|Cape Cod wetlands. Image courtesy of Ms. Hanna's Room 202.|
These were the goals of a New England research team that recently utilized a series of aerial photographs to analyze the effects of anthropogenic disturbance on Cape Cod wetlands between 1939 (the date of their first photograph) and 2005. They obtained a total of 12 images (from 1939, 1976, 1994, and 2005) that they could use to track the state of the wetland over the years; their focal area was the amount of habitat covered by marsh vegetation in the original photograph.
An examination of the earliest photos revealed that the vast majority (>95%) of mosquito ditches had been created prior to 1939, laying the groundwork for significant changes to the Cape Cod habitat. Where these ditches were installed, high marsh plants were replaced by low marsh cordgrass, which is eaten by purple marsh crabs. Populations of these crabs are normally kept in check by predators such as blue crabs and striped bass, but these species are particularly desirable to fishermen--and fishermen were steadily increasing in the area: The period between 1939 and 1976 was marked by a near-tripling of the human population, not to mention a huge amount of coastal development (defined here as the installation of homes, docks, and marinas). In fact, >95% of human infrastructure was installed during this time; further construction was only halted by 1976 legislation forbidding any additional development.
|Denuded marshland. Photograph courtesy of the Bertness Lab at Brown University.|
The most interesting thing about these findings is not when, or the extent to which, this denuding occurred, but where. All 12 of the researchers' study sites had experienced some amount of mosquito ditching, but development and die-off only affected some of those sites. Specifically, sites with high levels (>5%) of development had much greater die-off than sites with low levels (<5%) of development. Further, analyses showed that there was a synergistic relationship between mosquito ditching and development: Areas with the worst die-offs were those with both high levels of ditching and lots of development. These were the areas where anglers could fish most easily, and where the suddenly predator-less purple marsh crabs could find the largest amounts of low marsh cordgrass to eat.
|Schematic showing how recreational fishing removed key predators of the purple marsh crab, thus allowing it (and other herbivorous relatives) to proliferate and eat its way through the low marsh cordgrass. Image courtesy of the Bertness Lab at Brown University.|
Based on these results, the scientists emphasize how important it is to see human impacts as "interactive effects of multiple disturbances," rather than looking at each disturbance in isolation. This is increasingly the mindset of researchers working in other habitats, as well--for example, urban ecologists who are now appreciating the combined effects of light, noise, and chemical pollution, along with physical habitat disturbances such as traffic and construction.
|According to another publication by the current study's authors, invasive green crabs can help stem the Cape Cod die-offs--though these exotic animals may also have additional, unwanted, effects on the ecosystem. Image courtesy of Northrup Photography.|
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Coverdale, T.C., Herrmann, N.C., Altieri, A.H., and Bertness, M.D. 2013. Latent impacts: the role of historical human activity in coastal habitat loss. Frontiers in Ecology and the Environment 11(2):69-74.