Detrital traits affect substitutability of a range‐expanding foundation species across latitude

Abstract

Climate‐driven range shifts of foundation species could alter ecosystem processes and community composition by providing different resources than resident foundation species. Along the US Atlantic coast, the northward expanding foundation species, black mangrove Avicennia germinans, is replacing the dominant salt marsh foundation species, marsh cordgrass Spartina alterniflora. These species have distinct detrital attributes that ostensibly provide different resources to epifauna. We experimentally examined how detritus of these species affects decomposition and community composition in different habitat contexts at regional and local scales. First, we manipulated detritus identity (Avicennia, Spartina) at 13 sites across a 5° latitudinal gradient spanning mangrove, mixed marsh‐mangrove and salt marsh habitats. Across latitude, we found that Avicennia detritus decomposed 2–4 times faster than Spartina detritus, suggesting that detrital turnover will increase with mangrove expansion. Epifaunal abundance and richness increased 2–7 times from south to north (mangrove to salt marsh) and were equivalent between Avicennia and Spartina detritus except for crabs, a dominant taxonomic group that preferred Spartina detritus. Second, to examine the whether changing habitat context affected regional patterns, we manipulated detritus identity and surrounding habitat type (mangrove, salt marsh) at a single mixed site, also including inert mimics to separate structural and nutritional roles of detritus. Epifaunal richness was similar between the two detrital types, but crabs were 2–7 times more abundant in Spartina detritus due to its structural attributes. Surrounding habitat type did not influence decomposition rate or community patterns, which suggests that latitudinal influences, not surrounding habitat, drove the regional community patterns in the first experiment. Overall, mangrove expansion could alter epifaunal communities due to the lower structural value and faster turnover of mangrove detritus. As species shift with changing climate, understanding foundation species substitutability is critical to predict community change, but we must account for concomitant environmental changes that also modify communities.

Coastal carbon processing rates increase with mangrove cover following a hurricane in Texas, USA

“…These conditions likely favored higher breakdown rates of labile organic matter (S. alterniflora and green tea litter), but we did not observe this pattern for recalcitrant litter (A. germinans). In contrast, higher breakdown rates of A. germinans in mangrove-dominated plots suggest a possible "home field" advantage of metazoan and microbial communities adapted to and preferentially consuming recalcitrant organic matter, which has been observed in other wetland and aquatic ecosystems (Kominoski et al, 2012;Smith et al, 2019;Yeung et al, 2019). Smith et al (2019) found A. germinans litter k was two to four times higher than that of S. alterniflora litter irrespective of cell vegetation type (marsh, mangrove, and mixed-species), and it was suggested that this was because A. germinans has a lower molar C:N ratio than S. alterniflora.…”

“…In contrast, higher breakdown rates of A. germinans in mangrove-dominated plots suggest a possible "home field" advantage of metazoan and microbial communities adapted to and preferentially consuming recalcitrant organic matter, which has been observed in other wetland and aquatic ecosystems (Kominoski et al, 2012;Smith et al, 2019;Yeung et al, 2019). Smith et al (2019) found A. germinans litter k was two to four times higher than that of S. alterniflora litter irrespective of cell vegetation type (marsh, mangrove, and mixed-species), and it was suggested that this was because A. germinans has a lower molar C:N ratio than S. alterniflora. However, C:N of A. germinans litter from our sites was higher (66-72) (Charles et al, 2020) than that has been reported elsewhere (47-55) (Lawton-Thomas, 1997;Twilley et al, 1986), making it more similar in nutrient content to Spartina detritus (65-92) (Breteler et al, 1981;White & Howes, 1994), and making this an unlikely mechanism in our study.…”

Evidence for the genetic similarity rule at an expanding mangrove range limit

“…We documented a correlation between mangrove host-tree genetics and fungal community differences, but does this relationship generate variation in stress tolerance among mangrove hosts? If so, this insight could broaden the current discussion of how a shift from salt marsh to mangrove dominance may shape these coastal communities (e.g., Kelleway et al, 2017;Johnston and Gruner, 2018;Smith et al, 2019;Armitage et al, 2020) by including mangrove intraspecific variation as a factor that could influence population resilience at these high-stress range limits.…”

The impacts of mangrove range expansion on wetland ecosystem services in the southeastern United States: Current understanding, knowledge gaps, and emerging research needs

“…Similarly, macrofaunal communities differed in marsh versus mangrove-dominated vegetation along the Atlantic coast of Florida, due to the influence of fine-scale plant structural differences and stand-level habitat attributes (Johnston & Gruner, 2018). Along the Atlantic coast, studies have noted differences in detrital-based epifaunal communities (Smith et al, 2019) Beyond just the direct effects on animals, mangrove expansion can also affect microbial (Barreto et al, 2018) and rhizosphere communities (Chen et al, 2020), which can affect abiotic conditions, biotic interactions, and biogeochemical cycling. The effects of mangrove expansion on coastal food webs and fisheries may also increase with time due to the influence of the forest developmental stage (Barimo & Serafy, 2003;Scheffel et al, 2018).…”