September 1, 2009
Written by Kelli L. Edwards
The salt marsh community is regulated by a trophic cascade. Although some communities are typically structured in a pyramid, some consist of an inverted pyramid. These inverted pyramids are most likely to be associated with pristine aquatic communities, where there are much larger animals and fewer trophic levels. Despite the differences, each trophic cascade consists of the primary producers, primary consumers, secondary consumers, and so on.
Each one of these trophic cascades is regulated by either top-down regulation or bottom-up regulation or both. Top-down control is determined by how many predators or consumers are present, while bottom-up control is determined by how many resources are present (Silliman and Bertness, 2002). Plant biomass is greatly controlled by predators and their feeding ativities. Beyond other environmental factors such as nutrient availitiblity and salinity, top-down control may be a key determinant of marsh grass growth.
In a 2002 paper, Silliman and Bertness investigated the effect of top-down regulation with the periwinkle snail Littoraria irrorata.
This snail feeds on a fungus which grows on the Spartina grass. With enough snails, they can devour a Spartina population. The periwinkle snail does have natural predators which help to indirectly regulate the consumption of Spartina.For our class activity we investigated the principle of top-down regulation. We are to assess the effects of density-dependent snail grazing on Spartina grass growth by maintaining constant snail densities in isolated cages.
Snails were put into cages at low (13 per meter square), medium (600 per meter square) and high (1200 per meter square) densities. Over the course of the next 2 weeks, we will monitor the growth and consumption of Spartina alterniflora.Low density:
Medium density:
High density:
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