OPTIMAL DISTRIBUTION OF HERBIVORY AND LOCALIZED COMPENSATORY RESPONSES WITHIN A PLANT

Because the impact of induced resistance on herbivores is often too sm all to have a considerable impact on herbivore populations, it has bee n suggested that the function of defenses is not necessarily to affect herbivore densities per se, but to spread the damage evenly throughou t a plant. Some observations suggest that evenly-distributed herbivory results in a smaller decrease in growth and reproduction than the sam e level of herbivory concentrated on only one part of the canopy. One possible reason for this is that plant parts are able to compensate fo r small amounts of local damage spread all over the plant but not for larger concentrated damage of the same extent because of the of resour ce distribution patterns in a plant. Models were constructed to analyz e how to optimally distribute damage among modules so that the total p erformance (growth, subsequent reproduction) of a plant, measured as t he sum of the module performances, is maximized. According to the mode ls, spreading damage is profitable when the performance of all or most modules decreases slowly with small damage levels and then faster as damage increases. In addition, the pattern of resource movement betwee n modules must be tolerant to damage. It is proposed a hypothesis that the resource distribution system is often tolerant to at least small or moderate total damage levels, since there are several alternative p athways of movement for assimilates and water through the plant's stru cture. It has been suggested that defense and compensatory growth are alternative strategies for a plant in the struggle against herbivores. The present models suggest that they can also work together to increa se fitness under herbivore pressure.