Variation in methylglucosinolate and insect damage toCleome serrulata (Capparaceae) along a natural soil moisture gradient.

We tested the hypothesis that plant loss to insects, and thus the relative fitness of an annual, was inversely related to spatial variation in the concentration of its characteristic secondary compound, methylglucosinolate, a mustard oil precursor. We found that methylyglucosinolate concentrations decreased significantly and linearly from the dry to the wet end along short soil moisture gradients in dry shortgrass prairie. Both leaf damage and capsule predation increased from the dry to the wet end. Thus, the glucosinolate appears to function defensively. Plant growth and flower production were favored at the wet end of the gradient; yet plants in the wet portion of the gradient were also more vulnerable to significant insect damage. The net result was that seed production by individual plants after predation was actually greater in the drier, harsher half of the gradient. The outcome strongly suggests that environmentally related variation in secondary compound concentration along environmental gradients can mediate and influence host-plant population abundance and distribution.