Optimal distribution of herbivory and localized compensatory responses within a plant

Abstract

Because the impact of induced resistance on herbivores is often too small to have a considerable impact on herbivore populations, it has been suggested that the function of defenses is not necessarily to affect herbivore densities per se, but to spread the damage evenly throughout a plant. Some observations suggest that evenly-distributed herbivory results in a smaller decrease in growht and reproduction than the same level of herbivory concentrated on only one part of the canopy. One possible reason for this is that plant parts are able to compensate for small amounts of local damage spread all over the plant but not for larger concentrated damage of the same extent because of the of resource distribution patterns in a plant.

Models were constructed to analyze how to optimally distribute damage among modules so that the total performance (growth, subsequent reproduction) of a plant, measured as the sum of the module performances, is maximized. According to the models, 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 between 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 pathways of movement for assimilates and water through the plant's structure.

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 increase fitness under herbivore pressure.