Habitat size is considered one of the primary driving forces for species richness within patches. However, the relationship between species richness and area - as predicted by the species-area relationship (SAR) - are affected by biotic and abiotic factors occurring within patches and in the surrounding matrix. In a naturally fragmented ecosystem with high levels of endemism in southeast Missouri, efforts to conserve this habitat could change the strength of SARs by altering connectivity, patch size, and/or species interactions. We investigated the change in SAR of both bees and flowering plants to a) increasing connectivity and b) the presence of a top predator (eastern collared lizard, Crotaphytus collaris). Bee and plant richness were recorded in 24 habitat patches of varying sizes. To quantify the effects of connectivity, measures of matrix canopy cover were recorded. To measure the effects of presence of a top predator in study two, half of the 24 glades had restored top predators. We measured the effect of a possible trophic cascade on pollinator behavior by observing 40 flower patches within each habitat patch, half of which contained a predation threat (i.e., a crab spider model).
Results/Conclusions
We found bee richness to be most strongly predicted by interactions between size and connectivity while plant richness responded solely to patch size. This suggests increasing connectivity has differential effects on different trophic levels and management level effects may depend on the group. The presence of top predators weakened SARs for bees, but had no effect on flowering plant SARs. The effect on bee SARs may be due to a cascade of trophic effects mediated by differential predation of eastern collared lizards on spiders. By consuming larger spiders, eastern collared lizards may relax intraguild predation pressure on flower-dwelling spiders which, in turn, has negative consequences for pollinator communities. This is further suggested by observations of pollinator behavior where pollinators exhibited more anti-predator behavior (i.e., fewer flower visits) as habitat area increased if collared lizards were present. The results of this study illustrate the complex interactions between SARs and biotic and abiotic factors and the importance of connectivity and cascading trophic effects in restoring degraded habitats to achieve management goals.