Climate change is altering freshwater habitats in the United States by increasing water temperatures and changing stream flows. These habitat changes can directly impact survival and geographic ranges of ectothermic freshwater organisms through thermal stress and habitat loss, and indirectly through changes to life-history traits such as generation time. Thus, it is likely that many freshwater organisms will be vulnerable to extinction under climate change, and many more will undergo distributional shifts to track their climatic niche, driving large-scale alterations to freshwater biodiversity in the United States. I investigated the effects of climate change on freshwater biodiversity at multiple levels of ecological organization and spatial scales. In Chapter 1, I examined how climate change could alter the survival of predators with different foraging traits. I used experiments to quantify the effects of temperature on predator-prey interactions of freshwater insects and zooplankton living in Southwest Michigan ponds. I found that freshwater insects increased their feeding and growth rates at higher temperatures. Further, predators with sit-and-wait foraging strategies may outperform insects with active strategies in a warming world, because active predators may be more likely than sit-and-wait predators to starve if their prey populations decline as a result of climate warming. In Chapter 2, I investigated how increases in predator feeding and growth rates with climate warming could alter populations of freshwater insects with complex life cycles. Using data from experiments in Chapter 1 and field surveys, I parameterized a consumer-resource population model for damselflies (consumer) and (zooplankton) resource. I used this model to simulate changes in populations with climate warming and found that damselflies shifted their life-history from one to two generations per year and increased their population size. In Chapters 3 and 4, I scaled up my research by investigating the effects of climate change on freshwater insect biodiversity in the contiguous United States. I addressed the limitations of existing biodiversity databases for freshwater insects in Chapter 3 by assembling a database, Freshwater insect occurrences and traits for the contiguous United States (Freshwater insects CONUS). This database contains 2.05 million genus occurrence records for 932 genera in the major freshwater insect orders, at 51,044 stream locations, and life history, dispersal, morphology, and ecology traits for 1,007 insect genera. In Chapter 4, I paired this database with spatial environmental data on climate, hydrology, land cover, and topography of watersheds to assess the climate sensitivity of insect genera across the contiguous United States. Of the 488 insect genera that I examined, insects with erosional flow preference, cold-cool eurythermal preference, and univoltine life-histories, and insects in the orders Plecoptera and Trichoptera are likely the most sensitive to climate change. I also found that watersheds with the highest proportions of sensitive taxa are in the mountain West and Appalachian regions of the United States. These insect genera and regions should be prioritized for further research and protection against losing freshwater biodiversity. I addressed two knowledge gaps that have hindered efforts to conserve freshwater biodiversity in a changing climate, including understanding physiological and demographic consequences of increasing temperatures and relationships between climate and geographic distributions of freshwater insects. Conservation practitioners can use the results of my research to prioritize freshwater organisms and watersheds for protection in a changing climate.
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