Fifty—four communities were developed from repeated introductions of 28 phytoplankton species in three different orders of invasion at three different rates. Variation in species richness values in these communities was assignable to invasion order, invasion rate, and the timing between interspecific invasions. Invasion rate was most influential, explaining 21.8—78.8% of the variation on any give date; invasion order never explained >4.3%, and timing explained between 19.7 and 74.9%. All three facets of invasion had significant effects. One of the low—rate invasion categories developed a unique community structure that was dominated by Chlamydomonas. Communities in this category were invulnerable to the invasion of many species and as a result had low species richness values. The relative abundance patterns of species in categories having low or moderate invasion rates but identical invasion orders clustered more closely to each other than they did to their high invasion rate counterparts. In this sense, invasion order is more influential where immigration rates are relatively low (e.g., on islands) than it is where dispersal from outside sources is high (e.g., on continents). Because invasion history was controlled, the methodology used here provides interpretable data concerning the potential importance of chance historical events that occur during community assembly. The data indicate that idiosyncrasies in community structure often may be explained on the basis of the random invasion patterns of organisms to different habitats.

Fifty four microcosmic communities were assembled over 4 months from a 28-species source pool of phytoplankton using nine different invasion patterns each replicated six times. Three communities from each set of replicates then were invaded with a cladoceran that feeds on phytoplankton. All communities were then treated identically for an additional 4 months. In all nine invasion categories species richness was greater in predated communities. Predation opened communities to invasion by increasing the representation of infrequently sampled species at the expense of more common species. Invasion rate was four times more influential than predation and over eleven times more important than either invasion order or the timing pattern of interspecific arrivals in determining species richness in this system of communitites.