Ecologists work to understand population dynamics of important species. A common working model used by many ecologists considers food available to a focal species and the focal species as food for other species. This model has been inadequate to explain many field observations, including the 1000-fold differences in numbers of wooly bear caterpillars at Bodega Marine Reserve over the past 20 years. One possible solution is to make simple tritrophic models more realistic by including additional species interactions or by including spatial dynamics. The objective of this study is to provide a first field test of this these new models by conducting field experiments and surveys evaluating additional complexity at each trophic level. This research will evaluate the relative importance of: 1) added species at the resource (plant) level, 2) added species at the herbivore/omnivore level, and 3) movement and spatial dynamics at the predator level.
Most ecological research is conducted for up to three years in a single location. A longer perspective over a larger area is essential to capture a realistic view of how and why populations change. This project will provide tests of current general theories about factors that organize and stabilize terrestrial communities. This information will allow management of undesirable species and evaluations of anthropogenic changes to useful species and ecosystems.
Many herbivorous insects are of considerable economic concern to Americans as pests of food crops and forests. Understanding the conditions that cause populations of these insects to increase or decrease is of considerable value. Our current models of herbivore populations rely on a simple model involving the host plant of the herbivore, the herbivorous insect species, and the predator and parasites of the herbivore. When we applied this simple model to understand 30 years of population data for a caterpillar that we had been studying we found that this model was inadequate. Adding more species to this model will improve our ability to predict caterpillar numbers but they could easily become too complicated to be useful. We sought to determine which additional factors were necessary to improve our understanding without making the model too cumbersome and complex. We found that we couldn’t predict caterpillar numbers by considering only a single field site – we needed to include multiple sites and include movement among sites in our model. Different processes were important in different sites and we needed to consider a diversity of habitats to get an informative picture for this species. We also found that resources that were not simply who eats whom were important. In our case, the depth of the litter layer was a critical resource in allowing the caterpillars to survive. The litter layer prevents small caterpillars from desiccating and allows them to elude ant predators. Other herbivores that we assumed were likely competing with our species were actually beneficial because they indirectly increased the depth of the litter layer. In general, these kinds of interactions that don’t involve consumption may be more widespread and important than we had realized in the past. Caterpillars are attacked by parasitoid flies that are generally assumed to kill their hosts. However, we found that caterpillars were able to survive these attacks approximately 50% of the time. Caterpillars changed their food preferences when they were attacked by parasitoids and may be medicating themselves to improve their chances of surviving. Populations of these parasitoids followed those of their caterpillar hosts rather than driving them. More important than these conspicuous parasitoids were relatively inconspicuous ants that preyed on small caterpillars. This observation supports the contention that small inconspicuous interactions rule the world rather than trophic interactions between larger species that are more apparent to humans.
