The Effect of Structural Complexity, Prey Density, and “Predator-Free Space” on Prey Survivorship at Created Oyster Reef Mesocosms

Interactions between predators and their prey are influenced by the habitat they occupy. Using created oyster (Crassostrea virginica) reef mesocosms, we conducted a series of laboratory experiments that created structure and manipulated complexity as well as prey density and “predator-free space” to examine the relationship between structural complexity and prey survivorship. Specifically, volume and spatial arrangement of oysters as well as prey density were manipulated, and the survivorship of prey (grass shrimp, Palaemonetes pugio) in the presence of a predator (wild red drum, Sciaenops ocellatus) was quantified. We found that the presence of structure increased prey survivorship, and that increasing complexity of this structure further increased survivorship, but only to a point. This agrees with the theory that structural complexity may influence predator-prey dynamics, but that a threshold exists with diminishing returns. These results held true even when prey density was scaled to structural complexity, or the amount of “predator-free space” was manipulated within our created reef mesocosms. The presence of structure and its complexity (oyster shell volume) were more important in facilitating prey survivorship than perceived refugia or density-dependent prey effects. A more accurate indicator of refugia might require “predator-free space” measures that also account for the available area within the structure itself (i.e., volume) and not just on the surface of a structure. Creating experiments that better mimic natural conditions and test a wider range of “predator-free space” are suggested to better understand the role of structural complexity in oyster reefs and other complex habitats

Results from each experiment showing the number of prey added (ind), mean number of prey removed (ind ± 1 standard error), mean prey survivorship, and two-way ANOVA results (F-value).

<p>Prey added represents the number of initially stocked individuals per trial. Prey removed represents the number of individuals remaining in the tank after each trial, and the prey survivorship is the number of prey recovered divided by the number of prey added. Expt 1 used a fixed number of predator and prey individuals across treatments of variable levels of structural complexity. Expt 2 used the same structural treatments as the first experiment, but scaled the density of prey across treatments. Expt 3 used the same prey density treatments as the second experiment, but altered the amount of ‘predator-free space’ (by increasing interstitial space) in each structural treatment. Control treatments had no structure. * p<0.01, ** p<0.001, *** p<0.0001.</p

Prey survivorship (no. prey removed/no. prey initially stocked) by experiment.

<p>Vertical bars represent prey survivorship (mean ± 1 standard error), or the proportion of prey that survived in each treatment. Different letters represent significant (p<0.05) differences among treatments. Expt 1 used a fixed number of predator and prey individuals across treatments of variable levels of structural complexity. Expt 2 used the same structural treatments as the first experiment, but scaled the density of prey across treatments. Expt 3 used the same prey density treatments as the second experiment, but altered the amount of ‘predator-free space’ (by increasing interstitial space) in each structural treatment. Control treatments had no structure.</p

Results from individual contrasts by experiment (e.g., Expt 1 ‘low’ treatment contrasted with Expt 2 ‘low’ treatment) of prey survivorship between treatments.

<p>Contrasts in Expt 1 and Expt 2 were conducted to examine the effects of prey density on prey survivorship across treatments of increasing structural complexity. Similarly, contrasts between Expt 2 and Expt 3 were conducted across treatments to examine the effects of ‘predator-free space’ on prey survivorship and whether these changed with structural complexity.</p