New dimensionless indices of structural habitat complexity: predicted and actual effects on a predator's foraging success

Ecologists have long known that complex habitats often provide prey with refuges from predation. This is true for a wide variety of habitat types in terrestrial, freshwater and marine systems. Despite the recognized importance of structural habitat complexity, ecologists have defined and measured complexity in many different ways. We propose 2 new indices of structural habitat complexity that are dimensionless, that can be applied across various habitat types and scales, and that directly measure how structural complexity interferes with a predator's foraging ability. These indices are: the total area of cover within a habitat divided by the total area of the habitat (C-t/A(t)) and the average inter-structural space size divided by the size of the predator (Sp/Pr). C-t/A(t) measures the amount of cover available within a habitat that interferes with a predator's ability to see, or otherwise sense,prey within the habitat. Sp/Pr measures the extent to which the structure interferes with a predator's ability to move through the habitat in search of, or while pursuing, prey. We predicted that prey survivorship should increase hyperbolically with increasing C-t/A(t), and that survivorship should decrease sigmoidally with increasing Sp/Pr. We also predict that both C-t/A(t) and Sp/Pr can influence survivorship independently, and that they form a survivorship plane. We tested our model in 3 laboratory experiments with the fish Fundulus heteroclitus as predator and amphipods as prey, and in 1 field experiment. The results of our laboratory experiments support our model for Sp/Pr, but are only suggestive for C-t/A(t). The results of the field experiment are consistent with our laboratory results, and our model.