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MIT12_009S11_lec22_24

MIT12_009S11_lec22_24 - 7 Ecological organization In these...

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7 Ecological organization In these lectures we investigate some ways in which organisms assemble them- selves into an ecosystem. We address two types of questions: Geometry. How can we characterize the topological assembly—i.e., the connectivity of the components of a complex system? Physics. How do rates of resource consumption influence the assembly? We begin with some observations about the flow of energy through ecosys- tems. 7.1 Energy flow References: Morowitz [36], Cohen et al. [37] An ecosystem is a biological community and its physical environment. The community is made up of the organisms living in a particular habitat (valley, lake, island, etc). Within and across communities, one can study the flow of matter and energy. In this way the ecosystem is not only a major component of the carbon cycle, but also a mechanism for energy dissipation: 133
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Between the sun and the dissipation of heat, and between the input and output of CO 2 , lies a chemical reaction network called a food web . Food webs describe which kind of organisms eat which other kinds. The simplest food web is a food chain . At the base of the food chain are producers , or autotrophs . At the next trophic level one finds consumers , or heterotrophs , which live off of primary producers. At the next higher trophic level are the consumers of consumers, etc. Given this simple description, we can ask a simple question: How long is a typical food chain? 134
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7.1.1 Food chains Reference: Hutchinson [38] In the simplest food chain, Individuals of species S 0 are eaten by those of S 1 ; those of S 1 are eaten by those of S 2 , etc. Schematically there is a flow of energy like S 0 S 1 S 2 etc. In such an idealization, S 0 is typically a plant, S 1 an herbivore, S 2 a carnivore, S 3 a bigger carnivore, etc. Now suppose that a fraction 0 < φ < 1 of the energy that passes from S i 1 to S i is available to pass to link S i +1 . After n trophic steps, available energy = φ n . A decent guess might be that φ = 0 . 2. Then S 4 has available to it only φ 4 = (0 . 2) 4 = 0 . 16% of primary production. To get a better idea of what that might mean, suppose that each predator has twice the mass of its prey. This is not so much: the linear dimension of the predator is then only 2 1 / 3 = 1 . 26 times greater than the prey. 135
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Then species S n is 2 n times larger than S 0 , and if we can equate its mass to the energy it provides the next trophic level, the ratio of its population P n to the population P 0 of S 0 is P n mass n / size n φ n / 2 n P 0 = mass 0 / size 0 = 1 / 2 0 = 10 n ( φ = 0 . 2) . Thus if there are n + 1 = 5 trophic levels, species S 4 at the top of the food chain has a population 1/10,000 of S 0 . This is the basis of the trophic pyramid , suggesting that populations and the energy available to them exponentially decrease with each trophic level.
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