Diverse plant communities may be more resistant to invasion because the communities contain a resident species that is functionally similar to the invader (limiting similarity/sampling effect), or the residents use the range of available resources more effectively (complementarity) than single species. However, the resistance of diverse communities to invasion appears to decline with increasing spatial and temporal scale, in a phenomenon known as the “invasion paradox.” We addressed two groups of hypotheses related to this paradox, broadly that: (1) functional diversity and functional identity resist invasion initially, via complementarity or limiting similarity; and (2) disturbance and propagule pressure interact to weaken the effects of functional diversity and identity on invader success through time. Using long-term data from experimental assemblages in a serpentine grassland of California, we examined the abundance of a high impact invader, yellow starthistle (Centaurea solstitialis), in response to functional diversity (functional dispersion), functional dissimilarity, extent of pocket gopher disturbance, and propagule pressure. Propagule pressure was estimated using a tilted Gaussian plume model of seed dispersal from the patches of yellow starthistle surrounding the experimental plots.
Results/Conclusions
Neither diversity, nor dissimilarity, nor disturbance significantly impacted the success of yellow starthistle at any point in the study. Instead, propagule pressure was the single most important predictor of yellow starthistle abundance. We argue the absence of effects of community composition and pocket gopher activity on invasion reflects time-dependent effects of ecological resistance and disturbance. Since previous work on these plots found strong evidence of community composition affecting initial invader success, our results suggest that propagule pressure may overwhelm ecological resistance through time. Such a trend implies that neutral processes of assembly may outweigh deterministic mechanisms when considering a longer-scale view of invasion. Overall, these findings support the idea that propagule pressure should be considered a null model for invader success. Conservation efforts should thus focus on preventing the introduction and establishment of invasive species, recognizing that even diverse communities are susceptible to invasion over time.