COS 55-1 - Invader species, not functional similarity, drives complex per-invader effects in a grassland community

Background/Question/Methods

Classic niche theory predicts that invaders that occupy a unique niche space are capable of accessing underutilized resources without strong competition with resident species. Such invaders may have higher establishment success but lower per-capita impact on the resident communities than functionally similar invaders that compete strongly with resident species. To test this hypothesis, we investigated the role of functional overlap in moderating changes in resident biomass per individual invader in California serpentine grasslands by seeding invader species into established small-scale experimental communities of annual plants of varying functional group composition differentiated by phenology. Two years after invader establishment, we tested how total resident community biomass changed as a result of invasion and how resource availability, functional overlap, and invader species identity altered the relationship between the number of invaders and changes in total resident biomass.

Results/Conclusions

We found that while invasion reduced the total biomass of resident communities, functional overlap between the invader and host community had only a marginally significant effect on resident community biomass and resource availability had no measured effect on resident community biomass. Instead, resident community biomass depended upon invader species, resident functional group, and the number of individual invaders, with both changes in the number of residents and resident size contributing to these changes in total resident community biomass. As invader density increased, effects on total resident community biomass intensified, diminished, or did not change, depending on the invader species and resident community composition. The mix of impacts with increasing invader density suggests that while some invader-resident interactions were characterized by competition, some late-season invaders may have facilitated early-season resident establishment success. These results highlight the need to consider both niche and fitness differences in predicting invader success and the complex nature of resident-invader interactions, as invasives produce a complex network of negative and positive consequences on their recipient ecosystems.