Differential responses of arthropod communities to habitat mixing in a dynamic marsh-mangrove ecotone
Along many coastlines worldwide, the boundary between tropical and temperate areas is marked by a habitat gradient between mangrove forests (tropical) and salt marshes (temperate). These gradients, or ecotones, are dynamic: their location and extent are seemingly impacted by climate trends, extreme weather events, and biotic interactions. In areas such as the Southeastern United States, mangrove-marsh ecotones are changing as mangroves expand their ranges poleward due to milder winters. Studying ecological patterns along these ecotones is of interest because 1) they are well suited for asking questions regarding range-expansions, habitat heterogeneity and climate change and 2) mangroves and salt marshes provide critical ecosystem services and support diverse animal communities. In the present study, we quantified the composition of terrestrial arthropod communities along the mangrove-marsh ecotone on the Atlantic coast of Florida, USA. In May and July 2014 we sampled arthropods from patches of black mangroves (Avicennia germinans), the marsh plant Batis maritima and the marsh plant Spartina alterniflora at 7 sites spanning 350km. Three sites were located inside the ecotone and four bordered the ecotone (2 north, 2 south). During both months we collected ten community samples per patch type at each site and identified specimens to order or suborder.
Arthropod communities on black mangrove trees were very different from those present in marsh plant patches, regardless of habitat type (ecotone or not). This was primarily due to a high abundance of psyllids (Hemiptera: Sternorrhyncha) on mangroves and their rarity in marsh patches. Arthropod diversity was greater in patches of marsh plants located in ecotone landscapes compared to patches in northern “mangrove-free” marshes (both B. maritima and S. alterniflora), while diversity did not differ between mangrove trees in the ecotone vs. trees in southern mangrove forests. However, arthropod community structure significantly differed between patches inside vs. beyond the ecotone for both mangroves and marsh plants, though the difference was subtle for mangroves and pronounced for marsh patches. Differences in diversity and community structure between ecotone and non-ecotone patches were driven primarily by taxonomic groups comprised of herbivores (Hemiptera, Orthoptera), implying that plant-herbivore interactions likely vary along the forest-ecotone-marsh continuum. Such spatial variation in herbivory may prove important for understanding mangrove range expansion. Our data suggest that the colonization of marshes by mangroves causes deterministic changes in local arthropod communities. We are currently conducting a transplant experiment to elucidate mechanisms behind these community shifts and predict some of their ecological consequences.