For a species to colonize a site it must both arrive there by dispersal from another site and maintain positive population growth in the local environment.  I experimentally tested the role of dispersal limitation in structuring the zooplankton communities of fishless ponds in southwestern Michigan.  An average of 12.9 new species of rotifers and crustaceans from the region were introduced at the beginning of the experiment into the intact resident communities of seven ponds in large in situ enclosures (the invasion treatment).  Introduced species were found at least once over three months in six of the seven ponds and one species consistently colonized all three replicate enclosures in three of the ponds.  However, the success rate of the introductions was low as >91% of the species introduced immediately became extinct.  In addition, introduced species remained rare throughout the experiment in every pond, comprising only 0-2.5 percent (mean = 1.1) of total zooplankton community biomass.  No effects of the invasion treatment were detected on total zooplankton diversity or biomass, the biomass of native species or on chlorophyll-a concentration, suggesting that relaxing dispersal limitation for the regional pool had minimal effects on local community structure.  Both the biomass of exotic species and the proportion of species introduced that successfully colonized the invasion treatment were negatively correlated with native species diversity.  These patterns support the hypothesis that diversity confers resistance to invasion.  The results of the invasion treatment indicated a minor role for dispersal limitation in structuring pond zooplankton communities.

To test the role of interactions with resident species in excluding potential invaders, a second experiment was performed in four different ponds the following year.  In addition to the control and invasion treatment, a second treatment (the resistance treatment) was imposed where the abundance of native species was reduced by filtering before the introduction of invaders.  The goal of this treatment was to weaken interactions between resident species and invaders while maintaining the abiotic environment intact.  Among the four ponds, 3.8 times more exotic species were found in the resistance treatment than in the invasion treatment and their total biomass was 16.4 times greater.  The contrast between the invasion and resistance treatments indicated an important role for species interactions in repelling invaders.  The two experiments showed that zooplankton communities were nearly saturated with species and that biotic interactions excluded many potential invaders.