PARENT SESSION
Poster Session #12: Modeling.
Tuesday, August 7, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Population thresholds as a mechanism to lessen extinction risk in a stochastic environment.

STAPLES, DAVID¹, TAPER, MARK¹, WARD, E¹, ¹

ABSTRACT- A stochastic Ricker model of density dependent population growth was fit to a time-series data set of cutthroat trout (Oncorhynchus clarki). Fitting of the model revealed a negative density dependent interaction with a high standard deviation in the growth rate. Over 1000 simulations of the model, the populations had an 85% probability of extinction within 100 years and a median time to extinction of 36 years. However, territoriality has been found to limit upper population size in stream salmonids, a maximum population number or "ceiling" was incorporated into the model to represent territoriality. This "ceiling" was set at 20% higher than the largest observed population size of the time-series data. The simulations showed a decrease in extinction probability from 85% to 15% over a 100-year period. This demonstrates specifically that an upper population threshold in the form of territorial behavior may lessen extinction risk for populations experiencing stochastic growth rates, and more generally that model fine structure may have profound impacts on population viability analysis.

KEY WORDS: population modeling, cutthroat trout, territoriality, oncorhyncus clarki