| Summary: | Abstract Woody plant encroachment (the replacement of grasses and forbs by woody shrubs and trees) is reshaping communities in both terrestrial and coastal environments, but little is known about its effects on basal consumers. We used interactions between fiddler crabs (Uca spp.) and the encroaching mangrove Avicennia germinans in Gulf of Mexico salt marshes to explore trophic interactions between basal consumers and encroaching shrubs. In a series of food preference trials, fiddler crabs consumed over five times more marsh (Spartina alterniflora) plant matter when given a choice between mangrove and marsh diets. In food quality trials, fiddler crabs stored 50% less energy, as measured by hepatopancreas lipid content, when restricted to a mangrove diet. These results demonstrate that mangrove encroachment may have negative consequences for salt marsh basal consumers, which may lead to deleterious population‐level effects and altered ecosystem carbon flows. As fiddler crabs are generalist consumers and mangroves share many physical and chemical characteristics with other encroaching woody species, negative consequences for basal consumers may be widespread in other environments undergoing woody shrub encroachment.
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