The biogeography of trophic cascades
Predators can indirectly benefit prey populations by suppressing mid-trophic level consumers, and these indirect effects can influence the biodiversity and functioning of coastal ecosystems. However, the strength and outcome of these predator effects may vary across physical and biological biogeographic gradients, and thus extrapolating results from one region to another is often problematic. Moreover, this mismatch between the scale of ecological studies and the biogeographic range at which species interactions occur fundamentally inhibits our understanding of linkages between community ecology and ecosystem processes, as well as our ability to manage these systems.
With collaborators from the University of North Carolina, the University of Georgia, and Northeastern University, we surveyed oyster reef communities across a 1000-km coastal region from North Carolina to northern Florida (the Southeast Atlantic Bight). We then conducted simultaneous manipulative field experiments across this region to ask:
- How do predators on oyster reefs alter ecosystem function through consumptive and non-consumptive effects?
- Are the effects of these predators on ecosystem function consistent across the biogeographic region?
We found that densities of oyster consumers were weakly influenced by predators at all sites. However, consumer foraging behavior varied considerably across sites in the presence of predators, and these behavioral effects altered the trophic cascade across space. Variability in consumer behavior was linked to regional gradients in oyster recruitment to and sediment accumulation on reefs. Specifically, asynchronous gradients in these factors influenced whether the benefits of suppressed consumer foraging on oyster recruits exceeded costs of sediment accumulation resulting from decreased consumer activity. Thus, although predation on consumers remains consistent, predator influences on behavior do not; rather, they interact with environmental gradients to cause biogeographic variability in the net strength of trophic cascades.
Mudcrab and oyster drill – the consumers on the mid-trophic level used in the field experiment.
Kimbro, D. L., J. E. Byers, J. H. Grabowski, A. R. Hughes, M. F. Piehler. 2014. The biogeography of trophic cascades on US oyster reefs. Ecology Letters 17 (7): 845-854