Can we use functional trait relationships to better optimize multiple ecosystem services in complex landscapes? Can it guide species selection in restoration plantings?
Community assembly research asks questions of species diversity and distribution. Why is a species present in one place but not another? Are there rules we can use to predict community composition? In relation to environmental restoration and management, can we manipulate community assembly to achieve our goals?
A community is composed of species that have arrived at the site, survived its environment and interacted successfully with other species. If this process is not random, then patterns of functional traits across a landscape can help us understand species distributions. Differences in seed size might limit species arrival, for example, whereas different nitrogen uptake strategies and growth rates may control species survival. Relating dispersal and environmental constraints to functional traits can indicate the mechanisms of community assembly.
Species also modify their environments, and a trait-based approach can describe these processes. Describing traits that affect the environment helps both to predict patterns of community assembly – nitrogen fixers, for example, may facilitate other species entering community – and to anticipate the ecosystem functions and services a species is likely to provide – plant with nitrogen-rich leaves, for example, are likely to make good forage for cattle.
Trait-based urban restoration
Strawberry Creek, which cuts through the UC Berkeley campus, has long been a focus of restoration efforts for the campus community. Much of this work has involved removing invasive ivy that coats the Creek’s banks. Removing invasive species without establishing other species however can leave a “weed-shaped hole” that invasive species can easily recolonize. The concept of “limiting similarity” suggests that planting species with similar functional traits may exclude invasive species. However, restoration plantings in the middle of an invaded landscape may experience contast invasion pressure. In these situations, the concept of “complementarity” may be more appropriate - by planting species dissimilar to the invasive species, it is more likely that species can coexist with the inevitable invasions. We used a trait screening of fifty riparian and woodland plant species to test these scenarios, finding that in highly invaded systems aiming for trait complementarity enhances restoration success. This work was lead by UC Berkeley undergraduates in conjunction with the Strawberry Creek Restoration Program.
Threshold models for restoration
A general principle of restoration ecology is to identify and reverse “thresholds”—levels of degradation that alter a system to the extent that it cannot recover without intervention. Thresholds can be abiotic (e.g., reduced water availability), or biotic (e.g., introduction of non-native species that suppress native species). After identifying thresholds, practitioners may alter them directly (e.g., irrigate; remove nonnative species) or restore species that can survive modified conditions (e.g., drought-tolerant species; good competitors). In the past we have worked with the Western Australian Department of Environment and Conservation and an NGO, Greening Australia, to monitor thresholds at ongoing old-field restoration sites.
Related publications
Hallett, L. M., D. E. Chapple, M. N. Bickart, A. Cherbowsky, L. Fernandez, C. H. Ho, M. Alexander, K. Schwab, K. N. Suding. 2017. Niche complementarity enhances native plant restoration in an invaded urban landscape. Ecological Restoration 35(2): 148-155.
Hallett, L.M., R.J. Standish, and R.J. Hobbs. 2011. Seed mass and summer drought survival in a Mediterranean-climate ecosystem. Plant Ecology 212(9): 1479-1489.
Hallett, L. M., R. J. Standish, J. Jonson, and R. J. Hobbs. 2014. Seedling emergence and summer survival after direct seeding for woodland restoration on old-fields in south-western Australia.Ecological Management and Restoration 15(2): 140-146.