The relative importance of competition and predation along a habitat size gradient in bromeliad mesocosms

Background/Question/Methods

When habitat patches vary in size, larger patches have more species -- but the 'threshold' size may vary widely among species. This may be caused by differences in environmental variables or by biotic interactions which vary with habitat size. Evaluating why species show various responses to habitat size is critical for understanding community structure, as well as for predicting the effects of habitat size changes.

Tank bromeliads provide a small freshwater habitat which varies widely in size and supports a diverse insect community. These insects show different responses to habitat size – sometimes even among congeneric species. We tested three mechanisms for this pattern: that each species responds differently to 1) environmental variables. 2) competition from the other species 3) predation.

Our design consisted of three treatments: a monoculture of each species, one (additive) polyculture, and polycultures plus one common predator. We established these four treatments in tubes and placed all four in either small or large bromeliads (20 total). We performed this experiment with a congeneric species pair (Diptera:Chironomidae:Polypedilum sp.).

Results/Conclusions

We found that the species do not show differences in performance when placed in bromeliads of different sizes in monoculture. Interestingly, each species responded positively to large bromeliads when in polyculture, suggesting a role for positive density-dependence. Species did differ, however, in their responses to predation. The species which preferred larger bromeliads also proved more resistant to predators (which are more common in larger plants). The presence of large predators in larger habitat patches may create the pattern of habitat preference we see among species.