The Wetzel Lab studies the ecological interactions among plants, insect herbivores, and predators and parasitoids. We focus on how variability—both biological and environmental—influences the dynamics of insect herbivore populations and their interactions with plants and predators. We strive to link patterns at population and community scales with mechanisms at the organismal scale. We do this by using mathematical and statistical modeling to integrate field and lab data across scales including plant chemistryinsect physiology and behavior, and population and community ecology. The lab also places an emphasis on using meta-analysis and synthesis to search for general answers to fundamental ecological questions.

We work in both natural and agricultural ecosystems and strive to answer fundamental biological questions that have implications for environmental problems. Our research in agricultural systems has suggested new ways to enhance agricultural sustainability and reduce reliance on pesticides. Our research in natural systems examines how climate change is influencing population dynamics and predator-prey interactions.

1. How does plant trait diversity and variability influence herbivore population dynamics and predator-prey interactions?

Herbivores in natural ecosystems have to cope with astounding diversity and variability in plant defenses and nutrients. Herbivores in agricultural ecosystems, in contrast, experience plant populations that are unnaturally homogeneous. Recent genetic studies indicate that plant genetic diversity has a profound and often negative influence on herbivore density. This work has contributed to calls for using plant diversity to manage agricultural pests as well as increasing attention to within-species genetic variation in natural systems. But the specific plant traits and the biological mechanisms underlying the effects of plant genetic diversity on insects, however, are poorly understood because the literature on plant-herbivore interactions focuses mostly on plant traits means. The Wetzel Lab is using lab, greenhouse, and field experiments, as well as meta-analysis, to understand how plant trait diversity and variability influence insect herbivore population dynamics and predator-prey interactions. We are linking patterns of population dynamics to diversity and variability in specific plant traits, representing a major advance in our understanding of the links between plant functional traits and insect ecology.

Figure 2 A and B
Performance curves for growth of (A) Heliothis virescens on various defensive plant compounds and (B) Helicoverpa zea on various plant nutrients.

In a meta-analysis of 53 species of insect herbivores, we showed that variance in plant nutrients reduces herbivore growth and survival via a mathematical phenomenon called nonlinear averaging. This occurs because relationships between plant nutrient concentrations and herbivore performance are consistently concave-down. In contrast, plant defense variance has no overall effect on herbivore growth or survival because relationships between plant defenses and herbivore performance are typically linear. These results demonstrate that plants suppress herbivore populations not only by having low average quality, as is typically thought, but also by having variable nutrient levels. This phenomenon could play a key role in the suppression of herbivore populations in natural systems, and increased nutrient heterogeneity within agricultural crops could contribute to the sustainable control of insect pests in agroecosystems. (Wetzel et al. 2016)

An  example of a temporal manifestation of the plant variability-gut acclimation hypothesis. Herbivores experiences changes in plant conditions (top). They respond via gut acclimation (middle). Many herbivore species are known to reduce feeding while acclimating and feed compensatorily following acclimation (bottom). The timing of predation risk (blue and red beetles) may interact with the timing of gut acclimation.

There is abundant evidence that ecosystems, especially agroecosystems, with higher plant diversity have higher densities of predators and parasitoids. Most work on this phenomenon searches for hypothesized direct, positive effects of plant diversity on natural enemies and the subsequent indirect negative effects on herbivores. An overlooked angle is how plant heterogeneity directly influences the physiological ability of herbivores to defend themselves against their enemies and quantifies the resulting changes in herbivore density and population dynamics. We proposed and are using field mesocosms to test a novel hypothesis: the plant variability-gut acclimation hypothesis. This hypothesis posits that plant chemical variability constrains herbivore anti-predator defenses by frequently requiring herbivores to change their gut morphology and physiology to keep up with changing plant defenses and nutrients. This gut acclimation process requires time and energy, and the plant variability-gut acclimation hypothesis proposes that these costs will constraint how and when herbivores can mount anti-predator defenses. One consequence of this hypothesis is stronger top-down control of herbivores in heterogeneous plant populations. The results of this work will reveal how plant diversity acts through a physiological mechanism to influence the dynamics of an important predator-prey interaction and has the potential to form the basis for a novel and sustainable insect pest management strategy. (Wetzel & Thaler 2016)

2. How does temperature variability influence the dynamics of plant-herbivore-predator systems?

Climate change is leading to changes in not just temperature averages but also temperature variabilities. In particular, extreme high temperature events—heat waves—are predicted to become more common. Most ecological climate change studies focus on the consequences of changes in temperature averages, overlooking the effects of temperature variability. This is a limitation because thermal stress is most important at temperature extremes. Moreover, the nonlinearity of thermal performance curves, especially for ectotherms like insects, leads temperature variability to influence performance through nonlinear averaging. The Wetzel Lab is using thermal experiments in the lab and field to parameterize thermally realistic models of insect predator-prey dynamics. This work is revealing the fundamental role of temperature variability in ecological dynamics and will allow us to predict how pest control in agricultural ecosystems will change because of the increased variability associated with climate change.

3. How do plants influence density-dependent herbivore population dynamics?

Wetzel 2014 Fig 3
Insect gall recruitment declines with increasing number of female flies on a plant. The rate at which recruitment declines depends on infestation history.

Density-dependent population dynamics—correlations between demographic rates and population density—is a central concept in ecology. Surprisingly, we are only beginning to learn how plants influence density-dependence in herbivore population dynamics and how this alters spatiotemporal patterns of herbivore abundance. We have addressed this issue with the sagebrush stem-galling fly (Eutreta diana). Like many species of herbivores that are specialized on just a few host-plant species, E. diana exhibits strikingly high variation in density among individuals of its host plant but low variation in density through time.  Using observational data and field experiments, we have shown that this spatiotemporal abundance pattern can be attributed to density-dependent herbivore survival that varies in intensity among individual host-plants at fine spatial scales. Heterogeneity in density-dependence among plants made the population dynamics on the average plant unimportant relative to the population dynamics on the rare plants with weak density-dependence. This result is surprising because ecologists typically focus on population dynamics parameters at large spatiotemporal scales, but this work shows that spatial variation in the strength of density-dependence within populations can also influence abundance patterns. Moreover, spatial heterogeneity in the strength of plant-mediated density-dependence serves as a potentially general explanation for the temporally stable and spatially variable pattern of population dynamics exhibited by specialist insect herbivores. (Wetzel 2014, Wetzel & Strong 2015)

4. How does herbivore modification of host plants influence arthropod communities?

Removing insect galls from oak trees makes the arthropod communities on those trees more similar, reducing their beta-diversity.

The focus of much of the work described above is on how plant traits influence herbivores, but herbivores can also dramatically alter the traits of the plants they attack. A major question in insect ecology how does herbivore modification of host plant traits influence arthropod community patterns? We have addressed this question using the California gall wasp (Andricus quercuscalifornicus) and its host, valley oak (Quercus lobata). These gall wasps modify the phenotype of oak trees by inducing large galls (circa 6-cm diameter) that remain attached to branches for years after the gall wasps emerge and depart, and there is high variation in levels of gall wasp attack among trees within a population. We showed that these galls are colonized by a high density of jumping spiders (Salticidae), voracious predators of other arthropods. We experimentally removed galls from trees and showed that trees with more galls have more spiders and several times lower herbivore species richness and density. Moreover, trees with galls were more likely to support unusual assemblages of arthropods—communities that were very different from the average arthropod community. In other words, galls increased arthropod community beta-diversity at the scale of entire populations of oak trees. Our results showed that heterogeneity in levels of attack by gall wasps led to heterogeneity in the abundance of gall habitat structures, which in turn led to heterogeneity in spider abundances and herbivore abundance and richness, and higher arthropod beta-diversity. This work demonstrates that the influence of plant traits on higher trophic levels depends not just on intrinsic trait values but also on how herbivores shape those plant traits. (Wetzel et al. 2016)

2017-04-22 10.41.07