Effects of mangrove encroachment on tidal wetland plants and epifauna: 2012-2020

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 and epifaunal 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. Starting in 2012, we
annually recorded vascular plant cover and diversity and recorded
snail (Littoraria irrorata) and fiddler crab (Uca spp.) density
along transects perpendicular to the shoreline. Marsh plant species
richness was 50% lower at sites with mangroves, and some species,
such as Sarcocornia spp. and Distichlis spicata, were relatively
rare or absent from sites with mangroves. The wetland plant
communities at these sites were relatively unaffected by Hurricane
Harvey (August 2017). Epifaunal snails and crabs were common at all
sites, with abundances that varied over time. Our results indicate
that coastal wetlands dominated by mangroves support different and
lower diversity plant assemblages than marsh-dominated areas. These
results were largely consistent with the results of a previous
manipulative experiment in the same area. 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.