Current Research: Jonathan Levine

Understanding Biological Invasions from Introduction through Impact

The invasion of species into new biogeographic regions is a process that has regularly occurred over geologic time. Over the last millennium, however, the human-mediated transport of species across the globe has increased the rate of invasion several orders of magnitude. Although most invaders fail to establish in their new range, the fraction that succeed have collectively exerted tremendous ecological and economic damage. Through competition, predation, and the alteration of disturbance regimes, biological invasions have caused massive changes in ecosystem structure, and are second only to habitat destruction in threatening imperiled species in the United States. In addition, from an economic standpoint, invasions cost the American economy tens of billions of dollars annually.

My research applies fundamental principles of population, community, and ecosystem ecology to understand the factors controlling the success and impacts of biological invasions. In addition, the research exploits the unique opportunities presented by species introductions to understand the processes organizing ecological systems. I employ methodologies ranging from pure theory to manipulative field manipulations, though it is the simultaneous use of these approaches that allows me to address challenging research questions.

Ecologists conceive of the invasion process as proceeding through a series of stages: introduction, establishment, spread, and impact. My current research examines the critical questions at each of these stages:

Introduction- How many new invaders should we expect in the coming decades?

Despite the severe ecological and economic impacts of exotic species, we have no quantitative forecasts for future invasions in the United States. Given that international trade, the source of most exotic species, is forecasted to grow 6% annually, one might predict a similar increase for invasions. However, my work suggests that the expected relationship between trade and invasions is more complex. This is because each unit of trade may transport some new invaders, but mostly transports species already carried by previous trade activity. Although this means that trade and invasions cannot be linearly related, this problem proves similar to that encountered in classic ecology when estimating cumulative species number from a series of samples. Each new plot placed in a meadow, for example, contains some new species, but also contains species sampled in previous plots. Ecologists have developed a range of "species accumulation models" to describe how species number increases with sampling intensity and predict species number in even larger samples.

In my work, I borrow the species accumulation models from classical ecology to relate the accumulation of exotic species to accumulated trade. These models provide excellent fit to historical invasions data, and when coupled with trade forecasts, predict future invasion rates. Predictions include a decline in the "per ship" probability of transporting new introductions with increasing trade. Nevertheless, because of the massive forecasted growth in trade volume, current rates of invasions should continue for the next several decades. Future work will apply this approach to different biogeographic regions.

Establishment- Are more diverse communities better able to resist biological invasions?

Concerns over the loss of species from ecological communities and the well-documented impact of exotic species on native diversity have focused ecologists on the classic hypothesis that diverse communities better resist invasions. Findings, however, have been controversial. Small scale experimental studies and theory have typically shown that high diversity inhibits invasions, whereas large scale observational studies have found that the most diverse communities are typically the most invaded.

My research attempts to reconcile these disparate results by incorporating principles of community ecology. In reviews, I have argued that the ecological factors known to promote native species diversity, such as high seed supply, intermediate disturbance, and environmental heterogeneity similarly promote exotic species invasions. Thus factors spatially correlated with native diversity across landscapes may also cause the most diverse natural communities to be the most invaded, maybe even despite the intrinsic or causal effects of diversity itself. What is needed are studies that measure natural patterns of species diversity and biological invasions, and in the same system, experimentally isolate the causal effects of diversity. This forms the rationale for my fieldwork in a northern California river system.

The plant community along the Eel River is contained on hundreds of mini-islands formed by a common sedge. The system provides an ideal natural context in which to explore the diversity-invasibility relationship, because the sedge islands vary widely in the number of native species they contain, and are also invaded by a number of exotic plants. By sampling an 8 km stretch of river, I have found that the more diverse islands are more likely to contain invaders. However, a direct in situ manipulation of local diversity shows that these patterns emerge despite the intrinsic negative effects of diversity on invasion operating at local scales. This latter result was demonstrated by experimentally "invading" island communities that had been randomly assembled with a range of species. This work, coupled with a current meta-analysis, begins to suggest that factors other than diversity explain the community-wide correlations between diversity and invasion.

Spread- Do feedbacks between invaders and their environment affect expansion?

Once invaders establish in new locations, they may modify the environment in ways that benefit their growth and reproduction. Exotic plants for example can cultivate specific soil fungi and bacteria that enhance their performance. Because native species in these habitats tend not to experience similar benefits, "plant-soil feedbacks" have been suggested as leading determinants of invasive spread.

My work challenges this commonly-held notion by demonstrating its contradiction to classic results from reaction-diffusion models of population spread. The spatial expansion of a population is constrained by its establishment and reproduction in previously unoccupied habitats, locations in which it is initially rare and where feedbacks have yet to operate. Preliminary results suggest that when empirically documented plant-soil feedbacks are incorporated into classic population models, they strongly influence the density of the population within the already occupied range, but have no effect on invader spread. Future work will develop this result, exploring in particular the influence of long-distance dispersal.

Impact- What factors regulate the impact of invasions once they are successful?

Though interactions with resident species may reduce the establishment of an exotic invader, my work suggests that such interactions cannot categorically prevent invasions, and more generally, that current rates of introduction will continue into the future. Thus, understanding what factors allow native species to persist with invaders once they have successfully established is important for native species preservation. My work borrows coexistence models from community ecology to suggest that environmental fluctuations may be essential for native plant persistence in invaded communities. This contrasts with work from a traditional conservation biology perspective, where fluctuations reduce long-term average growth rates and increase the probability of extinction.

In work on the California Channel Islands, I plan to examine how tremendous between-year fluctuations in rainfall influence the persistence of endangered annual plants growing in a matrix of exotic annual grasses. The entirely annual nature of the system makes possible the development of reasonable mathematical models and multigenerational field experiments. Preliminary results suggest that because the germination biology differs between the rare plants and their grass competitors, the former species may benefit from inter-annual fluctuations in the environment, a result completely counter to classic ideas in conservation biology. Future work involves obtaining model parameters by sowing plants into experimentally imposed rainfall "years" simulated in the field with rain shelters and hand-watering. Model predictions will be tested against the observed population dynamics of the plants through years varying naturally in rainfall.

Jonathan Levine | Research | Publications | Curriculum Vitae

<< Return to Faculty Page