Human land use in stream catchments and direct alterations to stream channels have degraded stream habitats across the world, which in turn has resulted in declines in biodiversity. In addition, climate change has and will continue to alter stream habitat and lead to additional changes in species distributions and characteristics of existing populations, which in some cases, may lead to species extinction. Given the current degraded condition of stream habitats and potential effects of climate change, effective conservation of stream species requires that decision-makers anticipate climate change and incorporate knowledge on its effects into conservation strategies (i.e., proactive conservation). Proactive conservation therefore requires that current influences of climate on streams and the organisms they support are understood. However, observed effects of climate on organisms depend on the scale over which they are examined. Studies at multiple spatial extents can increase our understanding of climate change effects on streams in different ways and can provide effective insights when they are complementary. The goal of my dissertation is to implement a research framework that increases understanding of climate influences on streams over multiple scales in support of proactive conservation. I applied this framework across the five largest Hawaiian Islands, where little research has been conducted on effects of climate on stream organisms. In these chapters, I examined the influence of climate on native stream organisms over two spatial scales, the entirety of study area, which has a wide range in rainfall and other natural landscape features, and within a region of Hawaii Island with little variability in most landscape features but a gradient in mean annual rainfall. I then used my results to inform a spatial prioritization approach that identified areas of high conservation value given potential changes in rainfall. I found that across the entire study area, influences of climate and other natural landscape features on species distributions could be used to characterize differences in stream habitats across Hawaii, while over smaller spatial extents, influences of rainfall on native stream species population characteristics (i.e., individual size, disease occurrence) were observed indirectly through influences of flow magnitude, variability, and the occurrence of low flows. The results of the spatial prioritization of stream habitats indicated that many areas capable of supporting unique stream habitats and taxa assemblages will continue to do so as rainfall magnitude changes through the 21st century, but some streams will experience annual or seasonal drying that may result in a shift in their ability to support taxa. My study generated useful information on potential effects of climate on species in Hawaii and demonstrated the value of considering changes in population characteristics of stream organisms to understand underlying mechanisms that may drive species loss with climate change. In addition, the multi-scale framework implemented in my study can be applied in other understudied regions that require information on influences of climate on stream ecosystems to inform proactive conservation decision-making.
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