Mikaela Huntzinger

THE SPECIFICITY OF EAVESDROPPING ON SAGEBRUSH BY OTHER PLANTS

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.

RELAXATION OF INDUCED INDIRECT DEFENSES OF ACACIAS FOLLOWING EXCLUSION OF MAMMALIAN HERBIVORES

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.

Effects of fire management practices on butterfly diversity in the forested western United States

Abstract In response to a policy of fire suppression since early in the 20th century, forest managers have recently initiated emergency programs of prescribed burning to reduce readily combustible fuel loads in many forests of the western United States. The effects of burning on woody plant composition and structure are relatively well understood; however, little is known about the impact of burning on other taxa. I tested the response of butterflies to fire reintroduction in the Rogue River National Forest and Yosemite National Park. I established replicated transects on three different types of prescribed burn treatment (forest burns, fuel breaks, and riparian burns), as well as control sites, to monitor adult butterfly richness and diversity. Two to three times as many butterfly species occur in forest burns as controls, 13 times as many in fuel breaks as controls, and twice as many in riparian burns as controls. The results of this study suggest that the reintroduction of diverse fire management methods, especially riparian burning, will benefit butterfly diversity in coniferous forests. Further study is required to examine potential proscriptions against riparian burning, including erosion and invasive species encroachment. Both area and density of gaps in the forest canopy were found to explain large amounts of the variation in butterfly richness (R2=0.64 and R2=0.80, respectively). This study demonstrates that using non-traditional taxa (e.g., butterflies instead of trees) to study ecosystem processes may help to provide valuable insights into alternative management strategies.

Diet mixing enhances the performance of a generalist caterpillar, Platyprepia virginalis

1. Platyprepia virginalis caterpillars are dietary generalists and feed on multiple host species within a single day. We conducted field experiments to evaluate their performance on diets consisting of only their primary food, Lupinus arboreus, or diets consisting of L. arboreus plus other acceptable host species.

NEGATIVE EFFECTS OF VERTEBRATE HERBIVORES ON INVERTEBRATES IN A COASTAL DUNE COMMUNITY

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.

Caterpillars escape predation in habitat and thermal refuges

1. Climate and, therefore, abiotic conditions, are changing rapidly, and many ecological interactions depend on them. In this study, how abiotic conditions mediate a predator–prey interaction were examined.