Testing habitat selection theory with slough crayfish (Procambarus fallax) across two vegetated habitats

Background/Question/Methods

The Ideal Free Distribution theory states that animals distribute between habitats to maximize individual fitness. The density distribution between habitats can be estimated by the growth-maximizing rule (g). However, if habitats have substantial variation in risk, density distributions are predicted to reflect individual decisions that minimize the ratio of habitat-specific mortality (μ) to habitat-specific growth (g).

Habitat selection of the slough crayfish (Procambarus fallax), within the ridge and slough landscape in the central Everglades presents a unique study system to test this theory. Crayfish have the choice of two adjacent habitats in the wet season, deeper sloughs with submerged aquatic vegetation or 15-20cm shallower sawgrass ridges with high levels of structure. We conducted field experiments with P. fallax to assess the components of expected fitness of individual crayfish. Density distributions were determined with throw-trap samples during the same time period. Habitat and density-dependent growth (a surrogate for resource availability) was addressed with a 23 day juvenile growth experiment in 1-m² cages stocked with a gradient of densities in each habitat. To determine predation risk in each habitat, 176 crayfish were individually tethered to poles at 5m intervals in transects across the two habitats and mortality was monitored for 5 days.

Results/Conclusions

Crayfish growth was higher in the slough than in the ridge (32% greater at low density) and growth declined with densities in both habitats, but only weakly in the ridge. Survival analysis of the tethering study showed that crayfish mortality increased by 62% in the slough over the ridge.

Applying the growth-maximizing rule, crayfish density is predicted to reach 29.3 individuals per m² in the slough before any animals use the ridge habitat. When habitat-specific risk is included in the model, the predicted slough density, at which crayfish enter the ridge, drops to 8.1 per m². The minimizing μ /g isodar is close to the actual distribution, suggesting crayfish are evaluating both resource and risk when selecting wetland habitats.