Dispersal–the movement of organisms from one breeding locale to another, also sometimes called migration–ranks among the most important life history characteristics in vertebrates. It allows populations to expand, flow from resource patch to resource patch and accommodate changing environments, among other things. In metapopulations–populations comprised of many geographically isolated subpopulations connected by dispersal–dispersal is easily defined as a permanent movement from one patch to another. We typically distinguish natal from adult dispersal. In the former, individuals move from their natal patch to another patch to breed the first time. Adult dispersal occurs when an individual breeds on one patch and then moves to another before breeding again.
Despite dispersal’s ecological importance, it remains something of an evolutionary conundrum. Most dispersing vertebrates assume risks that their philopatric competitors (those that do not move) avoid. But there is no guarantee that the patch to which they move is any better than the one they left, and worse, it might have fewer resources and (or) more competition. In the face of these risks, how does evolution shape dispersal strategy? This question drives the following projects.
Project 1: Dispersal in American pikas (Ochotona princeps)
Our model organism to study dispersal evolution in the field is the America pika, a small lagomorph that inhabits rocky talus in the mountains of western North America. Our initial studies focused on the population of pikas at Bodie State Historic Park. At about the turn of the 20th Century, pikas colonized the ore dumps scattered across the Bodie hills, forming an “artificially natural” metapopulation experiment. This study discovered that, because patches on this site are so small, demographic extinction supplies natural selection with sufficient spatial and temporal variation in patch density to favor a significant dispersal rate, and this predicted rate has not yet been approached by the pikas at Bodie. Therefore, we predict that this population’s dispersal rate is currently evolving to a higher rate, which will also buffer the entire metapopulation from extinction.
Project 2: Dispersal in Artificial Metapopulations of Confused Flour Beetles (Tribolium confusum)
Studying pika dispersal in the field has its drawbacks. Primarily, one cannot manipulate the population to test hypotheses and models. Therefore, we developed an experimental system using manufactured metapopulations of flour beetles. Early experiments confirmed earlier observations, primarily by Ogden, that these beetles have an innate (genetic) tendency to disperse, and this tendency varies among individuals. Therefore, the ingredients for evolution of dispersal tendency via natural selection are in place. We are just completing our first long-term experiment with which we will test some of our initial models of both metapopulation dynamics and evolution of dispersal.