To Be or Not to Be

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Picture of recruitment sampling at the GCE-LTER site. PVC pipes are used to sample barnacles, caged canisters filled with dead oyster shells (above right) to samples crabs and oysters, and caged, potted Spartina plants to sample snails. Inset: some of the dominant organisms that are monitored: littorine snails, crabs and acorn barnacles.
Pictures provided by James Nifong, Christine Holdredge and Brian Silliman
  • Why are there more organisms in some parts of the landscape than others?
  • Are there genetic differences between populations living in one area as opposed to another?
  • Is it because adults do better in some areas than others?
  • Or, in marine systems, is it because there are more larvae settling from the plankton in one place than another?

In the GCE-LTER study site on the Georgia coast, researchers have documented that the population densities of periwinkle snails, barnacles and oysters are all highest in marshes located on barrier islands close to the ocean, and decrease considerably in marshes further inland. In a genetic study investigating variation across this gradient, examination of DNA sequence nucleotide diversity (a measure of the amount of polymorphism harbored in a DNA sequence data set) indicated that populations of marine invertebrates closer to the ocean were also more than twice as diverse as populations closer to the mainland. In addition, the study showed that genetic diversity was positively correlated with species diversity across these sites. This pattern could emerge if the same processes that control species diversity also control genetic diversity within a species. Such a relationship has been predicted but to our knowledge this is the first case to be shown in a marine environment.

The genetic studies are consistent with the idea that more larvae are making it to the sites closer to the ocean. If so, the water that bathes the more inland marshes might be depleted of larvae, and therefore these marshes would end up with lower densities of marine invertebrates. Direct measurements of the recruitment rates of juvenile periwinkle snails, barnacles and oysters confirmed this conclusion, with many more individuals recruiting to marsh sites closer to the ocean than the mainland.

However, the story is not that simple. Studies of predators show that they too have an effect on the populations of marine invertebrates found in different areas. For example, despite high levels of recruitment on barrier island marshes, predation is so intense along creek banks that high densities of snails can never establish in these parts of the marsh. Instead, adult snails are found in the higher areas of the marsh, further away from the creeks, where predation is not as intense.

By understanding the interplay between recruitment and predation across the landscape, GCE scientists are beginning to unravel the mystery of why marine invertebrates are rare in some marshes and at very high densities in others. This type of information will help us understand variation in abundance of key fishery species, such as oysters, which will be potentially useful for deciding how best to place marine reserves to maximize population increases of these important species.

Comparison of the density of adult snails (top), the rate of recruitment of snail larvae to the adult population (middle) and the predation rate (bottom) for a gradient of marsh sites from the mainland out to the barrier islands. Grey bars are for measurements in the higher elevations of the marsh (short Spartina zone) where predation rates are low; black bars are for measurements at the creekbank (tall Spartina zone) where predation rates are high. Bars represent means and error bars +/- one standard deviation.
Data from Silliman, B.R., C. Holdredge, J. NIfong, M. Hensel, and S. von Monfrans. In prep. The Supply-side of salt marsh ecology: predicting large-scale population and community level patterns. Target journal: Ecology.
For further reading: 
Díaz-Ferguson, E., J. Robinson, B. Silliman, and J. P. Wares. 2009. Comparative Phylogeography of East Coast American Salt Marsh Communities. Estuaries & Coasts, DOI 10.1007/s12237-009-9220-6.
Robinson, J., E. Díaz-Ferguson, M. Poelchau, D. Bishop, S. Pennings, and J. P. Wares. 2009. Spatial variation of genetic and biotic diversity in the salt marsh ecosystem. Estuaries & Coasts, DOI 10.1007/s12237-009-9188-2.ssemblage. ISME Journal. (in press)
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