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The Determinants of Food Chain Lengths
S. L. Pimm and R. L. Kitching
Vol. 50, No. 3, Trophic Exploitation, Community Structure and Dynamics of Cyclic and Non-Cyclic Populations. Proceedings of a Symposium Held 25-27 May, 1985, at Hällnäs, Sweden (Nov., 1987), pp. 302-307
Stable URL: http://www.jstor.org/stable/3565490
Page Count: 6
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What little natural variation exists in food chain length is most likely explained by one of two hypotheses: that the number of trophic levels present in a community reflects the amount of energy entering the food web or that species atop long food chains cannot recover sufficiently quickly following disturbances. We first review natural variation in the food chains of phytotelmata - water-filled plant-bodies. Such comparative studies suggest that the role of disturbances may, indeed, be the major factor in determining their food chains. We experimented with a three trophic level system in south east Queensland, Australia. Placing water-filled pots on a rainforest floor creates communities similar to those in natural tree holes. We manipulated energy input by adding known quantities of leaf litter. The act of setting up the pots creates a small-scale disturbance by setting the densities initially to zero, though the organisms are present in the tree holes of the surrounding forest. Of the two predators, one, tadpoles of a small frog, actually avoids the most productive systems. The other predator, larvae of a chironomid, show a slight increase in numbers with the level of energy input. But most importantly, it was slow to colonize - waiting for its prey (species of saprophagous chironomids) to attain approximately constant numbers and size. We suggest that these data indicate that frequent disturbances would remove these predators from the system resulting in the two trophic level systems found in tree holes elsewhere.
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