Monday, 27 May 2013

The power of historical ecology: a case study in Cape Cod's wetlands

Since the colonization of North American in the 17th century, few ecosystems have been so routinely and extensively disturbed as the salt marshes of Cape Cod. Among other things, residents have dug drainage ditches throughout the wetlands in order to make them less habitable to mosquitoes; developed the shoreline to facilitate the introduction of industrial, maritime, and residential facilities; and harvested huge numbers of edible wildlife off the coast. Over the past century or so, the human population has increased by approximately six-fold, placing extreme pressure on the wetland habitat. The most recent evidence of this burden is the spread of salt marsh die-offs, or "loss of foundation plant species to herbivores as a result of trophic dysfunction." Besides being unattractive, these die-offs threaten biodiversity and ecosystem function and may compromise the structural integrity of the wetland habitat.

Cape Cod wetlands. Image courtesy of Ms. Hanna's Room 202.
Researchers are scrambling to understand what caused these die-offs, how they may be exacerbated by current activities, and what we can do in the future to stem or reverse their spread. Cumulatively, these goals are at the heart of a relatively new scientific field called "historical ecology." Recognizing that ecological interactions can be quite complex and take many years to have noticeable effects on the environment, historical ecologists seek to use information from a variety of sources (including, for example, tree rings and soil cores) to reconstruct an ecosystem's experiences. Armed with these data, they can figure out the chain of events that resulted in contemporary problems--and, hopefully, suggest relevant management solutions.

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.
Although the legislation was helpful for preventing further habitat loss, the damage had already been done. Though marshland die-offs had previously been thought to have begun in the 1980s, the researchers saw clear signs of denuding in the 1976 photographs; from then on, some areas lost >90% of their vegetation. All of this can be traced back to the purple marsh crabs, which, when released from predation pressure by the activity of fishermen, chomped their way through the encroaching low marsh cordgrass.

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.
The authors point out that, on its own, mosquito ditching was actually a "fairly benign" practice, and only became problematic when it was coupled with development. The introduction of the second type of disturbance created "diverging pathways" for the habitat, enabling crabs to enjoy the cordgrass buffet offered by highly-ditched, highly-developed areas that facilitated fishing activities. Without taking such a long-term, historical ecological view of the ecosystem, researchers might not have realized all the factors involved in the recent vegetation die-offs. The results highlight how some disturbances result in "unanticipated or novel outcomes" that can "remain wholly or partially unrealized and become apparent only following subsequent impacts or environmental changes." In this case, the latent disaster of the mosquito ditches only became a reality after nearly four decades of coastal development.

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.
The researchers do not offer any new management solutions for the Cape Cod problem--that is an issue they have already written about elsewhere (see here and here for examples). Instead, the main point of their current paper is to show how their historical ecological techniques allowed them to pinpoint three major phases in marshland disturbance, as well as highlight the interconnectedness of those three types of disturbance. These patterns, the researchers say, allow them to "demonstrate that the ability of ecologists to understand and predict the consequences of future development on ecosystems is dependent on an understanding of the accumulation of latent human impacts already affecting them."

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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.

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