Simultaneous evolution of competitiveness and defense: induced switching
in Arabis drummondii |
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Authors: | Tessa Jones Shannon Kulseth Karl Mechtenberg Charles Jorgenson Michael Zehfus Paul Brown David H Siemens |
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Institution: | (1) Biology, Black Hills State University, 1200 University Street, Spearfish, SD 57799-9099, USA;(2) Chemistry, Black Hills State University, Spearfish, SD, USA;(3) Environmental Studies, Trinity Western University, Langley, B.C., Canada |
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Abstract: | Optimality theory for plant defense against herbivores predicts an evolutionary tradeoff between the abilities to compete
and defend. We tested this hypothesis by studying the effects of genetic variation in competitiveness on defense expression.
Two closely related and differentially competitive congeners were compared for levels of resistance, tolerance, and secondary
metabolite production. In a growth room experiment, plants of Arabis drummondii and A. holboellii were grown in the presence and absence of the common bunch grass Boutelloua gracilis, the specialist herbivore Plutella xylostella, and generalist herbivore Trichoplusia ni. Tolerance to competition, measured as growth next to the grass relative to controls in the absence of grass, was greatest
for A. drummondii, the species that occurred in communities with higher densities of inter-specific neighbors. Measures of defense (resistance
to herbivores, tolerance to damage, and concentrations of glucosinolates) varied inconsistently between the Arabis, species, depending on type of herbivore, competition level, and type of defense. The better competitor A. drummondii was more resistant to specialist herbivores, as in the field, and exhibited greater herbivore- and competition-induced changes
in glucosinolate profiles. Further, when plants of A. drummondii were fed upon in competitive environments, the induced glucosinolate response was reduced while tolerance levels increased
in an apparent switching of induced strategies. We suggest that competitiveness and defense responses are sometimes positively
correlated because some defensive traits also function as competitive traits. A competitive function for defenses may also
explain why defenses were affected by competition. Alternatively, since the induced response did not increase estimates of
total glucosinolate content significantly, minimal defense costs might also allow the simultaneous evolution of competitiveness
and defense. Finally, when faced with both herbivory and competition, some competitive species, such as A. drummondii, may switch to growth-based rather than toxin-based strategies as recent theoretical models predict. |
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