Interaction of diffuse competition and insect herbivory in limiting brittle prickly pear cactus, Opuntia fragilis (Cactaceae)

We tested the effects of above- and belowground competition and evaluated insect herbivory on the? growth of a common cactus (Opuntia fragilis) in Sandhills prairie. Our purpose was to determine the independent and joint contribution of these factors to the variation observed in cactus size and local abundance. We manipulated ambient prairie vegetation and surface water availability in a full factorial design. When Opuntia fragilis was released from competition with surrounding vegetation, it grew significantly larger in the second growing season, both in terms of number and size of new cladodes. Ramets grown within live prairie vegetation averaged no net growth. These ramets were also more frequently fed upon by the larvae of two internally feeding cactus insects. As a result, insect herbivory reinforced the competitive suppression of the cactus by grasses. Water supplementation had no significant effect, either on cactus growth or on insect herbivory. We conclude that the mechanism by which dense prairie vegetation influenced net growth of O. fragilis was bath direct, through competition for nonwater resources such as light, and indirect, through its mediation of feeding by specialized insect herbivores. Furthermore, since the direct growth response to treatment did not precede the indirect effect of habitat-mediated variation in feeding by insects, our results contradict the current expectation that indirect effects require more time to be expressed than do direct effects.