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Vulnerability of cold-water and cool-water fishes to climate change within an anthropogenic context using boosted regression trees, decision scaling, and ecosystem services




Harrison-Atlas, Dylan, author
Theobald, David, advisor
Arabi, Mazdak, committee member
Goldstein, Joshua, committee member
Poff, N. LeRoy, committee member

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Across the globe, environmental changes are occurring in ways that are profoundly important for freshwater ecosystems with implications for the occurrence of species. Typically, ecologists have sought to understand the distribution of freshwater species using natural environmental gradients. However, because rivers and streams embody a wide range of conditions due to human activity, adequately characterizing modern day drivers of species occurrence requires assessing both natural and anthropogenic influences within the context of global change. In recent decades, growing concerns over climate change have further contributed to the need to assess contemporary drivers of species occurrence. Despite this urgency, forecasting ecological responses to climate change remains a key conservation challenge. The aims of my research were to: a) investigate the drivers of western US riverine fish species occurrence within the context of global change; and b) project range-wide and site-level vulnerability of cold-water fish species to climate-induced changes in stream temperature and streamflow and to alternative land use trajectories. In my assessment of contemporary drivers of cold-water and cool-water fish species distribution, I found that primary determinants of fish occurrence included human influences that accounted for a substantial portion of modeled outcomes among species. Sedimentation and nutrient enrichment were the two primary disturbance pathways by which human activities influence aspects of stream condition that drive patterns of species occurrence. I also found that species had variable responses across anthropogenic gradients, suggesting that future efforts to characterize species-environment relations consider approaches that can capture nonlinear and threshold responses that occur along continuous gradients. In a second analysis, I evaluated the range-wide vulnerability of cold-water fish species to projected climate change in the western United States and assessed site-level vulnerability to varying degrees of exposure to climate change and additional environmental stressors. I focused on rainbow trout (Oncorhynchus mykiss sp.) and cutthroat trout (Oncorhynchus clarkii sp.) -- two wide-ranging salmonids of significant conservation and economic importance. Using high resolution data on future stream temperature and mean annual flow, I projected climate-induced changes in suitable habitat across the historic native ranges of both species within the western United States. Projected declines in suitable habitat for cutthroat trout were substantial by 2080 and exceeded those of rainbow trout. A sensitivity analysis revealed that stream temperature warming was the primary driver of habitat loss for both species. Both cutthroat trout and rainbow trout exhibited regional variability in habitat loss that was consistent with the magnitude of projected warming for summer stream temperature. Cutthroat trout distributions are expected to shift upwards along an elevational gradient with warming causing fragmentation of contiguous habitat that will likely expose them to additional environmental disturbances. I conducted a complementary set of analyses using a decision-scaling approach to explore site-level vulnerability as a function of feasible climate futures and human-influenced environmental factors that have previously been implicated as key components of suitable habitat for cutthroat and rainbow trout. I uncovered important insights into species vulnerability including differential sensitivity to stream temperature warming among cutthroat trout and rainbow trout as well as predominant influences of land use on species vulnerability independent of climate. Under a hypothetical climate adaptation scenario, I found that increased riparian cover shifted the distribution of vulnerability of cutthroat trout towards less frequent extirpations and that these benefits were achieved throughout feasible climate space. My findings suggest that augmentation of riparian vegetation is likely to be a robust climate adaptation strategy in an uncertain future. I conclude by offering two complementary approaches for advancing climate adaptation for freshwater systems in the face of uncertainty. I also conducted a systematic review of hydrologic ecosystem services (HES) studies published within the past decade, finding compelling evidence that variability in methods used to quantify HES reflects an orientation towards decision making. I discuss implications of my findings on climate change vulnerability and consider ways to integrate an ecosystem services approach into the management and conservation of freshwater fish.


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