Effects of mangrove encroachment on tidal wetland plant, nekton, and bird communities in the Western Gulf of Mexico

Abstract

Woody encroachment is occurring in many marsh-mangrove ecotones across the globe, with multiple drivers contributing to an increase in mangrove cover. As a result, marsh plant species are often displaced, resulting in a striking regime shift from grass and forb-dominated habitats to taller, woody vegetation. Our goal was to quantify the bottom-up effects of mangrove woody encroachment into coastal wetlands on associated plant, nekton, and bird assemblages. In 2012, we established several large (>20 ha) survey areas at tidal wetland sites with or without black mangroves (Avicennia germinans) on the Texas (USA) coast in the Gulf of Mexico, an area highly susceptible to mangrove encroachment. From 2012 to 2015, we recorded vascular plant cover and diversity, sampled nekton at the water-vegetation interface using seine nets, and compared wintering shorebird and wader use among sites using the citizen science database eBird. Marsh plant species richness was 50% lower at sites with mangroves, and some species, such as Distichlis spicata, were absent from these sites entirely. The relative abundance and species richness of nektonic fish and invertebrates were similar between sites, but certain species (e.g., bay anchovies Anchoa mitchilli, grass shrimp Palaemonetes spp.) were more common at sites without mangroves. Citizen science data indicated that shorebird and wader species richness was 20% lower at sites with mangroves. Shorebird assemblages were markedly different between site types; six shorebird and three wader species were only detected in marsh sites. Our results indicate that coastal wetlands dominated by mangroves support different plant and animal assemblages than marsh-dominated areas. These results were largely consistent with the results of a previous manipulative experiment in the same area, lending considerable confidence to the conclusion that mangroves influence the associated plant and animal communities. Therefore, as woody encroachment continues and mangrove cover gradually increases, this change may lead to complex bottom-up effects on a range of ecosystem processes and services.