Monday, August 6, 2007

PS 1-3: The role of the decline of a foundation species, Eastern Hemlock (Tsuga canadensis), on consumer-resource interactions in adjacent aquatic ecosystems

Carrie A. DePalma and Christopher M. Swan. University of Maryland, Baltimore County

In forest communities, loss of foundation species by human-induced disturbances threatens the stability and performance of terrestrial and aquatic ecosystems. A contemporary example is the decline of Eastern Hemlock (Tsuga canadensis) by the invasive herbivore, hemlock woolly adelgid (Adelges tsugae). Considering that hemlocks thrive in riparian forest communities, hemlock mortality may result in increased wood to adjacent streams, consequently influencing stream ecosystem processes. Since the primary energy source in small streams is allochthonous, aquatic detritivores play a fundamental role in organic matter decomposition, altering their contribution to decay in response to litter quality. We investigated the influence of leaf litter mixed with a gradient of hemlock wood in order to understand the detritivore behavioral response to increased in-stream wood. We conducted a laboratory experiment where we offered a common leaf-shredding detritivore (Pteronarcys sp.) a constant amount of Red Maple leaf litter (1 g), which was combined with five hemlock wood treatments, ranging from 0-8 grams. Detritivores were excluded in five replicates of each treatment to serve as controls for calculating the microbial and fungal contribution to mass loss. Our results revealed that as hemlock wood increased, mass loss of leaf litter declined significantly, suggesting that detritivore feeding rates were hindered when offered a refractory resource such as wood. No significant trend emerged from mass loss of wood among detritivore or wood treatments, indicating that preferential feeding on wood did not occur. We conclude that increasing hemlock wood can drive detritivore feeding rate down, potentially causing a decline in detritivore contribution to leaf litter decomposition in hemlock-drained streams.  This work provides insight into how loss of a foundation species results in changes to consumer-resource interactions, and the implications for a critical ecosystem process.

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