Current Projects

Plant-soil feedbacks and the spread and impacts of invasions

Clark Cowan imposing artificial rainfall treatments on Santa Rosa Island.

Although biological invasions incur tremendous economic and environmental impacts worldwide, the fraction of introduced species that spread through native ecosystems is surprisingly small, between one and ten percent. Recent studies suggest that successful plant invaders may benefit from positive plant-soil feedbacks, where plants change the mutualistic and pathogenic components of the soil microbial community in ways that benefit themselves. Considering that native plants often experience negative feedbacks, plant-soil feedbacks are currently a leading hypothesis to explain why invaders rapidly spread through native communities, and how they may do so more aggressively in their exotic versus native range.

Using simple models of invasive spread, we (Leeza Pachepsky, Bruce Kendall, Stephanie Yelenik, Janneke Hille Ris Lambers, and myself) have demonstrated that counter to prevailing ideas among ecologists, empirically documented plant-soil feedbacks should not affect invasive spread. We derived the conditions under which feedbacks affect invasion velocity and then show that such conditions are never observed in empirical studies. Though unimportant for spread, feedbacks in our models do affect the density and therefore potential impact of exotic plants after they have invaded. Our work emphasizes the importance of distinguishing factors affecting spread versus impact for predicting the most damaging invaders.

In related field work, Stephanie Yelenik, is leading a project examining the role that plant soil feedbacks play in regulating native shrub recolonization of exotic annual grasslands on Santa Cruz Island. Livestock grazing coupled with the introduction of European annual grasses in the early part of the last century converted island habitats from native coastal sage scrub to non-native grasslands. Although grazing has been removed, native scrub has not recolonized. Stephanie is investigating how native shrub effects on nutrient cycling and soil biota differ from the dominant exotic grass, and how these differences feedback to affect shrub reinvasion. This work is among the few studies testing the importance of plant-soil feedbacks in regulating invasion dynamics in the field.

Germination, resource uptake, and the dominance of exotic annual grasses over native annuals in California

Preparation for experimental serpentine outcrops at Sedgwick Reserve.

In the nineteenth century, California grasslands were converted from native perennial to exotic annual grassland. While the advantages of an annual life history may explain this conversion, why native annuals were and remain subdominant members of grasslands is unclear. Janneke Hill Ris Lambers is leading a project to test the role of resource uptake and tolerance to grazing in determining exotic annual grasses dominance over native annuals. Specifically, we are growing native and exotic annuals in monoculture and mixture settings, in the presence and absence of cattle grazing.

Another potential determinant of exotic annual dominance over native annuals pertains to differences in their germination biology. Many of the native annual plants in California require cold temperatures at the time of seed wetting to germinate. This peculiarity of their germination biology may strongly determine year to year variation in species diversity and productivity, maybe even more so than season-long rainfall. This germination requirement is somewhat puzzling given the Mediterranean climate of coastal California, where germinating with the first major rains seems advantageous, regardless of the temperature. Indeed, the highly successful exotic annual grasses in the region possess no cold temperature requirements for germination, which may contribute to their dominance. Margaret Mayfield and I are initiating laboratory tests and field experiments directly manipulating the temperature of the first rains to test the role of germination cues in explaining dominance of exotic grasses in California. Particularly interesting is the possibility that the native annuals possess these germination cues because they evolved in a desert environment where such cueing was particularly advantageous.

Interannual variation in precipitation and the persistence of native annuals with exotic grass competitors

Though interactions with resident species may reduce the establishment of an exotic invader, work in our lab 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. In collaboration with Mark Rees, we have used models based on annual plant dominated grasslands of California 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. With these models as a backdrop, we are conducting fieldwork on the California Channel Islands to examine how large between-year fluctuations in rainfall influence the persistence of endangered annual plants growing in a matrix of exotic annual grasses. This work involves measuring plant demographic parameters in plots subjected to direct manipulation of rainfall with rain shelters and hand watering. Results suggest that because germination cues differ between the rare plants and their grass competitors, the former species may benefit from inter-annual fluctuations in the environment.

Climatic variability and the coexistence of perennial grasses

Rainshelters and Lasthenia on the North Shore of Santa Rosa Island.

Ecological theory suggests that temporal fluctuations in the environment may be critical to the coexistence of species observed in nature, but empirically testing this idea has proven difficult. Considering that climatic variability is forecast to change over the next century, understanding the influence of temporal variability on coexistence is particularly important. In work lead by Peter Adler, in collaboration with Janneke Hille Ris Lambers and myself, we have analyzed a remarkable long-term dataset to show that interannual variability stabilizes the coexistence of three Kansas prairie grass species. We have demonstrated that the dynamics of all three species satisfy the theoretical prerequisites for temporal variability to maintain diversity, and that climate is well correlated with interannual variability in plant performance. Moreover, this variability greatly enhances long-term low density growth rates, helping species avoid competitive exclusion. Our results contrast with recent work questioning the importance of stable coexistence in natural systems and emphasize that stabilizing mechanisms may regulate the impacts of climate change on species diversity.

Stable versus neutral coexistence of serpentine annual plants

The maintenance of diversity in ecological communities, or more formally, coexistence, results when stabilizing processes, which give species an advantage when rare, over compensate for the differences between species in average fitness. Ecologists have long thought that because species co-occur for long periods of time, stabilizing processes must be overcompensating for species differences in net fecundity. However, recent years have seen the rise of the "neutral theory" of community organization, arguing that species coexist not because of strong stabilizing mechanisms or trade-offs, but rather, because their average fitnesses are not as large as commonly believed. This has raised tremendous controversy that has been difficult to resolve in part because the focal species for the debate are long-lived. In collaboration between myself and Janneke Hille Ris Lambers, we have designed an experiment to rigorously test the role of stabilizing versus equalizing (or neutral) processes in regulating plant diversity. We have an ideal study system in the diverse community of annual plants that develop on serpentine outcrops in Santa Barbara county. We have created, in the field, 50 1 m x 1 m serpentine soil hummocks, and seeded them with the annual communities. These plots will then be subjected to several treatments where we manipulate seed production and survivorship so that we eliminate stabilizing processes or augment species differences in average fitness.