Effect of Community Structure on Invasion Success and Rate

Although invasion has long been recognized as an important ecological process, there are very few experimental studies of invasion in natural communities and virtually no studies that determine how trophic structure affects the probability of invasion. We introduced novel protozoans and rotifers into the natural communities found in the water-filled leaves of the pitcher plant Sarracenia purpurea. The communities were manipulated in a factorial design of removal of predators (larvae of the mosquito Wyeomyia smithii) and addition of resources (dead insects). Three of the six protozoan species successfully established populations when introduced into pitchers, suggesting that these species are migration limited. The other three protozoans and a rotifer did not successfully invade established communities, although all four are naturally found in these inquiline communities. Of the three successfully invading protozoans, two were more likely to invade when resources were added and one of those even more frequently when predators were removed. Invasion by the third was unaffected by these experimental manipulations. Similar effects of predators and resources were found on population sizes of these three species. This study is one of very few that have addressed invasion experimentally; its results suggest that a variety of factors, including migration, predation, and resource availability, can have different influences on invasion by fairly similar protozoans.