The effects of a large-bodied apex predator cascade through a salt marsh food web

Background/Question/Methods

In the past 30 years, myriad of non-manipulative studies have provided evidence that large-bodied apex predators can have pervasive interactions that can profoundly affect community structure and ecosystem function.  Few studies, however, have experimentally isolated the effects of apex predator presence on mesopredator abundance and behavior and quantified the cascading effects on mesopredator prey.  Here we use stomach content analysis, surveys of previous studies, and a mesocosm experiment to answer the following questions regarding the effects of the American alligator (Alligator mississippiensis) within salt marsh food webs: What is the strength of alligator consumptive interactions? Is there potential for the propagation of non-consumptive effects? Can alligators generate a trophic cascade in a simplified salt marsh food web?  In our mesocosm experiment, we isolated the cascading effects of the American alligator on a model estuarine food web comprised of blue crabs (Callinectes sapidus), periwinkle snails (Littoraria irrorata), ribbed mussels (Guekensia demissa), and salt marsh cordgrass (Spartina alterniflora).  We measured the effects of alligator presence on blue crab survival, behavior, and consumption of snails and mussels.  Treatment effects were assessed using Generalized Linear Models (GLMs).

Results/Conclusions

Consistent with 10 previously conducted diet studies of estuarine-occurring alligators (range 20 to 83 %, mean ± SD = 47 ± 20 %), we found blue crabs were present in 35% of stomach contents of 25 adult alligators caught on Sapelo Island, GA.  In our mesocosm experiment GLM analysis revealed significant treatment effects of alligator presence on blue crab survival (P = 0.0009) and behavior; resulting in decreased foraging activity (P =  0.04) and increased refuge occupancy (P = 0.03).  The combined effects of reducing blue crab abundance and altering their behavior translated into increased survival of snails (~20%) and mussels (~190%) when alligators were present (P = 0.04 and P = 0.02, respectively).  Our findings provide the first experimental confirmation that a large-bodied, apex predator can 1) be an important regulator of mesopredator interactions, 2) elicit behavioral changes in mesopredator prey, and 3) elicit a trophic cascade in salt marshes that can affect both top-down and facilitative effects on a foundation species.