Faculty Research: Jonathan Levine

Research in my group examines leading questions at the interface of population, community, and ecosystem ecology. Though I am fundamentally an empirical ecologist, population models lie at the heart of our approaches, allowing rigorous links between theoretical concepts and the dynamics of field systems. Currently, our research is focused on three themes: (1) the controls over coexistence, (2) the influence of climatic variability on plant communities, and (3) the controls over the spread and impact of biological invasions.

Controls Over Coexistence

Beginning with papers published with two postdocs, Peter Adler and Janneke Hille Ris Lambers, our most significant advances over the last several years have been in the area of coexistence (Adler et al. 2006, 2007, Levine et al. 2008, Levine and Hille Ris Lambers 2009, MacDougall et al. 2009, Mayfield and Levine in revision). Among the most enduring mysteries in ecology is how multiple plant species, all competing for the same handful of limiting resources, coexist. Classic theory shows that stable coexistence requires competitors to differ in their niches, and this finding has motivated countless investigations of ecological differences presumed to maintain diversity. That niche differences are key to coexistence, however, has recently been challenged by the neutral theory, which explains coexistence by the equivalence of competitors. The ensuing controversy has motivated calls for better understanding the collective importance of niche differences for the diversity observed in natural systems.

In a soon to be published paper in Nature (Levine and HilleRisLambers 2009), we show that niche differences strongly stabilize the coexistence of serpentine annual plants. Following a coexistence framework put forth by Peter Chesson, we use models to predict how rapidly species diversity would decline without the stabilizing effects of niche differences. We then compare model predictions to observed community dynamics in the field. After two generations of community change in the field, species diversity was over 50% greater in communities stabilized by niche differences relative to those exhibiting dynamics predicted by the null model. This work provides some of the best evidence that niche differences strongly stabilize coexistence, and helps locate communities along the continuum between niche and neutral theory.

In other coexistence work, we have used the Chesson framework to help resolve debate between niche and neutral theory (Adler, Hille Ris Lambers, and Levine 2007), propose new tests for the importance of niches (Levine et al. 2008), unify the diversity of hypotheses controlling biological invasions (MacDougall, Gilbert and Levine 2009), and refine the predictions of phylogenetic community assembly (Mayfield and Levine, in revision).

Plant Community Response to Climatic Variability

Numerous studies have explored how forecasted changes in mean climatic conditions will influence plant communities. Far less understood are the potential impacts of forecasted changes in climatic variability. In collaboration with Peter Adler and Janneke Hille Ris Lambers, we have been exploring how between-year climatic variability maintains plant diversity. Working with long term datasets, Bayesian hierarchical models, and simulation, we have found beneficial effects of climatic variability for plant diversity in a central U.S. grassland (Adler et al. 2006), but little influence on a sagebrush steppe community (Adler et al. 2009). These analyses, among the most comprehensive tests of the storage effect, suggest that forecasted increases in climatic variability may, but not always ameliorate negative effects of climate change on plant community diversity.

As our storage effect work exemplifies, climate change directly affects species by altering their physical environment and indirectly affects species by altering the competition they face. These indirect effects may amplify or counteract the direct effects of climate change, but their importance is poorly understood. In factorial manipulations of precipitation and competition in grasslands of the California Channel Islands and the central United States, we have been evaluating indirect effects of climate change mediated via altered competition. Although these indirect effects are often hypothesized to operate, we failed to find strong evidence for their importance in either system (Levine et al. 2009, Adler et al., in revision). In fact, in the Channel Islands system, plant populations fluctuate widely, but these fluctuations are driven more by temporally variable germination cues than fluctuations in the dominant competitors or total rainfall (Levine et al. 2008).

The Spread and Impacts of Biological Invasions

The severe ecological and economic impacts of biological invasions have motivated many ecologists to explore how exotic species aggressively spread through native ecosystems. Our work is currently focused on the demographic controls over population spread, and how invader impacts on ecosystem processes feed back to influence spread and dominance.

Much of our understanding of the demographic traits controlling invader spread comes from mathematical models of populations expanding through continuous environments. A large body of theory, ranging from reaction-diffusion to integrodifference equation models, has shown that the spread rate for an invader depends on its dispersal ability and its population growth rate when rare. This classic result underlies our 2006 finding that plant-soil feedbacks that take time and density to develop have little effect on invader spread (Levine et al. 2006). More generally, in the absence of Allee effects, factors that regulate population growth at high density have little influence on invasion speed through homogeneous landscapes (Levine 2008).

Working with postdoc Leeza Pachepsky, we have found that the demographic controls over invasions in classic models break down with the simultaneous inclusion of two features of real invasions: discrete populations and movement through heterogeneous landscapes of suitable and unsuitable habitat. By including these features in individual based simulation models, we have shown that an invader's ability to reproduce at high densities can be key to its spread. These results suggest that different demographic traits may be responsible for local spread through suitable habitat versus regional spread through patchy landscapes (Pachepsky and Levine, in prep).

Complementing this theoretical work are our empirical efforts to understand the mechanisms regulating the rate at which native plant species reinvade exotic-dominated landscapes. Through a series of field experiments and soil analyses lead by my recently graduated doctoral student, Stephanie Yelenik, we have been examining how plant species effects on nutrient cycles influence native plant recovery. We have found that native shrub species of the California Channel Islands differentially alter soil nutrient cycling in ways that either enhance or impede their recovery in exotic grasslands (Yelenik and Levine, in revision). Importantly, the species modifying soils to its benefit is the only one recovering (Yelenik and Levine 2009), suggesting an important role for plant-soil interactions in the recovery process.

Jonathan Levine | Research | Publications | Curriculum Vitae

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