Past Projects

What factors affect the dynamics of plant-microbe symbioses?

Populations of grasses vary in the frequency of endophyte infection. Our work has demonstrated that factors extrinsic to the grass-endophyte symbiosis, namely mammalian and insect herbivory, explain significant variation in the long-term dynamics of plant-microbe symbioses. In experimental populations composed of 50:50 mixtures of infected and uninfected plants, we found a greater increase in endophyte frequency (from 50% to 80%of plants infected) under ambient levels of herbivory than when herbivory was experimentally reduced (from 50% to 62%).

Do indirect interactions influence plant evolution?

Plants have a number of strategies to defend against herbivory, including attracting the predators and parasitoids of their herbivores — an indirect defense. Research has largely focused on direct interactions between plants and herbivores, with less consideration of the role of indirect interactions in plant evolution. To examine selection on indirect defense, I used a wild species of cotton (Gossypium thurberi), which bears extrafloral nectaries that attract ants (see photo). The ants consume and disturb herbivores, thereby increasing cotton fitness. Unlike some of the ant-plant mutualisms made famous by Janzen and others, the ant-cotton association varies in the degree to which ants are beneficial. It is therefore more representative of the complex and variable indirect interactions that typically occur between plants and predators. By combining quantitative genetics techniques with field experiments that altered ant and herbivore densities, I showed that extrafloral nectary traits exhibit heritable variation, that ants increase plant fitness by reducing herbivory, and that the availability of extrafloral nectar mediates the benefits of ants (Rudgers 2004, Ecology).

What is the relative importance of consumptive versus non-consumption interactions in predators' control of prey? In addition, work with wild cotton suggests that ants may be an overlooked resource in the biological control of cultivated cotton pests. The mechanisms by which ants benefit wild cotton are not exclusively through their consumption of herbivores. Ants also cause herbivorous caterpillars to drop from leaves and reduce their feeding rates. In fact, ants confer greater benefits to plants by modifying herbivore behavior than by consuming caterpillars (Rudgers et al. 2003, Oecologia )

Do plants face trade-offs in allocating among anti-herbivore defense traits? In exploring the evolutionary consequences of indirect interactions, it is important to consider how selection on one plant trait, such as extrafloral nectar production, may constrain the evolution of other traits. In collaboration with Sharon Strauss and Jonathan Wendel, I tested for constraints on defensive traits in the cotton clade (Gossypieae). Using a recent phylogeny to control for shared ancestry, we found that direct plant defenses (toxic leaf glands and leaf hairs) and the indirect defense of extrafloral nectar appear to evolve independently in the Gossypieae, a pattern that may reflect the facultative nature of indirect defense (Rudgers et al. 2004, Am. J. Bot.).

How does geographic variation in species interactions affect coevolution?

Experiments conducted in multiple populations of wild cotton revealed that the defenses conferred by ants varied geographically. Recent theory on coevolution has embraced this commonly observed biological reality: the outcomes of species interactions vary across space. Understanding how this spatial variation affects selection will enhance our ability to predict how species traits are shaped by coevolution. By experimentally manipulating the putative agents of selection (ants) in three populations, I showed that ants were responsible for geographical variation in selection on extrafloral nectary traits . This work is some of the first to manipulate interactions experimentally across multiple sites and thereby document that geographically variable selection, mediated by a mutualist, can shape the evolution of plant traits (Rudgers and Strauss 2004, Proc. Roy. Soc.).

How important is plant genetic variation to the distribution and abundance of species?

In addition to investigating factors that affect genetic diversity within plant populations, I am also interested in the ecological consequences of this genetic diversity, specifically how the loss of diversity influences the ecological community. Because a nthropogenic disturbances and agricultural monocultures often reduce diversity, an understanding of the ecological function of diversity (including both species diversity within communities and genetic diversity within species) can inform ecosystem conservation.

For this question, I used the shrub Baccharis pilularis, which has an architectural dimorphism with both prostrate and upright genotypes. The two genotypes sustain different ecological interactions in coastal California . My experiments showed that only the prostrate genotype facilitates the establishment of another dominant, coastal shrub, Lupinus arboreus (Rudgers and Maron 2003, Oikos). This is one of the first studies to suggest that plant genotype is important to facilitative interactions between plants, revealing the potential for facilitation to affect plant evolution. In addition, a four-year herbivore exclusion and common garden experiment showed that the two genotypes support different assemblages of insect herbivores. This differential herbivory, in turn, may affect the maintenance of the dimorphism within the Baccharis population (Rudgers and Whitney 2006, J Ecol). These studies confirm the importance of population-level genetic diversity to both plant and arthropod communities. I have conducted similar work with ants and cotton, addressing whether variation among plants in the amount of extrafloral nectar affects the diversity of an arthropod assemblage (Rudgers and Gardener 2004, Ecology).