Factors influencing long-term population dynamics of pronghorn (Antilocapra americana): evidence of an Allee effect

Invasion in patchy landscapes is affected by dispersal mortality and mate-finding failure

Range expansions are a function of population growth and dispersal, and nascent populations often must overcome demographic Allee effects (positive density dependence at low population densities) driven by factors such as mate-finding failure. Given the importance of individual movement to mate finding, links between landscape structure and movement may be critical to range expansion; however, landscape effects on other factors including mortality may be equally or more important. In one of the most comprehensive investigations of the interactions of these processes to date, we combined field experiments, simulation modeling, and analysis of empirical spread patterns to investigate how landscape structure affected the spread of the gypsy moth in Virginia and West Virginia. In experiments designed to assess how landscape attributes affect mate finding, we found adult males resisted leaving forest patches and the probability of locating a pheromone source declined more rapidly over distance in non-forest matrix than in forest. We used these findings to develop individual-based simulation models of gypsy moth population dynamics and spread in complex patch-matrix landscapes. The models produced an Allee effect that strengthened with reductions in forested area, but owing more so to dispersal mortality than to effects on mate location. Predicted maximum rates of population spread grew with increases in forest area due to increasing success of long-distance transport events. Evaluations of empirical data showed relationships between spread rates and landscape structure largely consistent with model predictions. We conclude rates of spread were largely driven by long-distance dispersal events, the success of which was influenced primarily by dispersal mortality of larvae in unsuitable matrix, and that landscape effects on mate location played a secondary role. Though influences of landscape structure on mate location appear to be unimportant to the spread of the gypsy moth, we predict they would have stronger effects on more dispersive species.

Bioeconomic analysis supports the endangered species act

The United States Endangered Species Act (ESA) was enacted to protect and restore declining fish, wildlife, and plant populations. The ESA mandates endangered species protection irrespective of costs. This translates to the restriction of activities that harm endangered populations. We discuss criticisms of the ESA in the context of public land management and examine under what circumstance banning non-conservation activity on multiple use federal lands can be socially optimal. We develop a bioeconomic model to frame the species management problem under the ESA and identify scenarios where ESA-imposed regulations emerge as optimal strategies. Results suggest that banning harmful activities is a preferred strategy when valued endangered species are in decline or exposed to poor habitat quality. However, it is not optimal to sustain such a strategy in perpetuity. An optimal plan involves a switch to land-use practices characteristic of habitat conservation plans.