Experimental Invasions Using Biological Control Introductions: The Influence of Release Size on the Chance of Population Establishment

Abstract

Introductions of biological control organisms offer a unique opportunity to experimentally study the process of invasion by exotic species. I used two chrysomelid beetles, Galerucella calmariensis and Galerucella pusilla, which are currently being introduced into North America for the biological control of purple loosestrife (Lythrum salicaria), to determine how the initial size of a release affects the probability that the introduced population grows and persists. I released both species into stands of their host plant at 36 sites scattered throughout central New York State using four release sizes: 20, 60, 180, and 540. I returned to these sites over the next 3 years to census the populations. For both species, the probability of population establishment increased with release size. Population growth rates also depended positively on release size. The implication from these results is that the demographic factors whose influence depends on population size or density such as demographic stochasticity, Allee effects, and genetics play important roles in the establishment of invading populations. A second set of releases was used to determine if it was at all possible for a single gravid female to found a population. Out of twenty individual females released, one female (a G. calmariensis) founded a population that persisted until the end of the study (3 generations).