Evol Ecol Res 2: 231-249 (2000)     Full PDF if your library subscribes.

Variable chemical defences in plants and their effects on herbivore behaviour

Angela L. Shelton

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

e-mail: angies@cats.ucsc.edu


Most plants exhibit very high chemical variability from scales of within individuals to between populations and between species. This extraordinary variation has not yet been adequately explained. One potential explanation is that variability may benefit the plants by making them a more unpredictable environment for herbivores. I explored this possibility with a dynamic state variable model. I assumed a concave benefits curve and modelled the behaviour of a herbivore on two types of plants. Each plant type had the same mean level of toxins, but one had a variable distribution of toxins while the other was completely predictable. In each time step, the herbivore either foraged or reproduced. If it foraged, it could either accept or reject the plant encountered. The model predicted that, when herbivores had intermediate energy levels, they would reject variable-toxin plants, as long as there were constant-toxin plants in the environment and the benefits curve was sufficiently concave. This emphasized the importance of herbivore choice in plant defence and demonstrated that variability can be an additional level of defence for some plants.

Keywords: chemical variation, dynamic state variable model, herbivore choice, herbivory, plant defence theory, secondary metabolites.

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