Multispecies invasions: Evaluating individual invader impacts, invasional meltdown, and management efficacy

Background/Question/Methods

It is well recognized that introduced plants can have substantial impacts on native communities.  Yet few studies have actually quantified invader impacts at the community level, and none have done so while accounting for the effects of multiple invaders.  In reality, most communities experience not one but many invaders simultaneously, potentially compounding invader impacts through additive or interactive effects.  Understanding community-level impacts requires addressing individual invader impacts, including potential nonlinear effects, while also accounting for potentially interactive effects of multiple invaders.  Here, we demonstrate a method for quantifying invader impacts on native plant abundance (percent cover) in multiple invader communities by applying Parker et al.’s (1999) equation (impact = range x local abundance x per capita or per unit effect) to plot-level survey data from 620 1m² plots distributed over 31 grasslands and 400 km²in westcentral Montana.  We also conducted a literature review to examine the efficacy of management actions in the context of multiple invaders.

Results/Conclusions

We found that 25% of the herbaceous flora across these grasslands was introduced (48 exotic and 141 native species).  Exotics averaged 13 species/grassland, or 40% of the average species richness.  Of the 25 exotics sufficiently abundant for analysis, 11 species showed significant negative correlations with native plant cover suggestive of local-scale impacts.  This analysis controlled for additive effects of other exotics on native plant cover, but we found no evidence for interactive effects such as invasional meltdown.  We also found little evidence for non-linear relationships between invader abundance and impacts.  These results suggest that management actions that reduce invader abundance should reduce invader impacts monotonically in this system.  However, we caution that the relationship between invader abundance and invader impacts may differ across spatial scales and mitigating invader impacts can be complicated by many factors including the presence of multiple invaders that vary in sensitivity to management actions.  In reviewing the literature addressing management suppression of dominant invaders, we observed that most outcomes resulted in secondary invasion – another invader(s), rather than native species, replacing the previously dominant weed.  In comparing our empirical rankings of regional-scale invader impacts to the state noxious weed list, we found that the noxious weed list captured 45% of the high-impact invaders and assigned the lowest risk category to the highest impact invader.