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Restoration through reassembly: plant traits and invasion resistance

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One of the greatest challenges for ecological restoration is to create or reassemble plant communities that are resistant to invasion by exotic species. We examine how concepts pertaining to the assembly of plant communities can be used to strengthen resistance to invasion in restored communities. Community ecology theory predicts that an invasive species will be unlikely to establish if there is a species with similar traits present in the resident community or if available niches are filled. Therefore, successful restoration efforts should select native species with traits similar to likely invaders and include a diversity of functional traits. The success of trait-based approaches to restoration will depend largely on the diversity of invaders, on the strength of environmental factors and on dispersal dynamics of invasive and native species.

Section snippets

Trait-based community assembly as a framework for the restoration of invaded systems

As ecosystems worldwide are degraded by human activity, ecological restoration plays an essential role in maintaining biodiversity and critical ecosystem functions. The invasion of exotic species poses a special challenge to ecosystem restoration, as invaders often both contribute substantially to ecosystem degradation and hinder efforts to restore systems [1]. An essential component of restoration is the reassembly of plant and animal communities following ecosystem degradation or the removal

Community ecology theory

For decades, ecologists have debated whether communities assemble following a set of nonrandom rules 14, 15, 16, 17. Much recent debate has focused on whether community assembly is more strongly limited by the availability of environmentally suitable sites (niche limitation) or the likelihood of plant species reaching those sites (dispersal limitation) 18, 19, 20, 21. A more integrated view of community assembly separates niche and dispersal limitation into individual ecological filters, where

Identifying relevant functional traits

Perhaps the biggest challenge in applying functional traits to restoration design is deciding which functional traits are relevant and measurable for a given habitat. Functional traits are typically classified as response or effect traits 29, 30, but these designations are not mutually exclusive. The functional significance of various traits to plant fitness (response) and ecosystem processes (effect) are presented in Table 1.

When selecting species for a restoration project, the relative

Applying the concept of limiting similarity to ecological restoration

The concept of limiting similarity predicts that invasive species will be unlikely to establish if there are native species with similar traits present in the resident community or if available niches are filled. Several plot-scale and mesocosm studies suggest that limiting similarity does indeed confer invasion resistance 42, 43, 44, 45 (but see Ref. [46]). In a European grassland, the invasion of exotic legumes was low in native legume communities relative to communities composed of native

Incorporating ecological filters into the design of restored communities

Ecological filters influence community assembly at multiple scales (Box 1). Evaluating the trait space of native and invasive species within a community might suggest methods to alter ecological filters to prevent invasion in restored communities. This evaluation could be particularly important where limiting similarity will not work, as when there are no native species in the regional species pool that are functionally similar to invaders (e.g. no suitable natives share resource-use traits

Functional diversity and community invasibility

Whereas native communities that are functionally similar to invasive species might be more resistant to invasion through direct competition for resources (limiting similarity), native communities might be vulnerable to invasion if their traits are not well dispersed over available niche space and resources are available for new species to establish. Thus, functionally diverse native communities might be less susceptible to invasion because there are few vacant niches. Trait space analyses can

Conclusions

The increasing need to reassemble communities through ecological restoration provides an opportunity to apply a community assembly framework to help restored communities resist invasion by exotic species. As many restoration efforts are stymied by invasive species, the trait-based framework that we advance here will have wide-reaching implications for restoration success. Large-scale restoration projects are needed to test this conceptual framework. Specifically, projects that select native

Acknowledgements

This work developed from an Ecological Society of America symposium entitled ‘Plant Functional Traits as Tools for Ecological Restoration.’ We thank David Hooper, Richard Hobbs and one anonymous reviewer for comments on the manuscript. J.L.F. was partly supported by the National Parks Ecological Research Fellowship Program, a partnership between the National Park Service, the Ecological Society of America and the National Park Foundation which is funded through a generous grant from the Andrew

Glossary

Competitive exclusion
two species competing for the same resources cannot stably coexist, if the ecological factors are constant.
Complementarity
two or more species use resources in complementary ways such that together they more effectively use available resources than either does alone [7].
Ecological filter
dispersal, environmental or biotic factors that limit which species are present in a community.
Ecological restoration
the process of assisting the recovery of an ecosystem that has been

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