An experimental test on the effects of dispersal from different habitat sources on community structure

Biological interactions, disturbances, and demographic stochasticity often drive population declines and local extinctions. Dispersal can counterbalance these drivers by rescuing small populations or facilitating recolonization. Using freshwater zooplankton in experimental mesocosms, we tested three hypotheses: (1) isolated sites would experience declines in species richness, with ecological drift causing communities to lose different species and become more dissimilar over time; (2) communities connected by dispersal from similar habitats would maintain their species richness and composition, as arriving species balance losses through rescue effects and recolonization, thereby halting community differentiation; and (3) dispersers originating from different sources may establish themselves in recipient communities through mass effects, resulting in higher species richness compared to communities receiving dispersers from similar habitat sources. Thirty 500-L tanks were initially colonized with zooplankton from lake A, and 10 tanks with colonizers from lake B, which had partially distinct species composition. Tanks were kept isolated for 50 days, after which 10 tanks initially colonized by lake A began receiving dispersers from paired tanks also colonized by lake A (treatment Aa). Another 10 tanks colonized by lake A received dispersers from paired tanks colonized by lake B (Ab). We found that isolated communities (A0, B0) tended to lose species over time and differentiate from one another, indicating differential local extinctions. Communities receiving dispersers from the same habitat (Aa) halted species losses and maintained their species richness, whereas those receiving species from a different habitat (Ab) not only halted species losses but also accumulated additional species over time. Treatments receiving dispersers (Aa, Ab) exhibited beta diversity (among replicates within treatments) similar to levels observed prior to dispersal events. Comparisons of paired source-recipient tanks (A0-Aa, B0-Ab) further supported the finding of differential extinctions in isolated communities. Our results demonstrate that dispersal counteracts declining species richness and increasing differentiation caused by differential local extinctions in isolated communities, either through rescue or mass effects.