Detecting impacts of climate change and fishing on shelf ecosystems
Creators: name: Kitchel, Zoë J.
Publisher: name: No Publisher Supplied
Year: 2023

description: Climate change, natural resource extraction, industrialization, and the spread of non-native species has led to the reorganization of biological communities worldwide. The loss of essential abiotic and biotic habitat characteristics has led to species extirpations. At the same time, the proliferation of newly available niche space has led species to expand into previously inhabited spaces. Inconsistencies in how species disappear from communities and move into newly accessible habitats both across species and across a species’ range lead to the coexistence of novel groups of species. How this unprecedented confluence of stressors drives changes in global biodiversity remains poorly understood. In addition, these changes affect how humans interact with the environment and natural resources. Key questions include which factors drive local and regional changes in species richness, how community structure is changing, and the implications of altered species distributions and interactions. The metrics and data we use paired with the scale at which we ask questions shape our baseline comprehension of biodiversity change. Marine communities are responding faster on the whole to recent and rapid environmental change, and therefore the ocean provides an ideal context to study biodiversity change. In this dissertation, I leveraged multiple long-term scientific surveys to expand our knowledge of the mechanisms and consequences of species reorganization in the ocean. In Chapter 1, I tested temperature as a driver of regional species gains and losses for &gt;600 fish and invertebrate species on North American continental shelves. I found that regions were more likely to gain species in years of low temperature seasonality. Species losses also responded to low seasonality years but were lagged by three years on average. These findings suggested that the seasonal range of environmental conditions experienced in a region can limit which species can thrive. Additionally, our observation that species losses lag species gains across temperate shelf communities suggested that many communities were in a transient state of high richness, which will likely be followed by a decline in richness when the rate of environmental change slows. Disentangling the alternate drivers of species gains and losses can improve our ability to predict future changes in species distributions. Chapter 2 investigated the prevalence of biotic homogenization and differentiation across 37 scientific surveys of tropical, temperate, and sub-arctic shelf communities of demersal fish. I calculated annual pairwise Bray Curtis dissimilarity for each survey in each year. Rather than exhibiting consistent directional change in dissimilarity over time, most regions exhibited high multi-annual and decadal variability in community structure. This finding highlights the dynamic nature of demersal fish communities and suggests that these communities may respond quickly to changing habitat conditions. However, eight surveys were differentiating and six were homogenizing. There were no clear links to survey size, duration, sampling frequency, species richness, or sampling depth, though all examples of homogenization occurred in the northern hemisphere. Neither temperature nor fishing pressure explained overall variation in dissimilarity, but some individual surveys exhibited strong relationships with these drivers. These findings suggest that—unlike in many terrestrial and freshwater systems—biotic homogenization is uncommon in demersal fish communities. Chapter 3 expanded on the spatial beta diversity analyses of the previous chapter by investigating how trends in dissimilarity across time vary by spatial extent, temporal frequency, and study duration. I conducted multiple experiments by subsampling observations from the long-term Eastern Bering Sea scientific bottom trawl survey. I compared patterns in total Bray Curtis dissimilarity through time for the full region and three ecologically distinct subregions. Additionally, I subsampled the full 36-year time series into sequential series of two to 35 years in length to examine how the dissimilarity trend varies by length of the study period. Finally, I explored how sampling frequency across a range of study duration shapes observed trends in dissimilarity. I found that beta diversity trends vary by geographic scope, sampling frequency, and study duration. Between 29-32 years of the full time series were required for most observed trends of that length to match the long-term trend. Because of the high sensitivity of beta diversity trends to study design, I recommended caution in identifying communities as homogenizing or differentiating without a careful consideration of spatial and temporal scale.In sum, this body of work advances our knowledge of the mechanisms behind changing species distributions, the consequences for broader community structure, and the importance of matching study design to research questions in biodiversity research.