PARENT SESSION
Oral Session #66: Plant-Animal Interactions: Webs and herbivory.
Presiding: J.N. Holland
Wednesday, August 7. 1:00 PM to 4:45 PM. Apache Meeting Room, TCC.
Experimental test of the Defense-Stress Benefit hypothesis.
Siemens, David*,1, Maggiulli, Nicole1, Roy, Bitty1, 1 University of Oregon, Eugene
ABSTRACT- It is generally assumed that plants have evolved defenses against insect herbivores. However, damaged plants and herbivores are common. One explanation for lower than expected defense levels is that there exist costs associated with the evolution of high defense levels. Although some studies indicate that defense costs exist, several do not. Proponents of costs have suggested that costs may increase and become more readily detectable in stressful environments where resources are limited. This stressful environment hypothesis for defense costs has not been supported in our recent studies with Brassica rapa or Arabis perennans grown under competitive stress. We have thus proposed the defense-stress benefit hypothesis (DSB), which states that defenses may also function in competitiveness to reduce net defense costs under competition. To test the DSB hypothesis we have begun to study populations and species of Arabis that have evolved to be differentially competitive. Arabis perennans plants from the cinder fields of Sunset Crater in Northern Arizona, where understory competitors are absent, had lower relative growth against the bunch grass Bouteloua gracilis than plants from the Lake Mary population where potential competitors are numerous. We compared these populations for costs of resistance and tolerance under competition in a greenhouse experiment and found that as predicted by DSB costs of resistance were only evident under competition for the poor competitors. However, this was not found to be the case for tolerance and we suggest that because the cinder fields represent a stressful abiotic environment that general stress tolerance benefits plants under competition and thus outweighs costs of tolerance to damage. Although cost/benefit analysis is currently the best framework we have to study the evolution of defenses, little is known about the dynamics of costs.
KEY WORDS: cost-benefit analysis, plant defense and competition, resistance and allelopathy, tolerance to herbivory and stress