COS 85-1
External chemical defenses in plants: Tests of abiotic and insect herbivore community consequences
Plant defenses may be physical or chemical and act directly or indirectly on herbivores. I propose and test a further dichotomy when examining chemical defenses: whether they are internal or external to the plant (e.g. on plant surfaces; volatiles constitute a distinct and separate entity). I hypothesize these ‘external chemical defenses’ (ECDs) differ ecologically from internal defenses by (1) being more prone to removal via environmental factors and (2) promoting different feeding guilds of herbivores, affecting community structure. I tested hypothesis (1) experimentally by simulating rainfall on a population of Atriplex rosea. I controlled for this water addition in an arid landscape by adding the same amount of water at the base of a second treatment group and having a third group as a real (no water) control. I tested hypothesis (2) by extracting over a thousand lepidopteran host-association records from the British Museum’s HOSTS database of plants with ECDs and closely-related or ecologically-similar non-ECD plants for community comparisons between these defense strategies.
Results/Conclusions
I found (1) plants subjected to simulated rainfall suffered significantly higher rates of chewing herbivory than plants in control and manipulation control groups. This result suggests that environmental removal of ECDs via environmental forces affects plant-insect interactions. Plants defended with ECDs (2) hosted Lepidoptera with body size distributions significantly smaller (left-shifted) compared to both close relatives and ecologically-similar species without ECDs. As lepidopteran larvae are constrained to be less tissue-selective with increasing size, this suggests selective feeding on less-defended internal tissues probably contributes to the smaller body sizes observed on plants with ECDs. These new results indicate that ECDs have complex interplay with environmental factors and contribute to differences in insect community structure. The physical location of chemical defenses has ecologically-important consequences and should be considered in future plant-herbivore interactions.