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Experimental Test in Lowland Tropical Forest Shows Top-Down Effects through Four Trophic Levels

D. K. Letourneau and L. A. Dyer
Ecology
Vol. 79, No. 5 (Jul., 1998), pp. 1678-1687
Published by: Wiley
DOI: 10.2307/176787
Stable URL: http://www.jstor.org/stable/176787
Page Count: 10
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Experimental Test in Lowland Tropical Forest Shows Top-Down Effects through Four Trophic Levels
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Abstract

Approximately 50% of the variation in productivity in lakes is hypothesized to depend upon the cascading effects of top predators. A relative paucity of evidence for such trophic cascades in terrestrial systems has prompted proposals that resource availability (donor control) is more critical than are top-down forces in structuring terrestrial communities, and that trophic cascades with top-down dominance will be restricted to systems of low species diversity. To test the effects of a fourth trophic level on successive lower trophic levels under different levels of plant resource availability, we used Piper ant-plants and their associated arthropods as a model system in lowland, tropical rain forest. This interacting web of four trophic levels is composed of small trees, various herbivores, predaceous ants, and specialist clerid beetles as predators of ants. Three-hundred-sixty Piper cenocladum cuttings were randomly assigned to 36 plots with a factorial design: predator treatment (three levels), light treatment (two levels), and soil type (two levels). We monitored indicators of ant-colony size (percentage petioles occupied per tree), herbivore loads (leaf area loss), and tree biomass (total leaf area per tree) for 18 mo. When the top predator Tarsobaenus letourneauae was added experimentally to the three-trophic-level ant-plant system, the average abundance of Pheidole bicornis ants was reduced fivefold, average herbivory to Piper cenocladum leaves was increased nearly threefold, and tree leaf area was reduced by nearly half. Direct effects of predatory beetles on ants were more pronounced and more rapid than were indirect effects accumulating to the second and first trophic levels. Bottom-up effects of light and soil quality tended to be mitigated by these top-down cascades. Neither potential leaf-area accumulation by trees nor actual leaf area (left after herbivory) showed a direct response to relatively high or low availability of soil nutrients or light at different sites.

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