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Realistic diversity declines reduce invasion resistance , grassland NPP, and resource use through loss of a key functional group.

Zavaleta, Erika*,1, Hulvey, Kristin1, 1 University of California-Santa Cruz, Santa Cruz, CA, USA

ABSTRACT- Consequences of progressive biodiversity declines depend on the functional roles of individual species and the order in which species are lost. Most studies of biodiversity-ecosystem functioning relationships tackle only the first of these factors. We used observed variation in neighborhood-scale grassland diversity to design an experimental test of how realistic species losses affect invasion resistance. We investigated the effects of non-random variation in diversity, with species occurring at realistic (field based) abundances, on grassland production, resource use, and resistance to intentional and naturally occurring invasions. We report results after two years. In year one, progressive richness declines led to decreases in invasion resistance, NPP and use of soil moisture and light in spite of the relative rarity of added species. Three lines of evidence point to the importance of intensified competition within a single functional group – late-season forbs – in explaining diversity effects on invasion resistance. First, reduced success of naturally invading species in more diverse communities consisted mainly of reductions in invading late-season forbs. Late-season forbs were resident only in the more diverse communities, suggesting a role for them in resisting invasion by similar species. Second, biomass of intentionally introduced yellow starthistle (Centaurea solstitialis, a late-season forb) was negatively related to resident biomass of late-season forbs, both within and across diversity levels. Finally, the addition of yellow starthistle reduced biomass of other late-season forbs but not of any other functional group. In year two, realized species richness departed more strongly from the original diversity treatments, reducing the differences among treatments in richness and composition. Realized richness and original treatment diversity were significantly correlated. However, functional group diversity no longer varied among treatments, with nearly all microcosms containing all late-season forbs. While biomass of invading yellow starthistle tended to increase with both declining treatment diversity and realized richness, neither effect was significant in year two. –

Key words: species loss, rare species, invasion resistance, California grassland

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