Variation Thresholds for Extinction and Their Implications for Conservation Strategies

Abstract

We examine the degree to which fitting simple dynamic models to time series of population counts can predict extinction probabilities. This is both an active branch of ecological theory and an important practical topic for resource managers. We introduce an approach that is complementary to recently developed techniques for estimating extinction risks (e.g., diffusion approximations) and, like them, requires only count data rather than the detailed ecological information available for traditional population viability analyses. Assuming process error, we use four different models of population growth to generate snapshots of population dynamics via time series of the lengths commonly available to ecologists. We then ask to what extent we can identify which of several broad classes of population dynamics is evident in the time series snapshot. Along the way, we introduce the idea of "variation thresholds," which are the maximum amount of process error that a population may withstand and still have a specified probability of surviving for a given length of time. We then show how these thresholds may be useful to both ecologists and resource managers, particularly when dealing with large numbers of poorly understood species, a common problem faced by those designing biodiversity reserves.

The Variability Among Populations of Coho Salmon in the Maximum Reproductive Rate and Depensation

“…This results in a difficulty in practice in applying Bayesian population dynamic models that include depensation. Considerable effort has been devoted to the development of both analytical and simulation models that estimate extinction probabilities of natural populations (Lande 1993, Ludwig 1996, Fagan et al 1999). On the whole, these models suffer from a lack of plausible parameter values, as there is often very little data available.…”

Umbrellas and flagships: Efficient conservation surrogates or expensive mistakes?

“…The databases we used contain incidence data only, and lack information on the boundaries or sizes of local populations. Such data, in general, are critical to current techniques for evaluating the extinction likelihood of populations (32)(33)(34). As such, our analysis can evaluate the potential effectiveness of surrogate schemes at the level of species coverage only.…”

Extinction vulnerability in marine populations

“…A key question is how common is self-recruitment? Fish population dynamics can be divided into three categories based on intrinsic rate of population growth (r), environmental carrying capacity (K) and interannual variation in population size (s) (Fagan et al 1999(Fagan et al , 2001.`Persistent'species exhibit such low variability relative to their growth rates that extinction is unlikely.`Refuge-dependent' species exhibit such high variability relative to their population growth rate that long-term persistence depends upon refugia or rescue dispersal from nearby populations.`Carrying capacity-dependent' species exhibit low variability and low-growth rates, with larger populations better able to withstand higher levels of variability (Fagan et al 1999(Fagan et al , 2001). An analysis by (Fagan et al 2001) suggested that 21% of 91¢sh populations exhibited`carrying capacity dynamics', and 75% of ¢sh populations exhibited extinction pronè refuge-dependent'dynamics.These populations £uctuated so much that, in the absence of dispersal or refugia, extinction was determined to be a possibility over a 100-year time frame.…”