Vernal pool ecology and restoration
Our research focuses on the ecology and restoration of native vernal pool plant communities in central California. These communities are particularly appropriate for hypothesis testing in community ecology because of their dramatic temporal and spatial variation in plant species composition. Species composition is highly variable even under relatively similar physical conditions within study sites. This observation suggests that chance occurrences, such as past colonization events, and biotic factors, such as competition and pollination may strongly influence plant community composition. As spatially discrete habitats, vernal pools are especially well suited to field experiments that investigate factors influencing colonization and persistence of species in the formation of ecological communities. Moreover, colonization of vernal pools is likely mediated by their spatial position in the landscape, as predicted by ecological theory. Given that many vernal pool plants have limited dispersal capabilities, pools near a source of propagules should be colonized more frequently than those further away.
[Learn more about our research by checking out the list of selected publications at the bottom of this page, or on the publications page to download selected articles as pdf files.]
Background: Vernal pools in California
Vernal pools are temporary wetlands with abrupt boundaries that form on relatively level sites underlain by an impervious hardpan soil layer (Zedler 1987, Holland and Jain 1988, Keeley and Zedler 1998). They are typical of Mediterranean climates and are characterized by winter and spring inundation of pools, followed by complete drying in summer. Like vernal pools elsewhere in the world, California’s pools are important biological reservoirs for amphibian and aquatic invertebrate species (Witham et al. 1998), but they are distinct in that they support a unique, highly endemic, and largely annual flora (Stebbins 1976, Holland and Jain 1981). Vernal pools were once widespread along the Pacific coast and Central Valley of California (Barbour et al. 1993), but due to urban expansion and agricultural development, most habitat has been destroyed; it is estimated that only 3-10 percent of pools remain (Leidy and White 1998) and many vernal pool endemics are now legally protected (Witham et al. 1998).
The overall objective of this research effort is to clarify the relative importance of historical factors and pool spatial position in controlling plant community composition in vernal pools, and to investigate possible abiotic and biotic mechanisms for observed patterns in species composition during the process of colonization. The role of historical factors and pool spatial position is examined via seed addition treatments and natural colonization of created vernal pools in a large field experiment, while mechanisms are investigated through an additional series of greenhouse experiments, field observations, and field experiments with vernal pool plant species. The specific goals of our research are to 1) characterize population and community trajectories of plants in vernal pools, 2) analyze effects of hydrological characteristics of vernal pool plant communities, 3) quantify contribution of the seed bank to population dynamics, 4) model population and community dynamics of five vernal pool plant species, and 5) model metapopulation and metacommunity dynamics. The outcomes of these experiments are making meaningful contributions to our understanding of the basic ecological forces that govern population establishment, persistence, and community composition. We are also learning key steps in successfully re-establishing native vernal pool plant communities.
Efforts to understand the factors controlling the composition of local communities have played a central role in the development of ecology (Diamond 1975, Strong et al. 1984, Pimm 1991, Weiher and Keddy 1999, Hubbell 2001, Chase 2003, Tilman 2004, Holyoak et al. 2005). An outstanding question involves the relative contribution of ‘local and deterministic’ versus ‘regional and stochastic’ factors in structuring ecological communities. Recent attention to this issue is driven by the urgent need for guidance in the restoration of functional ecological communities (Young et al. 2001, 2005, Suding et al. 2004, Falk et al. 2006). The question is, can the restoration of a local community proceed independently of the metacommunity? A ‘metacommunity’ is broadly defined as a collection of communities connected by dispersal (Hanski and Gilpin 1991, Holyoak et al. 2005), and a ‘community’ is a collection of individuals that interact either directly or indirectly by partitioning the resources within a patch of shared habitat (Hubbell 2001). Two main branches of ecological theory relate to the structure of local communities and metacommunities. Community ecology focuses mainly on local, especially deterministic interactions such as predation and competition to explain the structure of local communities. Biogeography and metapopulation ecology focus on regional, largely stochastic dynamics such as dispersal, disturbance and extinction to explain the regional distribution and coexistence of species. Our study addresses recent calls for the unification of community and metapopulation ecology (Hubbell 2001, Holyoak et al. 2005), so that we can begin to resolve the importance of local versus regional factors in structuring communities. We use an established ecological restoration project to test key hypotheses regarding the relative importance of colonization patterns in community composition.
Progress to date
The purpose of this vernal pool field experiment and restoration project is to preserve and restore the natural vernal pool ecosystem at the Travis AFB Aero Club site. The project’s long-term objectives are to preserve existing vernal pools at Travis AFB, and to construct vernal pools that exhibit similar structure and function to currently existing, protected vernal pools at the Aero Club. The viability of this vernal pool ecosystem is particularly important to prevent extinction of the federally endangered plant, Lasthenia conjugens (Contra Costa goldfields: Asteraceae), and to maintain and enhance vernal pool ecosystems in central California, which have declined to less than 10% of their original extent.
As part of the vernal pool mitigation requirements for Travis AFB (Collinge 1999), 256 pools were constructed at the Aero Club in November 1999. Because this project’s ultimate goal is to reconstruct viable vernal pool communities, in addition to Lasthenia conjugens, the constructed pools were seeded with four other species found in vernal pools at the study site, including Eryngium vaseyi (Button celery, Apiaceae), Deschampsia danthonioides (Annual hairgrass, Poaceae), Layia chrysanthemoides (Tidy tips, Asteraceae), and Plagiobothrys stipitatus (Popcorn flower, Boraginaceae). In December 1999, seed addition treatments were randomly assigned to each of the 256 constructed pools. Pools in the control treatment were left unseeded to assess the rate of natural colonization of created vernal pools.
Since 2000, we have quantified abundance and distribution of all plant species in seed plots of constructed pools and naturally occurring reference pools at our study site. Our results to date show that as expected, cover of seeded species has been significantly higher in seeded than control pools in every year of the study. The smallest observed difference in seeded species cover occurred in 2009. Furthermore, sampling plots in seeded and reference pools were similar through time in cover of seeded and other native species, as well as exotic species. Cover of seeded species in constructed pools was best modeled by seeding treatment and maximum depth of inundation. Pool depth and duration of inundation were similar among seeded, control, and reference pools through 2002, but by 2009 reference pools were significantly deeper than restored pools during the period of peak inundation. The depth of restored pools has decreased apparently as a result of climate-driven increases in the cover of exotic species that contribute to rapid thatch accumulation. These combined results suggest that vernal pool restoration can be facilitated by ensuring appropriate hydrological conditions and by seeding constructed pools with native species. However, our observed effects of climate on the outcome of competition between native and exotic species suggest impending effects of climate change, especially if exotic species facilitate their own dominance through thatch accumulation. We are currently expanding our research to investigate the role of thatch accumulation, particularly by exotic plant species, on native species performance (see Akasha Faist’s research).
Kids do Science!
Sixth graders are engaged in conducting experiments related to our research. One of our key research goals is to understand seed germination requirements and persistence of seeds in the soil. In spring 2011, the students performed a seed germination experiment with the endangered species, Lasthenia conjugens. They assessed how different watering treatments would affect germination of seeds. The students observed that the highest number of seeds germinated from the bowls with intermediate watering levels. Their results help us to interpret our observations of native vernal pool plant species in our restoration experiment.
Selected vernal pool publications
Collinge, S.K., C. Ray, and F. Gerhardt. 2011. Long-term data on vernal pool plant communities reveal formerly cryptic effects of biotic resistance to exotic species invasion. Ecological Applications 21:2105-2118.
Collinge, S.K. and C. Ray. 2009. Transient patterns in the assembly of vernal pool plant communities. Ecology 90(12):3313-3323.
Ramp, J.M., T.A. Ranker, and S.K. Collinge. 2008. Conservation of rare species with an island-like distribution: a case study of Lasthenia conjugens (Asteraceae) using population genetic structure and distribution of rare markers. Plant Species Biology 23:97-110.
Gerhardt, F. and S.K. Collinge. 2007. Abiotic constraints eclipse biotic resistance in determining invasibility along experimental vernal pool gradients. Ecological Applications 17:922-933.
Ramp, Jennifer M., Tom A. Ranker, and Sharon K. Collinge. 2006. Restoration genetics of the vernal pool endemic Lasthenia conjugens (Asteraceae). Conservation Genetics 7:631-649.
Gerhardt, F. and S.K. Collinge. 2003. Exotic plant invasions of vernal pools in the Central Valley of California, USA. Journal of Biogeography 30:1043-1052.