The influence of variance in quality within plant populations on herbivore dynamics and spatial distributions

The quality of individual plants as food for herbivores (and thus the mean quality of plant populations or communities) can influence both individual herbivores and herbivore populations. However, assemblages of plants vary not only in mean quality, but also in the variance around that mean (CV). In the hopes of gaining a better understanding of how plant quality affects herbivore populations, I have recently been working on experiments and models that focus on variance in plant quality, rather than the mean.. In particular, I am exploring the effect of variance in quality among plants on the movement and long-term population dynamics of insect herbivores. I have been working with a spatially-explicit computer model to examine how herbivore movement influences the interaction of variance in plant quality with herbivore population dynamics in space and time. This model will provide a framework for understanding how variance in quality affects herbivore populations in systems where quality varies among phenotypes, genotypes or species. I am also using field experiments at the Bodega Marine Reserve (Sonoma Co, CA) to examine how the degree of genetically based variation in quality in wild strawberry populations affects strawberry aphid movement and population dynamics. I am looking at  how aphid movement responses to individual plant quality are affected by population variance in plant quality, and using longer-term experiments to follow aphid population dynamics on plant populations with different levels of quality variance. Aphid dynamics on these populations are being characterized by direct observation of multiple generations and by fitting population dynamics models to the observed time series. These experiments should indicate how aphid population dynamics on monocultures of individual plant genotypes combine to produce dynamics on mixtures of genotypes of different qualities. Understanding the consequences of among-plant variance in quality for herbivore population dynamics may allow us to extend our knowledge of the effects of plant-quality characters on individual herbivores to an understanding of how populations or communities of plants affect populations of herbivores in space and time. Results of this research should also allow critical examination of the hypothesis that agricultural mixtures of plant varieties (genotypes) should host lower pest populations than monocultures. Ultimately, I am thinking of linking genetic variance in plant quality and herbivore population dynamics as a step towards incorporating plant population genetics into the study of plant-herbivore dynamics, thus allowing connections between ecological and evolutionary processes in these systems.

Poster presentation on correlation between aphid dynamic parameters across plant genotypes from ESA 2004

 Population of wild strawberries in dune habitat                                Population of wild strawberries growing in bluff habitat

   strawberry aphid (Chaetosiphon fragaefolii)                      Fragaria chioloensis (picture © Br. Alfred Brousseau, Saint Mary's College)