Habitat size moderates the influence of heterogeneity on species richness patterns in an experimental zooplankton community
Habitat heterogeneity is a primary mechanism driving patterns of species richness; heterogeneous habitats typically support more species than homogeneous habitats (termed the Heterogeneity-Diversity Relationship; HDR). Despite HDRs typically being positive, recent meta-analyses suggest that the spatial scale of investigation may lead to some studies finding neutral and negative HDRs. At very small spatial scales, heterogeneity may not elicit variable responses from different species, potentially leading to negative HDRs. Also, if the area of a given habitat is fixed (e.g., islands), increased habitat heterogeneity will lead to a decline in the total amount of each habitat type; known as the Area-Heterogeneity-Trade-Off (AHTO). The AHTO model predicts different responses of species to heterogeneity in large compared to small habitats, leading to a hump-shaped relationship between heterogeneity and species richness. However, few experimental tests exist assessing how habitat size alters the importance of heterogeneity for species richness. Additionally, researchers have had difficulty assessing the scale at which species respond to heterogeneity. To more explicitly examine the role of habitat size in moderating the HDR, and test mechanisms of the AHTO, I devised an experimental approach using a diverse zooplankton community in freshwater mesocosms. I used small and large mesocosms, and manipulated heterogeneity by dividing mesocosms in half with a porous barrier, and varying the presence of mosquito fish (Gambusia affinis), which are important predators that alter zooplankton species richness and composition.
Only large heterogeneous mesocosms had greater species richness than the homogeneous treatments, there were no differences in species richness among the small mesocosms. The increase in species richness within heterogeneous mesocosms resulted from a more even Relative Abundance Distribution in the large heterogeneous mesocosms, meaning rare, specialist species likely benefited from increased heterogeneity. Small heterogeneous mesocosms supported lower densities of individuals than large heterogeneous mesocosms, meaning rare specialist species likely went extinct more often, compared to large heterogeneous mesocosms. Therefore, heterogeneity can positively affect species richness, but only when the total area of each habitat type is large enough to support specialist species, as predicted by the AHTO. Also, species spillover effects in large, heterogeneous environments led to greater species density in a single patch of a heterogeneous environment, compared to species density in larger homogeneous habitats, which is inconsistent with the predictions of the AHTO. Further studies should investigate the mechanisms that alter the aggregation patterns of species, and investigate alternative mechanisms that increase species richness in heterogeneous habitats.