Total cover and cover quality: predicted and actual effects on a predator's foraging success

Abstract

I tested the importance of both total cover and cover quality to the foraging ability of large Fundulus heteroclitus fish in tanks. Total cover was measured as the combined areas of all the indi- vidual structures, viewed as shadows. I divided total cover by the bottom area of the tank to form Ct/At, an index that measures the total amount of structure available in a given area for prey to hide behind. Cover quality was measured as the width of each individual structure divided by the width of the indi- vidual prey organism (Cw/Pw). This index measures how visible a prey organism is when hiding be- hind an individual structure. Both of these complexity indices are dimensionless and potentially ap- plicable to any habitat type and habitat scale. I predicted that prey survivorship should increase hyperbolically with increasing Ct/At, because increasing the amount of structure within a habitat in- creases the amount of the habitat blocked off from detection, but with diminishing returns. Past ex- periments demonstrated that increased Ct/At does increase prey survivorship, but the shape of the re- lationship was unclear. I also predicted that prey survivorship should increase hyperbolically with increasing Cw/Pw. I predicted this by simulating a prey organism hiding behind a flat structure being viewed by a predator from all potential angles. In this simple simulation, the 'average amount of the prey hidden from view' increased hyperbolically with increasing Cw/Pw to an asymptote of 0.5. I per- formed 2 experiments to test these ideas. In the first, I kept Ct/At constant between treatments and var- ied the width of the individual structures relative to the width of shrimp prey Paleomonetes spp. In the second experiment, I kept the inter-structural space widths nearly constant between treatments and varied Ct/At. The prey for the second experiment were mobile amphipods. I tested the hypothesis that survivorship and treatment were independent using log-linear models for both experiments. I found that there was no significant difference in survivorship between treatments for the Cw/Pw experiment, and there was a significant difference in survivorship between treatments for the Ct/At experiment. The relationship between Ct/At and prey survivorship appeared to increase hyperbolically, as pre- dicted. Finally, I used the results of the Ct/At experiment, and results from a past study, to form a mod- eled regression equation of the survivorship surface for large F. heteroclitus feeding on amphipods for various levels of Ct/At and Sp/Pr (inter-structural space size/predator width). This equation was highly significant, and fit the data well. In this regression, amphipod survivorship increased hyperbolically with increasing Ct/At, and decreased sigmoidally with increasing Sp/Pr.