Snail and bivalve recruitment

Overivew To begin to assess the relative effects of habitat quality, larval transport, and predation in governing invertebrate densities across the off-shore, on-shore gradient, the Silliman lab is running 2 major studies in the summer and fall of 2008, 2009, and 2010 at 24 replicate sites of barrier island marshes (n=8, both tall and short zone), 8 mid-estuary marshes (n=8, both tall and short zone) and mainland marshes (n=8, both tall and short zone). 1) We are assessing recruitment rates using integrative methods and adult densities using quadrats for all major marsh invertebrates at all sites. For species-specific integrative recruitment, we use the following methods: I) for ribbed mussels and oysters, we use 6"x3" pvc pipes sealed with ¼" vexar at the ends and filled with 5 live oysters or ribbed mussels and bricks fastened just above the marsh surface; II) for barnacles, we use 2' tall ¾" PVC pipes and bricks fastened just above the marsh surface, and III) for periwinkle snails, in the first week of November we count recruits occurring in dead, furled leaves of Spartina in permanent 25x 25 cm plots that have been cleared of all recruits in August of each year. Each of these approaches is replicated 4 times at each site in both the short and tall Spartina zones. In total, there are 256 sampling devices deployed for each species. All of these methods build upon the work that D. Bishop began when monitoring invertebrate recruitment at the primary GCE sites. 2) To asses if some aspect of marsh condition excludes invertebrates from mainland marshes, we are transplanting tethered periwinkle snails, ribbed mussels and mud snails (n=4 cages per site with 5 animals of each species in each paired uncaged and caged area) across this gradient both with and without predator/competitor access (2009-2010). To assess the relative roles of recruitment and predation in controlling invertebrate densities, we will use a multiple regression model to compare separate and combined effects of recruitment rates and predation rates as assessed by tethering all invertebrates at all sites (n= 40 animals/ site/ zone) (2009-2010). When completed, this work will be integrated with work by the Pennings laboratory on grasshopper distributions to compare drivers of distribution between marine vs. terrestrial invertebrate taxa. In 2008, we surveyed these sites and found a strong impact of both site and zone on density of adult periwinkle snails and recruits of periwinkle snails and barnacles. The pattern was consistent across both species and life stages. Densities were highest in the barrier-island marshes followed by mid-estuary and then mainland marshes (P < 0.01, all cases). There was also a strong main effect of zone (P < 0.01, all cases), but no interaction between zone and site. In the case of adult snails, density was always higher in the short zone, while the opposite pattern was true for barnacle and snail recruits-recruits were always most abundant in the tall zone. These patterns suggest a strong recruitment shadow effect, where larval supply to marshes is highest closest to the ocean source and trails off toward the mainland. Combined with our tethering experiments (ongoing) which suggest strong predation in the tall zones at all sites, but lower predation rates in the short zone, these data suggest that high densities of marsh invertebrates occur in areas where recruitment is high enough overcome losses due to high predation, i.e., short Spartina zones on barrier islands.