Lecture 22

Lecture 22 - Trophic Levels and Food Chains Eutrophication...

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Trophic Levels and Food Chains Eutrophication and other impacts of human activity have tended to simplify complex ecosystems. This results from the removal of ecological niches, reducing the numbers of species that can survive there. To understand complex ecosystems, we must begin by understanding energy flow. This simple picture shows a grassland ecosystem food chain, in which the primary producers and three levels of consumers occupy different trophic levels. Because about 90% of the original solar energy put into the system is lost at each level, the biomass of organisms at each level is progressively reduced. Note that eutrophication leads to an overproduction of certain primary producers such as blue-green algae, which are extremely good at utilizing nitrogen. These algae tend not to be eaten by primary consumers such as protozoa. They reduce the light in the water, making it difficult for other primary producers to grow. This has a “ripple effect” throughout the ecosystem, lowering the abundance of secondary and tertiary consumers. Food Webs
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Predator-prey interactions can be represented in the form of a food web: You can see that a simplification of the number of plant species will also tend to simplify the food web. Note that this representation of a food web, complex as it seems, does not include pathogens and parasites. Contaminants of the Food Web In the US, many sources of severe contamination of food webs have been reduced since the Clean Water Act of 1977. But other trace contaminants are an emerging worry. Small amounts of primary messenger molecules such as hormones can trigger large changes in their target cells, and many compounds found in cosmetics and household cleaners can mimic hormone action. In Boulder Creek in Colorado, small fish called suckers have a 1:1 sex ratio upstream of a wastewater treatment plant. Below the plant, the sex ratio has shifted to 5 females to every male, and 10% of the fish are abnormal hermaphrodites. The multiplier effect of the trace contaminants mimics the multiplier effect of hormones and other primary messengers. Such influences of trace contaminants may contribute to the growing number of disturbances in our own reproduction. PCBs and other dangerous chemicals accumulate in the Arctic
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This note was uploaded on 07/13/2009 for the course BILD BILD3 taught by Professor Henterheather during the Spring '08 term at UCSD.

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Lecture 22 - Trophic Levels and Food Chains Eutrophication...

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