Resource limitation in seasonal environments

A model applying to species that are dependent on different resources durin g breeding and during non-breeding (over-wintering) periods is considered. The model is used to make both intra- and inter-species comparisons, predic ting the difference in population size in varying as compared with unchangi ng environments: expressed in terms of demographic parameters. Different po pulations of the same species can respond differently depending on context: populations in which numbers are primarily restricted by the availability of resources necessary for over-winter survival are typically more vulnerab le to environmental fluctuations than populations restricted by resources r equired for reproduction. Although species that have low fecundity and are short-lived are most vulnerable overall, species that have low fecundity bu t are long-lived are most sensitive to the season of the limiting bottlenec k. For species with a high basic fecundity the season of most concern depen ds on seasonal differences in density dependence responses. Thus one needs to know whether populations are limited by scramble competition or by terri torial behaviour, for example, and in which season. One cannot evade the im portance of identifying the limiting resource when assessing the vulnerabil ity of populations to environmental change.