Sea level rise may increase extinction risk of a saltmarsh ontogenetic habitat specialist

Abstract

Specialist species are more vulnerable to environmental change than generalist species. For species with ontogenetic niche shifts, specialization may occur at a particular life stage making those stages more susceptible to environmental change. In the salt marshes in the northeast U.S., accelerated sea level rise is shifting vegetation patterns from flood‐intolerant species such as Spartina patens to the flood‐tolerant Spartina alterniflora. We tested the potential impact of this change on the coffee bean snail, Melampus bidentatus, a numerically dominant benthic invertebrate with an ontogenetic niche shift. From a survey of eight marshes throughout the northeast U.S., small snails were found primarily in S. patens habitats, and large snails were found primarily in stunted S. alterniflora habitats. When transplanted into stunted S. alterniflora, small snails suffered significantly higher mortality relative to those in S. patens habitats; adult snail survivorship was similar between habitats. Because other habitats were not interchangeable with S. patens for young snails, these results suggest that Melampus is an ontogenetic specialist where young snails are habitat specialists and adult snails are habitat generalists. Temperature was significantly higher and relative humidity significantly lower in stunted S. alterniflora than in S. patens. These data suggest that thermal and desiccation stress restricted young snails to S. patens habitat, which has high stem density and a layer of thatch that protects snails from environmental stress. Other authors predict that if salt marshes in the northeast U.S. are unable to migrate landward, sea level rise will eliminate S. patens habitats. We suggest that if a salt marsh loses its S. patens habitats, it will also lose its coffee bean snails. Our results demonstrate the need to consider individual life stages when determining a species’ vulnerability to global change.

Habitat edge effects decrease litter accumulation and increase litter decomposition in coastal salt marshes

“…On July 14, 2011, a considerably hot day ([ 33 8C) with few clouds, we analyzed the microclimate of patches at each location with litter bags. Although this was a limited measure of microclimate, individual S. patens patches generally have consistent microclimates due to the decumbent growth and thick thatch layer found in S. patens (Gedan and Bertness 2010;Johnson and Williams 2017). We measured ambient air temperature and relative humidity as well as the air temperature and relative humidity in patches at the soil surface using a Traceable Humidity/Temperature Pen (Thermo Fisher Scientific, Waltham, Massachusetts).…”

“…Currently, the sizes of S. patens patches are declining, mainly from nutrient eutrophication and relative sea level rise which induce competitive interactions that favor S. alterniflora over S. patens (Warren and Niering 1993;Donnelly and Bertness 2001;Carey et al 2017). The transition to pure S. alterniflora monocultures is of critical importance for the animal and microbial communities endemic to S. patens, as well as the species that need both grasses in different life cycle stages (Raghukumar 2017;Johnson and Williams 2017;Wimp et al 2019). Further, S. patens has considerably lower decomposition rates than S. alterniflora, leading to an accumulation of litter (Frasco and Good 1982;Foote and Reynolds 1997;Elsey-Quirk et al 2011).…”

“…Further, S. patens has considerably lower decomposition rates than S. alterniflora, leading to an accumulation of litter (Frasco and Good 1982;Foote and Reynolds 1997;Elsey-Quirk et al 2011). The increased accumulation of litter in S. patens patches leads to a thick litter layer just above the soil that reduces salinity, increases humidity, reduces summer temperatures, and provides refuge from flooding for multiple animal species (DiQuinzio et al 2002;Gedan and Bertness 2010;Johnson and Williams 2017). Considering S. patens is more likely to experience changes in elevation, salinity, tidal inundation, and biological communities at the edge of its habitat (Denno 1983;Bertness 1991;Johnson and Williams 2017;Wimp et al 2019), one may expect S. patens litter decomposition to be altered by smaller habitat areas and increased habitat edge exposure.…”

Saltmarsh plants, but not fertilizer, facilitate invertebrate recolonization after an oil spill

“…In terms of structuring communities, marsh plants are strong facilitator species that enhance the survival of co‐occurring species by ameliorating physical stress (Bertness and Hacker , Bruno et al. , Whitcraft and Levin ; Johnson and Williams ). If facilitation is an important process in community regulation, then it may be an important mechanism in promoting animal colonization (Silliman et al.…”

“…Saltmarsh plants such as Spartina alterniflora are foundation species that regulate marsh structure (Kirwan and Megonigal 2013), ecosystem processes (Baustian et al 2012), and community structure (Johnson et al 2007). In terms of structuring communities, marsh plants are strong facilitator species that enhance the survival of co-occurring species by ameliorating physical stress (Bertness and Hacker 1994, Bruno et al 2003, Whitcraft and Levin 2007Johnson and Williams 2017). If facilitation is an important process in community regulation, then it may be an important mechanism in promoting animal colonization (Silliman et al 2015).…”