Negative competitive effects of invasive plants change with time since invasion

Competitive impacts of invasive species may vary across invaded ranges, owing to spatio-temporal gradients in adapted traits and abundance levels. Higher levels of interspecific competition in recently invaded areas may lead invaders to be more competitive. Here, using meta-analysis and home range estimation techniques, we examine how negative competitive effects of invasive species vary across different spatio-temporal invasion contexts. We conducted a meta-analysis of 26 studies that used greenhouse microcosm and common garden pairwise experiments to measure the growth response of native plants in the presence of terrestrial plant invaders (totaling 36 species), and compared this to the time since invasion at the collection site (number of years between the estimated year of initial invasion, by spread of the invader, and the time of collection for the study). We show that negative competitive effects decline across sites that had been invaded for longer periods of time, with effects of invasive grasses declining more rapidly over time than forbs, herbs and shrubs. To our knowledge, only two studies have directly measured competitive or consumptive effects of invaders across a gradient of time since invasion; our study is the first to identify a general pattern of temporal variation of competitive effects that may be attributed to intraspecific trait differences. Management efforts may be guided by such spatio-temporal patterns of invader impact, particularly for grasses.