Local and regional variation in effects of burrowing crabs on plant community structure JANET B. WALKER, 1,2,4 SHELBY A. RINEHART , 3 WENDI K. WHITE, 1 EDWIN D. GROSHOLZ, 2 AND JEREMY D. LONG 1 1 Biology Department, San Diego State University, 5500 Campanile Drive, San Diego, California 92182 USA 2 Department of Environmental Science and Policy, University of California, Davis, One Shields Avenue, Davis, California 95616 USA 3 Department of Ecology, Evolution, and Behavior, The Alexander Silberman Institute of Life Sciences, The Hebrew Universityof Jerusalem, Jerusalem 91905 Israel Citation: Walker, J. B., S. A. Rinehart, W. K. White, E. D. Grosholz, and J. D. Long. 2021. Local and regio- nal variation in effects of burrowing crabs on plant community structure. Ecology 102(2):e03244. 10.1002/ ecy.3244 Abstract. Burrowing animals can profoundly influence the structure of surrounding com- munities, as well as the performance of individual species. Changes in the community structure of burrowing animals or plants together with changing abiotic parameters could shift the influ- ence of burrowers on surrounding habitats. For example, prior studies in salt marshes suggest that fiddler crabs stimulate cordgrass production, but leaf-grazing crabs suppress cordgrass production. Unfortunately, testing this prediction and others are impeded because few studies have examined crab impacts on the plant community and across multiple sites, multiple years, or both. This challenges our ability to predict how burrowing animals will influence plant com- munity structure, and when and where these impacts will occur. We manipulated the densities of the dominant burrowing crabs in plant assemblages dominated by Pacific cordgrass (Spar- tina foliosa) and perennial pickleweed (Sarcocornia pacifica) at three sites in southern Califor- nia for three years (2016, 2017, 2018). Crab impacts on plant community structure differed among each of our three sites. In contrast to our predictions, (1) leaf-grazing crabs (Pachygrap- sus crassipes) had positive effects on cordgrass cover at one site and no effect on cordgrass pro- duction at a nearby site in the same marsh and (2) fiddler crabs (Uca crenulata) did not stimulate cordgrass production at another marsh. Because crabs affected traits of cordgrass, but not pickleweed, in the direction consistent with changes in cordgrass cover, we propose that marsh-specific crab effects on community structure were largely mediated through changes in cordgrass, as opposed to pickleweed. Importantly, crabs facilitated cordgrass during marsh-wide cordgrass loss, suggesting that crabs may mitigate environmental stress for this ecologically important plant. Because cordgrass abundance can be a critical measure of marsh functioning and is often a restoration target, we suggest that managing cordgrass populations would benefit from additional information about crab populations and their impacts among years, and among and within marshes. Key words: burrowing animals; cordgrass; crabs; plant–animal interactions; salt marsh; spatial scales; temporal scales. INTRODUCTION Burrowing animals, such as earthworms, crabs, prairie dogs, ground squirrels, ghost shrimps, and rodents, are often considered to be ecosystem engineers given their influence on plant communities (Whicker and Detling 1988, Laundre 1993, Mulder and Keall 2001, Wilby et al. 2001, Davidson and Lightfoot 2008, Castorani et al. 2014, Craven et al. 2017). These animals can alter the total abundance of plant species (e.g., reducing vege- tation cover), and their impacts on plants can vary both in size and direction. More subtle effects, such as the direct and indirect effects on individual plant species, can influence plant community structure and are less understood. This is surprising given that such plant- specific effects may frequently influence ecosystem func- tioning (Fields et al. 1999, Eisenhauer et al. 2009, Andri- uzzi et al. 2016). Few studies have tested these burrower effects across multiple sites, multiple years, or both, which challenges our ability to predict how burrowing animals will influence plant community structure, and when and where these impacts will occur. Changes in the plant community by burrowing ani- mals can affect important ecosystem functions, such as carbon and nutrient cycling (Vanni 2002, Andriuzzi et al. 2016), plant succession (Fields et al. 1999), and plant productivity (Eisenhauer et al. 2009). In temperate grasslands, earthworms can alter belowground competi- tion and aboveground production of plants, which can Manuscript received 22 March 2020; revised 3 September 2020; accepted 18 September 2020. Corresponding Editor: A. Randall Hughes. 4 E-mail: janwalker@ucdavis.edu Article e03244; page 1 Ecology , 102(2), 2021, e03244 © 2020 by the Ecological Society of America