Relationship between structural complexity, refuge abundance, and species diversity in an oyster reef system

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Kwanmok Kim, Marine Science, UC Santa Cruz, Santa Cruz, CA, Kwanmok Kim and Peter C. Frederick, Wildlife Ecology and Conservation, University of Florida, Gainesville, FL

Background/Question/Methods
Structural complexity of animal habitats plays a significant role in affecting predator-prey interactions by providing “nooks and crannies” that become refuge spaces for animals. However, the relationship between structural complexity, abundance of refuges, and their effect on species diversity have been assumed rather than demonstrated. This is mainly due to logistical issues such as finding a system with refuges that is shared by many species but at the same time available to quantify the refuge abundance. Moreover, observing and assigning species occupancy is difficult for many mobile species such as fish or birds.
I directly tested the predictions that (1) refuge abundance increases with structural complexity and (2) that refuge abundance affects macroinvertebrate species diversity. I tested this in an intertidal oyster Crassostrea virginica reef system where structurally complex oyster clusters generate many refuge spaces for macroinvertebrate species e.g., polychaetes, crabs, bivalves and gastropods. I randomly collected 54 oyster clusters from the field site and immediately stored them in dry ice to minimize animal activity. We also measured the habitat types surrounding the oyster clusters. Later, oyster clusters were CT-scanned to achieve 3D images and refuge spaces were accurately identified (based on the size of the chela of the main predatory crab species Panopeus obesus). In addition, structural complexity (roughness index) and species diversity (e.g., Shannon, Simpson, abundance, and richness) indices were measured in the lab.
Results/Conclusions
Our preliminary Generalized Linear Mixed Models (GLMM) analyses show no significant relationship between a measure of structural complexity (roughness index) and refuge abundance. However, other structural complexity indices (e.g., rugosity) will be further tested. Moreover, refuge abundance was not related to diversity indices but instead to total abundance of invertebrates. The results further suggest that refuge abundance may not be the limiting factor for occupancy in this ecosystem (53% of refuge spaces unoccupied), but that the presence of predators and other habitat characteristics, such as elevation or surface area of the habitat, may play a stronger role in shaping the local species composition.
This is one of the first studies to apply a 3D image analysis to test the relationship between structural complexity and refuge abundance, and how that translates to species diversity. This advances previous studies by accurately measuring the predator reach in a digitized format and quantifying structural complexity indices while also controlling for surface area.