal environmental variation is nonstationary, i.e. shows no long-term stability, calls into question the utility of traditional approaches to the study of populations and communities. Populations defined by the organisms at a particular place, such as a given area of land, are open to migration, and major population shifts may occur with long-term temporal environmental change. Study of a fixed location might give a misleading picture of the organisms’ environmental requirements, evolutionary pressures and population dynamics. It seems better to recognize populations as fluid on a landscape. A moving population might experience a very different environment than a static one, and environmental fluctuations might have very different roles in these two situations. Moreover, predictions about population dynamics and population persistence might be very different. These issues were investigated for models of populations experiencing nonstationary change on a spatially structured landscape using analytical and simulation techniques. New concepts and quantitative tools were developed for a better understanding of spatially fluid populations. Especially useful is the environment realized by a population as it moves on the landscape determined by weighting local environmental frequencies by local population densities.
The realized environment can be very different from the environment at anyone place, and from the average on the landscape. Generally, the realized environment is much more favorable to the population than the average environment, and shows a different pattern of temporal fluctuations. The realized environment can be stationary even though temporal variation at any given locality is profoundly nonstationary. This outcome can occur when the nonstationary temporal change has a spatial analogue. However, the stationary environment experienced by a population differs from the experienced environment under stationary change, and implies that the realized environment may often be suboptimal. More generally, realized environments remain nonstationary, but are closer to stationarity than local environments. A long-term view of populations that emphasizes fluidity on a landscape can lead to a very different view of population dynamics, population viability and species interactions. Although this population concept raises major challenges for empirical study, it might resolve some of the paradoxes and conceptual difficulties of populations defined by a place. Extending this idea to communities is likely to lead to even more insight as the fluid nature of the associates of a species on a landscape, and its consequences, are better understood.