Negative outcomes of novel trophic interactions along mangrove range edges

Abstract

Tropicalization is a phenomenon that is changing the structure of ecosystems around the world. Mangrove encroachment is a particular form of tropicalization that may have cascading consequences for resident fauna in subtropical coastal wetlands. There is a knowledge gap regarding the extent of interactions between basal consumers and mangroves along mangrove range edges and the consequences of these novel interactions for consumers. This study focuses on the key coastal wetland consumers, Littoraria irrorata (marsh periwinkle) and Uca rapax (mudflat fiddler crabs), and their interactions with encroaching Avicennia germinans (black mangrove) in the Gulf of Mexico, USA. In food preference assays, Littoraria avoided consuming Avicennia and selectively ingested leaf tissue from a common marsh grass, Spartina alterniflora (smooth cordgrass), a preference that was also previously documented in Uca. The quality of Avicennia as a food source was determined by measuring the energy storage of consumers that had interacted with either Avicennia or marsh plants in the lab and the field. Littoraria and Uca both stored ~10% less energy when interacting with Avicennia, despite their different feeding behaviors and physiologies. The negative consequences of mangrove encroachment for these species at the individual level suggest that there may be negative population-level effects as encroachment continues. Many studies have documented shifts in floral and faunal communities following mangrove replacement of salt marsh vegetation, but this study is the first to identify physiological responses that may be contributing to these shifts.

A call to expand global change research in LTER coastal wetlands

“…These studies, each of which are briefly summarized below, evaluated a diversity of response variables, including dietary preferences and food quality, consumer species abundance and biometrics, and changes in ecological community structure. Goeke et al (2021aGoeke et al ( , 2021bGoeke et al ( , 2023 used feeding trials and food quality assays over a one year period to determine how key coastal wetland consumers, the marsh periwinkle (Littoraria irrorata) and mudflat fiddler crabs (Uca rapax), interact with the encroaching black mangrove (Avicennia germinans) in the Gulf of Mexico. They found that Littoraria avoided consuming Avicennia and selectively ingested leaf tissue from a common marsh grass instead, and that both Littoraria and Uca were found to store ~10% less energy when consuming Avicennia.…”

“…They found that Littoraria avoided consuming Avicennia and selectively ingested leaf tissue from a common marsh grass instead, and that both Littoraria and Uca were found to store ~10% less energy when consuming Avicennia. Though outside the scope of their study, Goeke et al (2023) note that the pattern of food preference and decreased physiological condition in two taxonomically distinct species may represent a general consequence of mangrove encroachment that is not species-specific, with significant implications for energy flow within these changing ecosystems. In particular, as Spartina alterniflora becomes less common, the attendant decrease in body condition of basal consumers may lead to a reduction in prey availability and destabilization of higher trophic levels (Goeke et al 2023).…”