Salt Marsh Ecosystems Need Quality Biodiversity, Not Just Quantity
December 7, 2013

Biodiversity Quality Is Key To Preserving Salt Marsh Ecosystems

redOrbit Staff & Wire Reports - Your Universe Online

Scientists have believed for years that preserving more species, no matter which ones, was a key component to enhancing how well an ecosystem performs. A research team from Duke University and the University of Massachusetts at Boston investigating biodiversity loss in a salt marsh says that this isn't quite right.

Their findings, published in Proceedings of the National Academy of Sciences, show that it's not just the total number of species preserved that matters, it's the number of key species. The authors suggest that if humans want to reap the benefits of the full range of functions that salt marshes and other coastal ecosystems provide, the right mix of species needs to be preserved.

"Having a group of distantly related species, representing markedly different ecologies and biology, is as important, or more important, than just having more species in general," said Brian R. Silliman, Rachel Carson associate professor of marine conservation biology at Duke's Nicholas School of the Environment.

"It's quality, not just quantity," said Marc J. S. Hensel, a Ph.D. student at the University of Massachusetts at Boston. "We need to preserve a wide variety of species."

A long list of ecological services are performed by salt marshes. For example, they buffer coastal erosion; filter runoff; reduce the risk of flooding; provide habitat for juvenile fish, crabs and shrimp; and store excess carbon, keeping it from re-entering Earth's atmosphere.

Silliman and Hensel conducted a tightly controlled eight-month field experiment in a salt marsh on Sapelo Island, Georgia, to understand how the loss of key species affects these functions. The researchers set up eight different experimental treatments. Each treatment had a different mix of three of the marsh's most abundant "consumer" species: purple marsh crabs; marsh periwinkle snails; and fungus. Initially, all three species were present, to mirror the natural conditions of the marsh. The researchers then began to sequentially remove species—first one, then two, then all three—to simulate extinctions.

They measured the effects of each species mix on three important salt marsh functions throughout the experiment: overall grass growth (productivity); the rate of dead plant removal (decomposition); and how fast tidal or storm surge water percolated through the marsh (filtration).

Because each species is effective at performing one or two functions, the effects of species removal on individual salt marsh functions varied considerably. They found, however, that when all three key species were present, the average rate of all functions - a measure of overall ecosystem health - rose simultaneously.

"Our study provides a rare, real-world example that the loss of key species can have profound impacts on the overall performance of an ecosystem," Silliman said. "It suggests that the ability of nature to perform well at multiple levels may depend not just on the overall number of species present, but on having many distantly related species, each of which performs a particular task that keeps an ecosystem healthy and allows it to provide the multiple benefits humans value."

"If we had only been looking at three different species of similarly functioning crabs, or only one marsh function, we would have missed that, and erroneously predicted that only one consumer species is needed to maintain high system performance," he said.