A paper in the scientific journal Ecology was just published about research near Cape Cod. The authors looked at the effects of removal of predators (striped bass, blue crabs, smooth dogfish) that eat herbivorous crabs, which in turn eat salt marsh plants. They suggest that the intensive recreational fishery on those predators allows the herbivorous crab population to remain so high that they are destroying the salt marsh, and that this effect may be occurring over much of the Atlantic coast.
Here's the citation and abstract from the paper.
Altieri, A. H., M. D. Bertness, et al. (2012). "A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing." Ecology 93(6): 1402-1410.
Overexploitation of predators has been linked to the collapse of a growing number of shallow-water marine ecosystems. However, salt-marsh ecosystems are often viewed and managed as systems controlled by physical processes, despite recent evidence for herbivore-driven die-off of marsh vegetation. Here we use field observations, experiments, and historical records at 14 sites to examine whether the recently reported die-off of northwestern Atlantic salt marshes is associated with the cascading effects of predator dynamics and intensive recreational fishing activity. We found that the localized depletion of top predators at sites accessible to recreational anglers has triggered the proliferation of herbivorous crabs, which in turn results in runaway consumption of marsh vegetation. This suggests that overfishing may be a general mechanism underlying the consumer-driven die-off of salt marshes spreading throughout the western Atlantic. Our findings support the emerging realization that consumers play a dominant role in regulating marine plant communities and can lead to ecosystem collapse when their impacts are amplified by human activities, including recreational fishing.
Up in Massachusetts, the distinction between the two is a bit fuzzier.
I've never heard of herbivorous crabs, but we have something similar down here in Maryland - herbivorous snail called periwinkles. When the crab population was collapsing (they eat periwinkles), the snails were everywhere, eating marsh grasses. With the crab rebound over the last 3 years, the population of perwinkles in my creek has fallen off dramatically.