Document: DAV-3-40-54

Effects of diversity on the invasibility of plant communities in California serpentine grassland.

HOOPER, D.U.* ¹, J.S.DUKES ² and J.VERVILLE ²

Western Washington University, Bellingham, WA 98225-9160 USA ¹
University of Utah, Salt Lake City, UT 84112 USA ²

Abstract:
More diverse communities are hypothesized to be more difficult for species to invade because greater species richness should leave less available resources for a potential invader to exploit. In this experiment, we addressed three fundamental questions: 1) Does plant diversity influence invasibility of communities? 2) Do functional attributes of invaders related to resource acquisition influence the success of their invasion? and, 3) Is there a greater probability of invasion success when the invader differs from community members in functional attributes related to resource capture? Using already-established experimental communities in California serpentine grassland, the experimental design allowed separation of effects due to differences in plant composition (which functional groups are present) from effects due to differences in functional group richness (how many groups are present). These experimental communities allowed control of several factors, including disturbance regime, soil fertility, and community history, which confound attempts to study these questions a posteriori in already-invaded ecosystems. The experimental communities consisted of combinations of four functional groups: early season (E) and late season (L) annual forbs, perennial bunchgrasses and nitrogen-fixers. Replicated plots were established in 1992 and have been maintained since. Invaders were seeded into experimental communities in known amounts, using several species from each of two functional groups (E and L; 6 species total). We measured abundance and biomass of invaders after the first year of introduction, in addition to resource parameters potentially relevant to invasion success (nitrogen, phosphorus, water and light availability). While success of some invading species decreased with increasing community diversity, overall invader success depended on interactions among initial community diversity, the functional type of the invaders relative to the functional types of species already present in the community, and the relative competitive abilities of invaders for limiting resources. These results suggest that there are not simple relationships between invasibility and species diversity, but that the functional characteristics of the species involved are equally important.

Keywords: biodiversity, species invasions, exotic species

This abstract is being presented at: 1:00 PM in session:
Oral Session #33: Plant Demography.