Habitat size mediates the importance of dispersal for patterns of species diversity

Background/Question/Methods

Dramatic losses of biodiversity have occurred in the last two centuries, due to habitat fragmentation and habitat loss, primarily driven by anthropogenic destruction of ecosystems. The rapid loss of biodiversity, coupled with increased habitat degradation has pushed researchers to investigate the importance of species dispersal for maintaining biodiversity in patchy landscapes. Many experimental manipulations have found that increased dispersal rates have a positive effect on species richness. Yet, ecologists have conflicting ideas regarding the spatial scale at which increased species dispersal positively affects species diversity. To investigate if habitat patch size influences the importance of dispersal for patterns of diversity, we manipulated habitat size and dispersal rates in a model zooplankton community, consisting of 72 rotifer, cladoceran and copepod species.

Results/Conclusions

Our results show that dispersal increased both the measured species richness and Chao’s extrapolated species richness in large and small habitats. Increased dispersal had a positive effect on species diversity in small mesocosms, but diversity in large mesocosms was not different among the low and high dispersal treatments. For our diversity metric, we used Hurlbert’s Probability of Interspecific Encounter, converted to an Effective Number of Species (ENS), which accounts for both rarefaction and differences in relative abundance distributions among communities. Accounting for sampling biases like the total number of individuals sampled, and differences in detection probabilities (due to different relative abundances) is important to accurately compare diversity metrics among communities that experience different environmental conditions. Our results indicate that researchers who found positive effects of corridors and assisted dispersal on species richness may have failed to investigate the importance of these factors at large spatial scales. That being said, habitat size rarely increases in human-dominated landscapes. Therefore, as habitat fragmentation and isolation continues, managers should consider the positive effects of facilitated dispersal and corridors for the diversity of species in small habitat patches.