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Weed management can result in unintentional secondary invasion

Date: August 19, 2016

A rapidly emerging problem is that of secondary invasion – an increase in non-target exotics following efforts to suppress targeted invasive plants


Herbicide treatment targeting the invasive plant, spotted knapweed, in Montana.
Herbicide treatment targeting the invasive plant, spotted knapweed, in Montana.
Exotic plant invasions present a world-wide threat to natural ecosystems and cost the global economy hundreds of billions of dollars annually. Substantial costs arise from efforts to suppress targeted invasive plants, yet the efficacy of these efforts in mitigating invader impacts remains unclear. Researchers at the Rocky Mountain Research Station and University of Colorado examined one factor inhibiting ecosystem recovery, secondary invasion by non-target exotics, in order to quantify the magnitude of the problem and identify mitigation strategies.

Researchers reviewed 168 studies from around the world that examined the efficacy of exotic plant management in terrestrial habitats. Of these studies, only 29 percent quantified community responses sufficiently to evaluate secondary invasion, suggesting a need for more community-level evaluation of management actions. 

Key Findings

Meta-analysis showed that management efforts strongly reduced target invader abundance overall. However, secondary invaders increased following control, with a mean effect size that was double that found for native plants, which increased only weakly. Of the secondary invaders identified in these studies, 89 percent were classified as noxious or invasive plants. Increases in secondary invaders were linearly correlated with target invader reductions, while other factors including control method and target invader growth form did not explain variation in secondary invader responses. 

These results suggest that suppression of target invaders drives the release of secondary invaders by creating a “hole” in the plant community. To mitigate this problem, management strategies should minimize disturbance and incorporate complimentary tools to suppress secondary invaders and fill opened space with desired species. Legacy effects of target invaders such as altered soil properties may also facilitate secondary invasion, requiring special mitigation. These findings emphasize the need for advanced revegetation approaches to improve invasive plant management.


  • Invasive plant management efforts often successfully suppress target invaders, but the result is largely secondary invasion by other pest plants.

  • Generally, the more successful the suppression of target invaders, the greater the response of undesirable secondary invaders.

  • To mitigate this problem, invasive plant management efforts must aim to minimize the size of the disturbance created and incorporate complimentary strategies for impeding not only target but also secondary invaders.

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Pearson, Dean E. ; Ortega, Yvette K. ; Runyon, Justin B. ; Butler, Jack L. , 2016