Article 13:

(Case Study) Vernal Pool Enhancement, Restoration, and Creation in Santa Barbara, California

By Wayne R. Ferren Jr., Maser Consulting

Vernal pools are seasonally-flooded wetlands, which in the Mediterranean climate of California means that they fill with winter rains and dry during spring, functioning as desiccated upland environments until the next winter rains (Zedler 1987). They are underlain by soils with clay layers that swell and become impervious when wet, causing ponding in shallow depressions. Vernal pools provide habitat for many specialized plants and animals that have important morphological and physiological adaptations for survival in these highly fluctuating ecosystems. In the Santa Barbara region, the last two centuries of land-use history include various forms of agricultural, military, institutional, and residential development, which has destroyed over 90% of the vernal pools in the region. All remaining pools have been degraded by a variety of disturbances, including fragmentation, soil compaction or disking, mosquito abatement practices, and colonization by invasive plants (Ferren and Pritchett 1988). There are no pristine vernal pools available as natural reference sites to guide restoration.

Del Sol Open Space and Vernal Pool Reserve, a 4 ha parcel in the urban setting of Isla Vista, 16 km west of Santa Barbara, is one of the remaining sites containing vernal pool habitat. Voters approved the purchase of the area in 1978, becoming one of the first communities in California to set aside land to protect vernal pools. At the time, all the extant pools were physically degraded or dominated by nonnative species. To enhance the pools at the reserve and to compensate for the dramatic decline in vernal pools in the area, the Del Sol Vernal Pools Enhancement Plan was implemented in 1986 (California State Coastal Conservancy 1986; Figure A.I). The goal of the plan, funded initially by the California Coastal Conservancy, was to preserve, enhance, and expand the Del Sol pools to compensate for the historic loss of habitat (Ferren and Pritchett 1988). Over a period of ten years and beyond, the staff of the Museum of Systematics and Ecology at the University of California Santa Barbara, in cooperation with the Isla Vista Recreation and Park District and the County of Santa Barbara, implemented a number of different phases of the enhancement plan (Ferren et al. 1998) in the context of a framework hypothesis (Ferren and Pritchett 1988; Ferren and Gevirtz 1990): Vernal pools at Santa Barbara can be enhanced, restored, and created: (1) to provide a broad array of ecosystem structure and function that occur within the variability of naturally occurring vernal pools; and (2) to be self-sustaining and thus mature or decline in ecosystem structure and function in patterns resembling natural vernal pools.


Figure A Creation of a vernal pool. (I) Excavation of vernal pool basin during dry period in 1986. The basin size, depth, and slope is based on topography of reference vernal pools in the region. (II) Seed bank material (soil + seeds + plant material + invertebrates) used to inoculate new pools is raked from small patches within dry, naturally occurring vernal pools. (III) Flooded winter view of created vernal pool. (IV) Desiccated, created vernal pool in spring 1996, 10 years following creation. Plant and animal communities in the created-inoculated pools are similar statistically to those of natural reference vernal pools.

The ecosystem structure or functions we evaluated include: topography (vernal pool size, depth, and slope); hydrology (ability to sustain inundation patterns consistent with vernal pool hydrology); native plant associations (ability to support predominantly native species in zonation, cover, densities, and richness consistent with donor or reference pools); native faunal associations (ability to support predominantly native species associations consistent with donor or reference pools); food chain support (presence of guilds of organisms that are found in presumed highly functional natural vernal pool ecosystems); and habitat for sensitive groups of organisms such as rare, endemic, and endangered species.

We used the least-damaged extant examples of vernal pools as reference sites for new pool design and post-construction monitoring protocols, as well as donors of vernal pool seed bank inoculum for new habitat. Extant pools were evaluated for topographic structure, hydrologic periodicity, vegetation zonation, and plant and animal species richness. We did not use live plants for revegetation, but rather “seed bank material” as an “inoculum” for new artificial pools or restored degraded pools. This was derived from raking the duff (dead plant material) and some topsoil from extant “donor pools.” The inoculum included natural substrate, organic debris, plant seeds, and eggs or resting phases of aquatic invertebrates.

The material was gathered late in the dry season (early fall; see Figure A.II) and spread over the new or restored substrates, which also had been raked to provide a loose subsoil cover to which the vernal pool inoculum was added before or immediately after the wet season began (mid to late fall). Sufficient amounts of rainfall resulted in germination of seeds and hatching of planktonic invertebrate eggs. In the initial experiment in 1986, three created pools received vernal pool inoculum and three created pools were left uninoculated with seed bank material. No inoculum was added to the enhanced pools, which remained relatively intact but had modifications in hydrology (elimination of drainage ditches), and minor amounts of debris and invasive plants removed.

Our analysis of a decade of data from pre- and post-construction habitat monitoring efforts supported the hypothesis we used as a guiding framework and demonstrated that enhanced, restored, and created-inoculated pools are self-sustaining (Ferren et al. 1998). Enhanced, restored, and created vernal pools also provide a broad array of ecosystem structure and functions similar to those of naturally occurring vernal pools (i.e., establishment of wetland hydrology, habitat for native plants and animals, habitat for sensitive species, food chain support, and functions in the grassland ecosystem; see Figures A.III and IV). At Del Sol Reserve, we concluded that vernal pools have been successfully enhanced, restored, and created.

The created-uninoculated pools, however, failed to establish some biological functions, particularly for native vernal pool plants. From these observations we concluded that the creation of vernal pools requires addition of seed bank material and/or plants to establish vernal pool structure and functions that are consistent with the variability of natural vernal pools, at least within the first decade after construction. This was the case even though newly created, uninoculated pools were in proximity to vernal pools with many ecosystem functions. Invertebrate fauna of modified and created pools was also similar to that of natural pools except for created-uninoculated pools, which had lower densities of aquatic fauna. We suspect that birds and mammals served as vectors to transfer invertebrates and some plant propagules between pools, including the uninoculated pools, thereby establishing new populations, which lacked the richness and densities of natural and inoculated pools.

We learned many important project-specific lessons as a result of these efforts. For example, similarity of plant richness, zonation, cover, and density was statistically significant for manipulated pools, except for uninoculated-created pools, as compared with natural pools within a five to ten-year period. Furthermore, similarity of invertebrate richness and density was statistically significant for manipulated pools, except uninoculated-created pools, within a ten-year period. Delays in invertebrate colonization rates were due largely to delay in colonization of sites by aquatic insects and not planktonic species that were introduced with seed bank inoculum. Insects had to find pools and lay eggs and aquatic larvae had to hatch and survive before species were established in the habitats.

Failure of uninoculated pools to establish plant cover in early post-construction years provides opportunities for invasive species (such as the nonnative annual grass Polypogon monspeliensis) to establish in great numbers, particularly if they are wind-dispersed. This colonization by undesirable species minimizes or perhaps prevents the future establishment of native vernal pool dominants, which could be dispersed by wind or birds. Extensive colonies of invasive species can then serve as the source population of propagules (e.g., wind-borne seeds) that colonize additional suitable habitat in the area.

In general, annual plant species are rapid colonizers of new habitat, whereas perennials take several years to achieve cover consistent with natural habitat because the extent of their cover is largely the result of vegetative growth. We recognized that stockpiled upland topsoil from vernal pool restoration or creation sites could be used to reestablish vernal pool edges and transition areas that might otherwise appear as bare zones for many years following construction. This practice facilitated a faster return to typical function for these habitats.

As a result of implementation of the Enhancement Plan, Del Sol Reserve was visited by an impressive number and richness of birds from different guilds that used the resources in a variety of ways. Anecdotal observations 15 to 20 years after initiation of the enhancement plan, and results from recent additional habitat manipulations in the region suggest that the enhanced, restored, and created pools continue to exhibit the structure and function of extant natural vernal pools in the region.

Many lessons broadly applicable to vernal pool restoration resulted from our studies:

Ecological restoration experimentation at Del Sol Vernal Pool Reserve has proven to be successful in large part because the landscape in which it occurs is not entirely destroyed in spite of two centuries of land use. This emphasizes the importance of protecting extant natural habitat. Although none of the remnant natural habitats are pristine, they offer the only hope for understanding the structure and function of natural habitat and the potential success of restorative actions. Use of extant examples of natural habitat as reference sites in the design and monitoring of artificial habitat and as donor habitats for the translocation of seed bank material, seeds, plants, and animals of local genotypes is essential in the effort to recover lost or damaged systems and declining species.

Literature Cited

California State Coastal Conservancy. 1986. Del Sol Enhancement. Staff Recommendation, 17 April 1986, File No. 86–020.

Ferren, W. R. Jr. and D. A. Pritchett. 1988. Enhancement, Restoration, and Creation of Vernal Pools at Del Sol Open Space and Vernal Pool Reserve, Santa Barbara County, California. The Herbarium, Department of Biological Sciences, University of California, Santa Barbara

Ferren, W. R. Jr. and E. M. Gevirtz. 1990. Restoration and Creation of Vernal Pools: Cookbook Recipes or Complex Science? In D. H. Ikeda and R. A. Schlising (eds.), Vernal Pools—Their Habitat and Biology, pp. 147–148. Studies from the Herbarium No. 8, California State University, Chico.

Ferren, W. R. Jr., D. M. Hubbard, S. Wiseman, A. K. Parikh, and N. Gale. 1998. Review of Ten Years of Vernal Pool Restoration and Creation in Santa Barbara, California. In C. W. Witham, E. T. Bauder, D. Belk, W. R. Ferren Jr., and R. Ornduff (eds.), Ecology, Conservation, and Management of Vernal Pool Ecosystems, pp. 206–216. Proceedings of 1996 Conference, California Native Plant Society, Sacramento, CA.

Zedler, P. H. 1987. The Ecology of Southern California Vernal Pools: A Community Profile. U.S. Fish and Wildlife Service Biological Report 85.