Urbanization alters food consumption patterns among lake-dwelling crayfish

You've probably heard of the saying, "You are what you eat," but what about "You are where you eat"? Using a method called carbon stable isotope analysis, which looks at the ratios of two forms of carbon (13C and 12C), scientists can tell where an animal's food comes from. In lakes, for example, food from open water areas has a depleted 13C:12C, food from the lake bed near the shoreline has an enriched ratio, and food from terrestrial resources (also called "subsidies," indicating items that fall, or are washed, in from land) has a ratio intermediate between these two values. Thus, by collecting tissues from animals throughout a lake, researchers can determine which area of the lake provides the food the animals are eating.

Recently, this technique was utilized by collaborators from the University of Washington in Seattle, Washington, USA, and Niigata University in Japan, in order to evaluate the impacts of shoreline urbanization on food resources in lakes of different sizes. The researchers collected samples from 14 lakes in the US and Japan and looked for relationships between 13C:12C, degree of urbanization, and size of lake. They focused their efforts on the signal crayfish, Pacifastacus leninusculus, a long-lived, omnivorous crustacean that likely plays an important role in "ecosystem engineering" because of the amount of detritus that it processes while feeding. Thus, any effects of urbanization on the diet of this species probably has wide-ranging impacts on the whole lake.

(A signal crayfish, Pacifastacus leninusculus. Although this species is native to western North America, it can now be found around the world.)

The researchers found that shoreline urbanization had an effect on 13C:12C, indicating that it influenced the amount of terrestrial resources entering the aquatic environment. However, the effects of urbanization differed depending on lake size. In large lakes, there was little impact on 13C:12C. However, in small lakes, crayfish had more enriched 13C:12C, indicating that they were eating more locally-generated food than food that was being washed in from land. This suggests that urban development had either diminished the availability of potential food items, or had prevented them from being deposited into the lakes (for instance, as a result of retaining walls or other blocking structures)--or both. Given that anthropogenic habitats are generally characterized by lower levels of plant material and high levels of impervious surface such as cement and blacktop, this is probably not surprising.

The authors report that these results are consistent with another recent study that found reductions in terrestrial subsidies to lake consumers in areas with shoreline development. These subsidies are important for stabilizing populations, communities, and food webs. When these are not available, organisms in lakes may face more competition for the resources that are generated within the lake itself, potentially leading to shortages. This can affect not only individual species, but also the ecosystem function of the lake as a whole. If the price of lakeside property wasn't already enough to put you off the idea of settling there, maybe the negative impacts of shoreline developments will be.

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Larson, E.R., Olden, J.D., and Usio, N. 2011. Shoreline urbanization interrupts allochthonous subsidies to a benthic consumer over a gradient of lake size. Biology Letters 7:551-554.

Thanks to http://www.essexbiodiversity.org.uk/Default.aspx?pageid=148 for providing the image used in this post.