The wrong kind of help: Positive associations with native shrubs are intense and important for an exotic invader but not the native annual community across an aridity gradient

Background/Question/Methods

Positive interactions influence the assembly of plant communities globally, particularly in stressful environments like deserts. However, few studies have measured the intensity (i.e., the absolute impact) and importance (i.e., the relative impact, compared to other factors) of positive interactions involving native and invasive species along aridity gradients. These measures are essential for predicting how dryland communities will respond to biological invasions and environmental change. Here, we measured the intensity and importance of positive associations formed between native foundation shrubs and the annual plant community – including Bromus madritensis ssp. rubens (“B. rubens”; a dominant invasive plant species) – and co-occurring native annuals along an aridity gradient across the Mojave and San Joaquin Deserts, USA. Along the gradient, we sampled metrics of abundance and/or performance for invasive B. rubens and the native annual community during peak flowering at 120 pairs of shrub and open microsites. From these measurements, we calculated RII (an index of interaction intensity) and Iimp (an index of interaction importance) values to characterize shrub-annual associations. For both indices, positive values indicate positive associations, and negative values indicate negative associations.

Results/Conclusions

Across the gradient and regardless of relative aridity, B. rubens consistently occurred at far greater abundance, cover, biomass, and fitness near native shrubs than away from shrubs, and RII and Iimp values for these metrics were always positive. In contrast and regardless of relative aridity, the native annual community occurred at greater abundance, cover, and species richness away from shrubs, and RII and Iimp values for these metrics were never positive. These findings suggest that B. rubens formed intense and important positive associations with native shrubs that consistently improved its abundance, cover, biomass, and fitness; and that B. rubens may be able to capitalize on positive interactions better than the native annual community. The risk of shrub-facilitated B. rubens invasion may be high across the Mojave and San Joaquin Deserts, regardless of relative aridity.