Effects Of Spatial Distribution Patterns On Plant Associational Defence Against Herbivory

Studies have shown that consumption of a focal plant by herbivores depends, not only on its own defence traits, but also on the characteristics of the neighboring plant. Plant associational defence has been considered at different spatial scales; nevertheless, the effect of spatial distribution pattern received less attention. We conduct a manipulative experiment to determine whether and how spatial distribution pattern of both preferred and less preferred neighboring plants affect associational defence. We used Lathyrus quinqenervious as a focal plant, Chloris virgata as a high preferred neighboring plant, and Kalimeris integrifolia as a low preferred neighboring plant. Sheep encountered two patches varying in spatial pattern (dispersed or clumped) and in patch quality (good-or bad-patch, where focal plant is neighbored to high or low preferred species respectively). Results showed that distribution pattern and patch quality affected intake of focal plant. Dispersed distribution pattern significantly reduced intake of focal plant as opposed to clumped pattern. When the two patches (good or bad patch) have different spatial pattern (good patch dispersed and bad patch clumped or vice versa) sheep foraging success was greatly reduced in dispersed spatial pattern irrespective of the patch quality. However, if both patches were dispersed, intake was reduced in bad patch than in good patch, corresponding to associational defence and susceptibility in bad and good patches respectively. On the other hand, sheep consumed focal plant in bad patch as much as in good patch when both patches were clumped, which indicated no associational defence between neighboring plants. Overall, sheep selectivity was lower in treatments with low contrast-both within and between patches-in terms of spatial pattern and patch quality leading to associational defence. We concluded that distribution pattern is as important as patch quality in predicting vulnerability of focal plant to grazing by generalist herbivores.