Does biological diversity of neighboring habitat control reintroduction success of endangered species?

Background/Question/Methods

Biological diversity is in decline worldwide, with potential negative effects on ecosystem functions and services.   Also, as diversity declines the number of endangered species in need of conservation increases.   Reintroduction of at-risk species may be necessary to protect them from extinction, provide connectivity between populations, and reach recovery goals under the U.S. Endangered Species Act.  One function that ecosystems serve is to provide habitat for these at-risk species.  We asked, can species richness in a plant community affect the success of endangered species reintroductions?  Working through a nonprofit organization in partnership with government agencies and with support from community volunteers, we conducted experimental reintroductions of golden paintbrush (Castilleja levisecta), a threatened plant, into grassland habitats in western Oregon where the species has been extirpated for over 70 years.  This species is a hemiparasite that establishes root-root connections with neighboring plants.  We planted 100 individuals at each of ten grassland sites to test for effects of site hydrology and neighbor-diversity on golden paintbrush growth and survival.

Results/Conclusions

Species richness within 10-cm of golden paintbrush had a significant effect on plant survival at most sites individually and overall (p<0.00001), indicating that the performance of golden paintbrush may depend on the nature of the plant community at the local microsite scale.  In addition, only 7% of golden paintbrush survived one year in wetland habitats, compared to 54% in uplands (p=0.001).  Reintroduction success of this species appeared to be strongly affected by biological diversity at the planting site with perennial plant diversity improving survival and annual diversity reducing survival.  This pattern with perennials may be the result of complementarity, meaning that having more kinds of neighbors is better because it increases the number of potential positive interactions.  Alternatively, it may be due to a sampling effect, with more species of neighbors increasing the likelihood that at least one of them is an appropriate host.  Both effects may operate simultaneously.  Annual neighbors (primarily invasive species) may be poor hosts because they do not provide resources to the hemiparasite beyond one season of growth.  Our approach to reintroduction emphasizes adaptive management through testing sites and assessing mechanisms.  With support of volunteers and scientists outside of standard academic institutions we are moving forward with additional reintroductions at upland sites with high local plant diversity.  To our knowledge this represents the first report of biodiversity increasing the success of at-risk species reintroductions.