Graduate Thesis Or Dissertation
 

Assessing the Effects of Contemporary Forest Harvest on Headwater Stream Food Webs in the Oregon Coast Range

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/f1881t79r

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  • Food webs are fundamental in ecology, as they offer a way to better understand the interrelatedness, connectivity, and complexity within a given ecosystem. The use of network analysis to model food webs can provide insights about the importance of physical templates on the organization of biological systems in streams using previously compiled observations. This approach can be used to assess temporal influence on stream food web structure, in the context of natural variation, and disturbance. In the Coast Range of the Pacific Northwest in North America, most headwater streams occur within forested systems that provide crucial habitat to aquatic biota and protect valuable fisheries and ecosystem services. Here, we propose the use of modeled stream food webs as tools that can integrate multiple processes and provide valuable insights to understanding the complex impacts of contemporary forest harvest on aquatic ecosystems. The Trask River Watershed Study (TWS) was a long-term, multi-disciplinary and experimental paired-watershed study aimed to assess the ecological impacts of contemporary forest harvest on stream ecosystems in the Coast Range of western Oregon. We used the composition of biological community data from empirical datasets of the TWS to model stream food webs pre- and post-harvest for over 10 years across 12 first- to second-order streams, including harvested and reference conditions. In addition, we used multiple covariates that include both abiotic and biotic factors that can help explain modeled food web structure over time. We modeled 114 stream food webs using R package ‘Cheddar’ and function ‘WebBuilder’ in R studio. We extracted multiple quantitative metrics that describe stability and complexity of food web networks (i.e., connectance, linkage density, average path length and fraction omnivory). We assessed the impacts of variable buffered streams (no buffer or leave trees only), compared to full riparian buffered streams across the upper Trask River headwaters. We found that forest harvest treatments resulted in variable responses in food web structure, that in some cases shift in directions outside of the natural temporal variation of systems not affected by forest management. We observed that the greatest effect of forest harvest did not necessarily occur immediately following harvest. We used trajectories over time from NMDS plots and showed that the food webs became less similar to pre-harvest over time at three sites post-harvest. Specifically, we found a detectable shift in food webs at three of the seven treated sites, with decreases in ‘connectance’ at one of the variable buffered sites and one of the uniform riparian buffered sites, along with an increase in ‘linkage density’ occurring at an additional one of the variable buffered sites. These results suggest a decrease in community stability for sites both treated with variable buffers and uniform buffers, and an increase in complexity for an additional variable buffered site, respectively. Our findings may indicate a potential response in stream food web structure to contemporary forest harvest based on detectable post-harvest shifts in food web metrics, more analyses of stream food webs will be useful to understand the implications of these metrics for stream ecosystems and forest management moving forward.
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  • Pending Publication
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  • 2022-03-23 to 2023-04-23

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