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# Abstract

The mechanisms structuring patterns of diversity and community composition can be difficult to identify, and much of our knowledge stems from study of local ecological systems.  Two candidate mechanisms include dispersal and environmental heterogeneity, which structure communities at local and larger scales by fostering species coexistence and niche partitioning, respectively.   It is important to understand these patterns and their drivers at larger scales, especially in the face of climate change and other perturbations. The Gulf of Alaska (GOA) has complex topography, climate-driven variability, and a well-studied groundfish community, making it an ideal study system.   We examined patterns of diversity and community composition in the groundfish community across 14 sites in the GOA using geostatistically modeled groundfish abundance and biomass from the Alaska Fisheries Science Center trawl survey data (1984 – 2015).  We found that species richness, and alpha, beta, and functional diversity varied little both within and between study areas, and were conserved across the central GOA.  Conversely, community composition varied significantly along a longitudinal gradient, with distinct groups of species in individual study areas.  These differences in community composition were driven by rare and lower-density species, while high-density species remained the same.   Thus, community structure was conserved despite variation in species identities.  Overall, environmental heterogeneity and community structure control groundfish diversity across the GOA large marine ecosystem.

# Introduction

The mechanisms driving patterns of diversity and structuring communities remain important topics of ecological study, but our knowledge of these concepts stems mostly from local ecological systems.  Less is known about what drives patterns of diversity and community composition in a large-scale, spatial context.
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Spatial stability in ecosystem properties and processes can be dependent on the spatial structure of biodiversity, and can occur across many scales, from between organisms to between ecosystems (Gonzalez 2009, Leibold 2004, Wang 2014, Wang 2016).  Species richness is one of the most fundamental diversity metrics and measures the number of species in a given space (habitat, ecosystem, etc.).  In a more explicitly spatial context, the effects of biodiversity are can be examined within local scale communities ($$\alpha$$ diversity), between local scale communities ($$\beta$$ diversity), and within regional scale communities ($$\gamma$$ diversity) (CITES,