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Invasions are driven by a combination of dispersal and population growth. I use age-structured models and spatial models to investigate the relative importance of factors that influence spread in invasive species. In Washington State, I found that reproduction in Scotch broom is strongly limited by pollinators (see Parker 1997). It also has eliasomes and is ant-dispersed; however, spatial models showed that the additional movement by ants is not very important relative to demographic factors (Parker and Reichard 1998). Interestingly, the sites where broom was most successful were the glacial outwash prairies, an ecosystem rich in native species that has been nearly eliminated by agriculture and development (Parker 2000).

I continue to pursue my interest in using models to understand the relative importance of different ecological factors or processes to spatial spread. As part of a working group run by Mike Neubert and Hal Caswell at NCEAS, we are applying stage-structured, analytical spatial models to link dispersal and demography in invading species.

In Washington, Scotch broom is a big, nitrogen-fixing shrub that invades grassland ecosystems that include no large native shrubs. Two undergraduates and I documented a change in plant diversity and species composition when Scotch broom invades prairie sites and old field sites (Parker et al. 1998). During my time as a postdoc in Carla DAntonio's lab at UC Berkeley, I teamed up with Karen Haubensak, the N-Fixation Queen, to investigate whether these changes in composition were accompanied by changes in nutrient levels in the soil. We found a significant increase in total nitrogen, total carbon, and net nitrogen mineralization.

When I moved to California, I was curious to see whether pollinator limitation of Scotch broom is a general phenomenon, or specific to the cold, rainy conditions of the Pacific Northwest. Karen Haubensak and I have been studying the reproductive success of Scotch broom and French broom in Marin County (Mt. Tamalpais State Park). These two exotic species are very similar, but French broom flowers are about 10 times smaller. Scotch broom is more attractive to pollinators, but we found that both species were pollen limited.

It is often stated that exotic species leave their natural enemies behind in their home range, and are therefore released from pest pressure and free to wreak havoc on native species. Plant pathologist and microbial ecologist Greg Gilbert and I are investigating the generality of this idea using clovers at the UC Natural Reserve at Bodega Bay. There we find 10 native and 8 exotic species of clovers living sympatrically, and we are studying effects of pathogens and herbivores on the fitness of individuals from all 18 species. We find strong evidence that herbivores do prefer native species.

Is a potential invader more likely to "escape" pathogens, or conversely to be clobbered by new pathogens to which it has no resistance? In experiments conducted in environmental chambers, Greg Gilbert and I are comparing the pathogenicity and evolution of virulence of California pathogens on native clovers, exotic clovers collected from California, those same species collected from their native range, and clovers that have never been grown in California. Many of the species are native to the Mediterranean, requiring two tedious field trips to France and Spain.

In California, Common mullein (Verbascum thapsus) is now found from sea level up to 7000+ feet in the Sierra Nevada. There are reports from resource managers at Yosemite National Park that it may be increasing its range upward. Does this broad geographic and environmental range reflect huge levels of phenotypic plasticity, or is there evidence for local adaptation in high elevation populations? Do reports of the increasing range of mullein simply reflect range expansion of existing genotypes, or adaptation, or possibly global warming? Stay tuned. See also Joseph's page.

The discovery and study of a novel crop disease has an obvious practical importance. It also represents an opportunity to study the dynamics of an invasion, and the relationship between spread and the distribution of the host. Porroca is a disease of coconut palms for which the causative agent is still unknown. In recent years, porroca has entered an epidemic phase of rapid spread along the Caribbean coast, from Colombia through Panama. With the help of the Kuna People of San Blas (Kuna Yala), Greg Gilbert and I are studying the biology of porroca: determining the cause, the vector, and the basic epidemiology, and investigating possible control options with landscape-scale, community-based controlled experiments. A main focus of our work is a set of detailed GPS/GIS maps of healthy and sick palms to follow and eventually model the dynamics of spread along linear plantations and patchy island systems.

Although ecologists commonly refer to the impacts of nonindigenous species, little formal attention has been given to defining what we mean by impact, or connecting ecological theory with particular impact measures. The resulting lack of generalizations regarding invasion impacts is more than an academic problem; we need to be able to distinguish invaders with minor effects from those with large effects in order to prioritize management efforts. Our working group at the National Center for Ecological Analysis and Synthesis (NCEAS) focused on defining, evaluating, and comparing the ways people measure impact (Parker et al. 1999). Are the results of complex multivariate methods adequately captured by simple composite metrics such as species richness? How well are impacts on native populations correlated with impacts on ecosystem functions, and which drive which? Are there useful bioindicators for invasion impacts? To what extent does the impact of an invasive species depend on the system in which it is measured? Three approaches would provide new insights in this line of inquiry: 1) studies that measure impacts at multiple scales and multiple levels of organization, 2) studies that synthesize currently available data on different response variables, and 3) models designed to guide empirical work and explore generalities.

UCSC

EEB Department

IMP Lab Home