ABSTRACT

Human-caused changes in the environment are by far the greatest threat to persistence of many species. Populations subject to long-term deterministic declines are certain to go extinct within a relatively short period, and large populations are expected to persist only a little longer than those that are small. Even populations that tend to grow from one year to the next, however, may persist for only a short time. If the year-to-year variability in population growth rate exceeds about twice the average annual growth rate, the population will decline to extinction over the long term, and large population size provides only a little extra protection against extinction. Persistence times of populations increase greatly with increasing population size only if there is relatively little year-to-year variability in population growth rate. Isolated populations are unlikely to persist unless either their annual growth rates are high relative to the variability in growth rate or they are carefully monitored and managed.

INTRODUCTION

Humans have an enormous influence on the earth, its ecosystems and its plant and animal populations, and it is our activities that are the primary cause of most species declines. Human activities are now responsible for more nitrogen fixation than all other biological and non-biological sources (Vitousek, 1994), they capture 54% of the accessible runoff in terrestrial ecosystems (Postel et al., 1996) and they have transformed between onethird and one-half of the earth's land surface (Vitousek et al., 1997).