Population ecology of the snail Melampus bidentatus in changing salt marsh landscapes

Abstract

We studied the population ecology of the snail Melampus bidentatus in relation to patch composition and landscape structure across several salt marsh systems in Connecticut, USA. These marshes have changed significantly over the past 40-50 years including loss of total area, increased areas of short Spartina alterniflora, and decreased areas and fragmentation of Spartina patens. These changes are consistent with tidal inundation patterns that indicate frequent flooding of high marsh areas. Melampus bidentatus densities were highly variable, both among different salt marsh systems and locations within specific marshes, but were generally similar among short Sp. alterniflora and Sp. patens patches within locations. Densities were lowest where the marsh was regularly inundated at high tide and only remnant Sp. patens patches remained.Almost no snails were found in bare patches. Areas that had large Sp. patens patches adjacent to short Sp. alterniflora supported the highest M. bidentatus densities.Population size-structure varied significantly among patch types, with higher proportions of large individuals in short Sp. alterniflora and hummocked Sp. patens patches than in large and remnant Sp. patens patches. This was likely due to size-selective predation and/or higher snail growth rates due to better food resource conditions in short Sp. alterniflora patches. Egg mass densities and the number of eggs per egg mass were highest in short Sp. alterniflora. Our results indicate that M. bidentatus is resilient to the level and patterns of salt marsh change evident at our study sites. Indeed, snail densities were significantly higher than reported in other field studies, suggesting that increased patch areas of short Sp. alterniflora and associated environmental conditions at our study sites may provide more favorable habitats than previously when marshes were dominated by extensive Sp. patens meadows. However, there may be threshold conditions that could overwhelm the ability of M. bidentatus to maintain itself within salt marsh systems where changes in hydrology, sedimentation and other factors lead to increased numbers of bare patches and ponds and loss of short Sp. alterniflora and Sp. patens. Studies of the responses of resident and transient fauna to salt marsh change are critically needed in order to better understand the implications for salt marsh ecosystem dynamics and services.

The effect of extreme climatic events on littorinid snails in two estuarine environments, temperate (NW Spain) and tropical (NE Brazil)

“…During extreme precipitation events, salinity usually decreases in estuaries (Parada et al, 2012), but there are no specific forecasts for these environments. However, increased sea level rise with frequent flooding of high marsh areas is consistent in predictive models (Stocker, 2013) and its effect on snails can be observed without any variation in salinity, modifying primary and secondary productivity that provides resources and habitat for organisms and leads to species extinction or changes in their geographical distribution (Mcmahon, 1988;Ng et al, 2017;Zajac et al, 2017).…”

Salt marsh climate change adaptation: Using runnels to adapt to accelerating sea level rise within a drowning New England salt marsh

“…These results may suggest that runnel effects, in combination with higher elevations, can potentially help prevent S. patens displacement by S. alterniflora, which would have positive implications on the vulnerable bird species, Ammodramus caudacutus or saltmarsh sparrow, that nests within S. patens habitat (Bayard & Elphick 2011). Without intervention, the impacts of accelerated SLR and excess tidal inundation may result in the continued decline in high marsh plants, suggested in Raposa et al (2015), which will be detrimental to the saltmarsh sparrow and other bird and invertebrate species that rely on high marsh habitat for survival (Bayard & Elphick 2011; Zajac et al 2017).…”

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

“…Food limitation is an alternative hypothesis to explain why snails in S. patens were smaller than those in stunted S. alterniflora (Zajac, Kelly, Perry, & Espinosa, ). While Melampus is considered a detritivore (Thompson, ) it also consumes benthic algae, a higher quality food than plant detritus (Pascal & Fleeger, ).…”

“…This emphasizes the importance of the thatch layer of S. patens habitats in protecting young snails from solar radiation and low humidity (Price, 1980). Food limitation is an alternative hypothesis to explain why snails in S. patens were smaller than those in stunted S. alterniflora (Zajac, Kelly, Perry, & Espinosa, 2017). While Melampus is considered a detritivore (Thompson, 1984) it also consumes benthic algae, a higher quality food than plant detritus (Pascal & Fleeger, 2013).…”