In Narragansett Bay they ran several tests during the summer at sites where die-off ranged from less than 5 percent to 98 percent. They determined the extent and progress of marsh death by examining aerial images of the sites from 1997, 2003, 2008, and 2012.
To gather data about whether tides were eroding the marshes away, the researchers made chalk blocks and placed them at each site so they could watch how quickly they dissolved. To determine whether local growing conditions were poor for the grasses, they planted healthy grass in each site and then protected it from all herbivores, including the crabs. To assess whether any site had too much nitrogen, they took leaves of the grasses at each site back to the lab, ground them up and analyzed their chemistry.
In the second week in July, they measured herbivory at each site by walking a 20-meter line at each site and then stopping every two meters to measure signs of crab damage on 100 cordgrass stalks. Later in July they measured Sesarma populations (based on how many they could trap). In August they tethered crabs to make them more vulnerable to predators and measured how much predation there was.
They even measured how hard the marsh soil was at each site.
When fall came, they analyzed all the data. The number crunching revealed that differences in herbivory at each site explained 73 percent of the variation in die-off from at each site.
The only other significant factor was the hardness of the soil. Why would that matter? "Substrate hardness influences crab herbivory by limiting crab burrowing in hard and soft substrates, leading to a peak in herbivory in medium hardness substrates where burrows can be easily constructed and maintained," they wrote in PLoS ONE.
Experiment on the Cape
Meanwhile on Cape C
|Contact: David Orenstein|