Testing the invulnerability of laboratory island communities to invasion.

One hundred communities were developed through the controlled introductions of microorganisms to beakers over a six month period of time. Following three months of development, a series of three separate previously unencountered species were introduced into each community. The persistences of these invaders were monitored and their relationships to invaded community complexity, composition and history of development evaluated. The null hypothesis that colonization success is independent of community complexity cannot be rejected. The rate at which species were introduced during development of these communities, as well as beaker size, influenced the invasion success of Dictyosphaerium, but not Staurastrum or Platydorina. The assembled communities were of two types: those dominated by Ochromonas and those domnated by Paramecium bursaria. Ochromonas dominated communities were invulnerable to the invasion of the above 3 species; no clear pattern of invulnerability was evident for P. bursaria communities.