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© 2017 by the Ecological Society of America Climate is widely recognized as an important factor that affects temporal and spatial patterns of occurrence and abundance of herbivorous insects, although the ecological mechanisms responsible are poorly understood. We found that precipitation and standing water were positively correlated with locations and years of high abundance of caterpillars of the ranchman's tiger moth, Platyprepia virginalis. We analyzed 30 years of survey data and found that the number of large rainfall events was a better predictor of caterpillar abundance than total annual accumulation. We considered three ecological mechanisms that could drive this relationship and conducted observations and manipulative experiments to evaluate these mechanisms. (1) Rainfall facilitates more plant growth, although we found no evidence that increased food quality or quantity was causing the positive association between precipitation and caterpillar abundance. (2) Large rainfall events cause predatory ground-nesting ants to be less abundant and we found that the number of ants that recruited to local sites was negatively associated with survival and abundance of caterpillars. (3) We found that litter from wet sites provided a refuge from ant predation; litter from wet sites was not beneficial to caterpillars in the absence of ants. Both abiotic factors (precipitation) and biotic factors (predatory ants) affected the temporal and spatial abundance of caterpillars directly and interactively. Climate models predict that rainfall will become more variable, suggesting that populations of this caterpillar may also become more variable in the future.

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 equationmodeling 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.

Sand entrapment on plant surfaces, termed psammophory or sand armor, is a phylogenetically and geographically widespread trait. The functional significance of this phenomenon has been poorly investigated. Sand and soil are nonnutritive and difficult for herbivores to process, as well as visually identical to the background. We experimentally investigated whether this sand coating physically protected the plant from herbivores or increased crypsis (e.g., decreased apparency to herbivores). We tested the former hypothesis by removing entrapped sand from stems, petioles, and leaves of the sand verbena Abronia latifolia and by supplementing natural sand levels in the honeyscented pincushion plant Navarretia mellita. Consistent with a physical defensive function, leaves with sand present or supplemented suffered less chewing herbivory than those with sand removed or left as is. To test a possible crypsis effect, we coated some sand verbena stems with green sand, matching the stem color, as well as others with brown sand to match the background color. Both suffered less chewing herbivory than controls with no sand and herbivory did not significantly differ between the colors, suggesting crypsis was not the driving resistance mechanism. Strong tests of plant apparency are rare; this experimental approach may be possible in other systems and represents one of few manipulative tests of this long-standing hypothesis.

© 2018 by the Ecological Society of America Climate change can have strong effects on species interactions and community structure. Temperature-dependent effects on predator–prey interactions are a major mechanism through which these effects occur. To understand the net effects of predator attack rates and dynamic windows of prey vulnerability, we examined the impacts of temperature on the interaction of a caterpillar (Arctia virginalis) and its ant predator (Formica lasioides). We conducted field experiments to examine attack rates on caterpillars relative to temperature, ant abundance, and body size, and laboratory experiments to determine the effects of temperature on caterpillar growth. We modeled temperature-dependent survival based on the integrated effects of temperature-dependent growth and temperature- and size-dependent predation. Attack rates on caterpillars increased with warming and ant recruitment, but decreased with caterpillar size. Caterpillar growth rates increased with temperature, narrowing the window of vulnerability. The model predicted that net caterpillar survival would decrease with temperature, suggesting that A. virginalis populations could be depressed with future climate warming. Theoretical work suggests that the net outcome of predator–prey interactions with increasing temperature depends on the respective responses of interacting species in terms of velocity across space, whereas the present study suggests the importance of effects of temperature on prey window of vulnerability, or “velocity” across time.

Recently plant biologists have documented that plants, like animals, engage in many activities that can be considered as behaviors, although plant biologists currently lack a conceptual framework to understand these processes. Borrowing the well-established framework developed by psychologists, we propose that plant behaviors can be constructively modeled by identifying four distinct components: (1) a cue or stimulus that provides information, (2) a judgment whereby the plant perceives and processes this informative cue, (3) a decision whereby the plant chooses among several options based on their relative costs and benefits, and (4) action. Judgment for plants can be determined empirically by monitoring signaling associated with electrical, calcium, or hormonal fluxes. Decision-making can be evaluated empirically by monitoring gene expression or differential allocation of resources. We provide examples of the utility of this judgment and decision-making framework by considering cases in which plants either successfully or unsuccessfully induced resistance against attacking herbivores. Separating judgment from decision-making suggests new analytical paradigms (i.e., Bayesian methods for judgment and economic utility models for decision-making). Following this framework, we propose an experimental approach to plant behavior that explicitly manipulates the stimuli provided to plants, uses plants that vary in sensory abilities, and examines how environmental context affects plant responses. The concepts and approaches that follow from the judgment and decision-making framework can shape how we study and understand plant-herbivore interactions, biological invasions, plant responses to climate change, and the susceptibility of plants to evolutionary traps.

Although competition has been a major focus in ecology for the past century, most empirical and theoretical studies in this area have emphasized interactions between closely related species. However, there is growing evidence that negative interactions among distantly related taxa also occur and may be far more important than previously thought. In this study, we took advantage of an 11-year-old replicated vertebrate-exclosure experiment in a coastal dune community in northern California, USA, to examine the effects of the two most common vertebrate herbivores ( jackrabbits and black-tailed deer) on the abundance of the three most visible invertebrate herbivores (two snail, a moth, and a grasshopper species). Our results indicate that four of the six possible pairwise interactions were significantly negative for the invertebrates. Jackrabbits reduced the abundances of snails by 44-75%, tiger moth caterpillars by 36%, and grasshoppers by 62%. Deer reduced the abundances of snails by 32%, increased the abundances of caterpillars by 31%, and had no measurable effect on grasshopper abundance. Our data also revealed that jackrabbits significantly decreased the volume of forbs and common shrubs and the flowering by grasses in our study plots. We were unable to detect an effect of deer on these measures of vegetation. These results suggest that by changing vegetation, jackrabbits may reduce invertebrate populations that are limited by food, protective structures, or microclimate provided by plants. Of these three mechanisms, only shade was strongly supported as limiting snail numbers in smaller-scale manipulations. In most systems, as in this one, the number of pairs of distantly related herbivores far exceeds the number of pairs of congeners. Since interactions among distantly related herbivores may be common in many cases, these interactions are likely to be important and should receive far more attention from ecologists.

Wild tobacco growing in a field near experimentally clipped sagebrush became more resistant to herbivores than tobacco plants near unclipped sagebrush. Here, we asked whether eavesdropping on sagebrush is a common phenomenon. Do three other forb species that grow near sagebrush experience less herbivory when their sagebrush neighbors are clipped experimentally? Similarly, we asked if sagebrush becomes more resistant when conspecific neighbors are clipped. We found that levels of damage to leaves of lomatium, lupine, and valerian were unaffected by experimental clipping of neighboring sagebrush. These three forbs share many generalist herbivores with sagebrush, as does wild tobacco. Sagebrush individuals near clipped neighbors experienced approximately half the leaf loss to herbivores over the summer as plants with unclipped neighbors. Most of this herbivory was caused by generalist grasshoppers and mammals. Success of a chrysomelid beetle that specializes on sagebrush was unaffected by clipping neighboring sagebrush. These results indicate that communication between sagebrush and tobacco is not a unique phenomenon since sagebrush also responded when neighbors were clipped. In contrast, other plant species generally respond less strongly to cues than does tobacco, indicating that not all species respond to relatively high levels of jasmonate in their environments. Eavesdropping may be restricted to those species, like tobacco, that are strongly inducible and to those species, like sagebrush, that release high concentrations of cues.

Many plants offer rewards to attract ants that provide indirect defense against herbivores. These rewards include nectar and swollen thorn domatia, and their production can be inducible. Theory predicts that costly rewards should be relaxed if the risk of herbivory is reduced, although this has not been previously demonstrated. Acacia drepan- olobium trees in East Africa produce two ant rewards, extrafloral nectar and swollen thorns. We compared the rewards offered by trees experimentally protected from mammalian brows- ing for seven years with those offered by unprotected trees. Protected trees produced 25% fewer nectaries per leaf and 25% fewer swollen thorns than did trees in browsed plots. Relaxation of indirect defenses when trees are protected from herbivores is consistent with the dual hypotheses that inducible defenses are dynamic and that plants can save costs by relaxing these defenses when they are not needed. Among trees exposed to mammalian herbivores, trees that were browsed within roughly the past year provided rewards for ants similar to those provided by trees exposed to herbivores but not browsed within the past year. This result suggests that relaxation of indirect defenses only occurs following more than one year of protection from mammalian herbivory.

Although many ecologists have discounted the possibility of communication between plants, recent work demonstrates that wild tobacco plants with experimentally clipped sagebrush neighbors suffer less leaf herbivory than tobacco controls with unclipped neighbors. In this report, we examine the fitness consequences of resistance induced by eavesdropping. Annual tobacco plants with clipped sagebrush neighbors produced more flowers and seed-bearing capsules than plants with unclipped neighbors although these performance measures varied considerably over the five years of the study. Tobacco plants with clipped neighbors also suffered more frost damage than controls in one year. There was no indication that eavesdropping was more beneficial to tobacco in years with high risk of herbivore damage. The potential adaptive benefits of eavesdropping remain unclear based on five years of data. However, the fact that eavesdropping had strong effects on herbivory and plant performance suggests that interactions between plant species may be richer than we previously suspected.

Herbivores and pollinators can simultaneously exert selective pressures on plant traits via direct and indirect effects. Net selection on plant traits, such as defensive chemistry, may be difficult to predict from studying either of these interactions in isolation. In this study, alkaloids were manipulated experimentally in the hemiparasitic annual plant Castilleja indivisa (Scrophulariaceae; Indian paintbrush) by growing these parasites with bitter (high-alkaloid) or sweet (low-alkaloid) near-isogenic lines of the host Lupinus albus (Fabaceae) in the field. To evaluate the effect of herbivores, half of the Indian paintbrush plants were randomly assigned to a reduced-herbivory treatment using insecticide, and the other half to a natural-herbivory treatment. Floral traits, bud and fruit herbivory, pollination, alkaloids, and plant performance were measured. These variables were used in a path analysis to dissect the direct and indirect effects of herbivory and pollination on lifetime seed set, and the direct and indirect effects of alkaloids on seed set via herbivory and pollination. Bud herbivory and fruit herbivory directly decreased seed production, whereas polli- nation had a direct positive effect. In addition, bud herbivory had negative indirect effects on seed set by reducing the number of open flowers, which reduced pollinator visits. Alkaloids directly reduced bud herbivory but did not significantly affect pollination or fruit herbivory directly. However, because bud herbivory indirectly reduced seed set by reducing pollinator visits to flowers, alkaloids also had additional indirect benefits for plants by increasing pollination. Overall, the net benefit of alkaloid uptake was due to both reduction in herbivory and an increase in pollinator visits to flowers. This study demonstrates the importance of considering multiple interactions simultaneously when attempting to under- stand the mechanisms underlying correlations between plant traits and fitness.

All plants pass through a series of predictable developmental stages during their lives, called phase changes. The phase change from juvenile to adult leaves is known to be associated with changes in resistance against plant pathogens and herbivores in several species. Virtually nothing is known about changes in resistance associated with the transition from embryonic tissue to autotrophic tissue in seedlings. We studied the consequences of transitions from cotyledons to juvenile true leaves to adult true leaves in cotton seedlings (Gossypium hirsutum) for their resistance to spider mites (Tetranychus urticae). Mite populations grew much more rapidly on cotyledons than on true leaves. However, there was no detectable difference in the population growth of mites on juvenile vs. adult true leaves. We suggest that population growth of mites is positively affected by the high rates of photosynthesis of cotyledons relative to true leaves, or by some process or attribute correlated with photosynthesis. Conditions that caused increased rates of photosynthesis (exposure to light and elevated concentrations of CO2) caused mite populations to increase. Greater mite population growth on cotyledons was not associated with stored reserves in the cotyledons, as the mites did poorly on cotyledons kept in the dark. This study indicates that phase changes can have profound effects on plant resistance to herbivores. Because the seedling stage is so vulnerable to herbivory and so critical to understanding plant population dynamics, a broader consideration of phase changes associated with seedlings is warranted.

Previous explanations for the evolution of induced resistance of plants to herbivory emphasized arguments based on saving costs when allocations to defense were not needed; these models met with limited empirical support. We offer a novel explanation based on induced resistance providing increased variability in defense. As long as maximal levels of defense are constrained, variability will increase the effectiveness of a given level of investment in defense. We show that variability can decrease herbivore performance if herbivore performance is a concave function of the level of resistance. In particular, if herbivores can choose among different plants and plant tissues, then variability created by induced resistance may benefit plants under attack and hence may be favored by selection. The key assumptions of this model are broadly supported by empirical data from many plant-herbivore systems.

Current models of parasite–host interactions generally emphasize considerations of parasite virulence and parasite transmission rather than host responses to attack. We describe a situation in which parasitism causes a host to change its feeding behavior and this change improves the expected fitness of the host and probably the fitness of its parasites. We have found that a tachinid parasitoid (Thelaira americana) often emerges from its caterpillar host (Platyprepia virginalis) without killing the host. Whether the host caterpillar survives its parasites depends in part on what it has been eating. Unparasitized caterpillars were more likely to survive to adulthood when feeding on lupine, whereas parasitized caterpillars were more likely to survive on poison hemlock. Development time and pupal masses of caterpillars (both parasitized and unparasitized) were not found to be affected by the host plants that they fed on. Survival of fly larvae in caterpillars that we determined were parasitized using ultrasound was not affected by host plant. However, fly pupae that emerged from caterpillars that had been reared on hemlock were heavier than those emerging from lupine-fed caterpillars. This was due primarily to the direct effect of diet on the flies and less so to the indirect benefit to flies whose host caterpillars survived their parasites by feeding on hemlock. Parasitized caterpillars were more likely to select hemlock, and unparasitized caterpillars were more likely to select lupine when offered both host plants in field tests. These results were consistent for the two years that the choice experiments were conducted. These results were also consistent with the hypothesis that caterpillars change their food plant choices so as to increase their conditional success. Conditional food choices that increase success depending upon parasite load are well accepted for humans, controversial for other primates, and unknown for insects. If caterpillars alter their host plant choices as a result of their parasite load, then this phenomenon could help to explain the evolution of host plant choices that have defied explanation in the past. Such a suggestion assumes that nonlethal parasitism is a common phenomenon. We believe that this may be the case since we did not detect the nonlethal nature of this interaction until we began the unconventional practice of rearing in the field. Other workers have described nonlethal parasitism for several tachinid–host systems, and many families of flies are similar to tachinids except that their hosts are vertebrates; these interactions are nonlethal. Future empirical work, as well as models of parasite–host interactions, should consider the possibility that hosts alter their plant choices depending upon their parasite loads.

Spatial dynamic theories such as source-sink models frequently describe habitat-specific demographies, yet there are surprisingly few field studies that have examined how and why interacting species vary in their dynamics across multiple habitat types. We studied the spatial pattern of interaction between a chewing herbivore and its primary larval host plant in two habitat types. We found that the interaction between an arctiid caterpillar (Platyprepia virginalis) and its host (Lupinus arboreus) differed in wet vs. upland dry habitats, as did yearly population dynamics for the caterpillar. In upland sites, there was a strong positive relationship between lupine cover and the abundance of caterpillars although this relationship was not apparent in wet sites. Additionally, in wet sites, caterpillar populations were larger and less variable across years. Caterpillars appeared to exhibit source-sink dynamics, with the time-averaged finite growth rate lamda > 1 in wet sites (sources), lamda < 1 in upland dry sites (sinks), and predominant source-to-sink movement of late-instar caterpillars. Populations in upland dry sites also went locally extinct in years of low regional abundance. Emigration from wet sites could potentially explain the lack of coupling of herbivore and host plant dynamics in these sites. These results indicate that movement and other factors affecting demography are habitat-specific and have important implications for trophic control. Acknowledging such complexity makes simple models of trophic control seem overly general but may allow us to formulate more broadly applicable ecological models.