Local and regional variation in effects of burrowing crabs on plant community structure

Abstract

Burrowing animals can profoundly influence the structure of surrounding communities, 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 influence 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 community 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 (Spartina foliosa) and perennial pickleweed (Sarcocornia pacifica) at three sites in southern California 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 (Pachygrapsus crassipes) had positive effects on cordgrass cover at one site and no effect on cordgrass production 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.

Predicting burrowing crab impacts on salt marsh plants

“…To study the impact of crabs on the plant community, we manipulated burrowing crab access to plots at both regions. Manipulations in southern marshes (KF1 and KF2) were conducted May 2016 to October 2018 (Walker et al 2020). Manipulations in northern marshes were started on different dates (May 2017, April 2018, and June 2018 at BOL, BOD, and TOM; respectively) and were run through October 2019.…”

“…To manipulate burrow density, we followed methods as detailed in Walker et al (2020). Briefly, square plots (0.7 9 0.7 9 0.6 m, l 9 w 9 h) were fenced in with plastic-mesh cages (mesh opening = 0.6 cm).…”

“…Control replication changed through time because we were initially limited on person hours to monitor additional replicates (Appendix S1: Fig. S2, Walker et al 2020). Two pitfall traps (empty tennis ball canisters: diameter = 7 cm, height = 20 cm) were installed at opposite, diagonal corners of all cages.…”

“…In some marshes, such losses have been positively correlated with drought and were hypothesized to be related to increased interaction strengths with snails and burrowing crabs (Silliman et al 2005. However, burrowers have also been found to mitigate cordgrass loss during a marshwide cordgrass decline (Walker et al 2020), suggesting that crabs could also facilitate cordgrass (Bertness 1985). The capacity of crabs to modify marsh loss may critically influence marsh functioning.…”

Burrowing crabs and physical factors hasten marsh recovery at panne edges

“…snail grazing in marshes with drought [64], pollinator and herbivore interactions with plants across environmental gradients [65], foraging behavior of coral reef fishes with distance from reef [66]). Crab effects on salt marshes have been shown to have such context-dependent variation when examined across different estuaries [34,38,67]. In a meta-analysis that included up to 42 studies assessing consumer effects of crabs on salt marsh plants, the average effect size (Hedges' g) for multiple response variables (e.g.…”