Evol Ecol Res 6: 709-726 (2004)     Full PDF if your library subscribes.

Variation in chemical defences of plants may improve the effectiveness of defence

Angela L. Shelton*

Department of Environmental Studies, University of California, Santa Cruz, CA 95064, USA

Address all correspondence to Angela L. Shelton, School of Computational Science and Information Technology, Florida State University, Tallahassee, FL 32306-4120, USA.
e-mail: angies@csit.fsu.edu


Chemical defences of plants exhibit great variation, but their precise patterns and effects on herbivores have not been examined, especially at small scales. Recent work has demonstrated very high spatial variation at small scales within leaves. Empirical tests of the effects of variation on herbivore fitness and behaviour are therefore particularly difficult. Here I present a series of dynamic-programming models of the effects of variation in chemical defences (glucosinolates) at several scales. All models include the same mean concentration of glucosinolates. They differ only in the presence or absence of variation at each of five scales: among plants, within plants due to induction of defences in response to herbivore feeding, among leaves on a plant, among parts of a leaf, and within leaf parts. I used empirical data from Raphanus sativus (Brassicaceae) and the generalist caterpillar Trichoplusia ni (Lepidoptera: Noctuidae) to parameterize the models. Model caterpillars behave to maximize their size at pupation by choosing each day either to feed in place or to move within a leaf, between leaves or between plants. Variation among plants and within plants due to induction dramatically decreased pupation rates. The smallest scale of variation – within leaf parts – also reduced fitness by slowing growth and delaying pupation. However, small amounts of this stochastic small-scale variation slightly increased pupation rates. The results suggest that variation in plant chemical defences has important effects on fitness and behaviour of herbivores.

Keywords: dynamic programming, glucosinolates, Raphanus sativus, variation.

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