Bottom-Up or Top-Down?

Prior to reading Silliman and Bertness’ paper I thought that most ecosystems were bottom-up systems and that physical and chemical factors had the strongest influence on their organisms. After all, the nutrients an organism is exposed to, the environment it lives in, and the food it consumes all greatly affect how it reproduces, survives, and interacts with other species. I did not think that the consumers would have that big of an effect on the structure or function of an ecosystem. However, Silliman and Bertness’ paper proved the importance and influence of top-down controls on ecosystems and primary production, specifically in salt marshes, which are the most productive systems in the world.

Silliman and Bertness performed three different experiments to test out the effects of Littoraria and a trophic cascade on cordgrass production in salt marshes. Their experiment concluded that Littoraria had a strong top-down control of cordgrass production and that simple trophic cascades regulate the structure of southeast marshes. They also concluded that by eliminating predators, salt marshes could be converted into mudflats by plant-grazing snails.

Scientists and environmentalists have said for years that overfishing is a serious problem, but I did not fully grasp its gravity until I read this paper. By fishing for the bigger fish in the sea, which happen to be the consumers of their own ecosystem, we are in effect altering the food web, primary production system, and interactions amongst the other organisms. Not only do we affect the organisms, but the structure and physical traits of their environment as well. Consumers have such a strong influence on the structure of an ecosystem and by depleting the consumer population we can alter the whole ecosystem.

Silliman and Bertness’ findings made me realize once again that people have such a strong influence on the environment and that we should use this knowledge to better protect the earth. Although many people enjoy fishing for recreational or economic reasons, we should also be aware of how this simple activity can influence the structure and function of other ecosystems and its habitants. If salt marshes do turn into mudflats, it will not only harm the other fish and plants in the ecosystem, but humans as well. In a sense we have a symbiotic relationship with the environment and in order for us to be able to benefit from it, we have to do our part and take care of it as well.