Many plants engage in protective mutualisms, offering resources such as extrafloral nectar and shelters to predatory arthropods in exchange for protection against herbivores. Recent work indicates that sticky plants catch small insects and provide this carrion to predators who defend the plants against herbivores. In this study, we investigated whether wild tobacco, Nicotiana attenuata, fits this sticky plant defense syndrome that has been described for other sticky plants. We developed a bioassay for stickiness involving the number of flies that adhered to flowers, the stickiest tissues. In surveys conducted over three field seasons at four sites, we found that the number of carrion that adhered to a plant was positively correlated with the number of predators that we observed foraging over its surfaces. The number of predators was positively correlated with the number of seed capsules that the plant produced, a measure of lifetime female reproductive success. Structural equation modeling indicated strong support for the causal path linking carrion numbers to predator numbers to capsule production. We investigated whether stickiness was an inducible trait and examined two potential cues. We found that experimental clipping of rosette leaves induced greater stickiness, although clipping of neighboring sagebrush leaves did not. Damage to leaf tissue is likely to be a more reliable predictor of risk than is damage to a neighboring plant. The sticky plant defense syndrome is a widespread protective mutualism; its strength and ecological relevance can adjust as risk of herbivory changes.

Grasses are major agricultural products worldwide and they are critical to ecosystem function in many terrestrial habitats. Despite their global importance, we know relatively little about their defenses against herbivory. Grasses tend to be tolerant of leaf loss because their valuable meristems are located underground, out of reach for above ground herbivores. Many grasses have unidirectional leaf hairs, prickles, and spines that make moving up the leaf blade easy, but make moving down, toward the meristem, difficult. We tested the hypothesis that unidirectional grass hairs direct small arthropod herbivores away from the meristems. In a field survey of the distribution of herbivore damage, we found that leaf tips received five times more damage than leaf bases for Avena barbata. Early-instar grasshoppers fed three times as often on leaf tops as on leaf bases of pubescent individuals in a common garden laboratory experiment. This effect was not observed for glabrous individuals where grasshoppers damaged leaf bases as often as leaf tops. A common generalist caterpillar, Heliothus virescens, was more than twice as likely to turn in the direction of the hairs, away from the meristems, when it encountered pubescent leaves of A. barbata. However, larger caterpillars of the generalist feeder Arctia virginalis showed no directional bias when they encountered pubescent leaves. In common garden experiments, selection on pubescence was weak and inconsistent over space and time. Under some circumstances, individuals of A. barbata with pubescent leaves were more likely to produce seeds than were individuals with fewer hairs. The surveys, behavioral experiments with small insects, and estimates of lifetime reproduction all support the hypothesis that unidirectional leaf hairs on A. barbata, and perhaps other grasses, serve as an unstudied defense that direct small herbivores away from the meristems.

Habitat-forming species provide refuges for a variety of associating species; these refuges may mediate interactions between species differently depending on the functional traits of the habitat-forming species. We investigated refuge provisioning by plants with different functional traits for dragonfly and damselfly (Odonata: Anisoptera and Zygoptera) nymphs emerging from water bodies to molt into their adult stage. During this period, nymphs experience high levels of predation by birds. On the shores of a small pond, plants with mechanical defenses (e.g., thorns and prickles) and high structural complexity had higher abundances of odonate exuviae than nearby plants which lacked mechanical defenses and exhibited low structural complexity. To disentangle the relative effects of these two potentially important functional traits on nymph emergence-site preference and survival, we conducted two fully crossed factorial field experiments using artificial plants. Nymphs showed a strong preference for artificial plants with high structural complexity and to a lesser extent, mechanical defenses. Both functional traits increased nymph survival but through different mechanisms. We suggest that future investigations attempt to experimentally separate the elements contributing to structural complexity to elucidate the mechanistic underpinnings of refuge provisioning.