Selection-mediated adaptive responses of native species to an invasive grass: Shade tolerance vs. shade avoidance

Abstract

Studies suggesting that native plant species adapt to invasive species are accumulating, but we still have little evidence of native species performance recovery over time, following phenotypic selection by invaders. The following sequential evidence is sufficient to infer selection-mediated native species performance recovery in response to the interaction with an invader: (i) a decrease in native species' performance in recently invaded areas, (ii) phenotypic selection on native species' functional traits within invaded areas, and in time (iii) trait changes in the direction of selection and (iv) recovery of native species' performance. In a field study in a Canadian grassland, we tested for the outlined sequential evidence in three native species in response to the invader Bromus inermis, whose growth habit significantly reduces light availability. Phenotypic selection was measured on two key traits associated with shade tolerance and avoidance: leaf area and plant height, respectively. Only one species, Artemisia ludoviciana, met all four criteria, thus showing evidence consistent with an adaptive response to B. inermis: increased shade tolerance in response to selection (increased leaf area), further recovering its performance. No such evidence was found for the other two species: we observed a lack of selection on key traits upon encountering the invader in Hesperostipa curtiseta and no response to selection in Achillea millefolium. Identifying the functional strategies plants can adopt to coexist with an invader is key to inform restoration and management strategies. The present study points to moving beyond the documentation of adaptation by native species to invaders. Specifically, following the proposed framework, future research would increase our understanding of when and how native species may adapt to persist and thrive within invaded areas, further identifying potential constraints to adaptive responses. Read the free Plain Language Summary for this article on the Journal blog.