Historical plant introductions predict current insect invasions

Abstract

Thousands of insect species have been introduced outside of their native ranges, and some of them strongly impact ecosystems and human societies. Because a large fraction of insects feed on or are associated with plants, nonnative plants provide habitat and resources for invading insects, thereby facilitating their establishment. Furthermore, plant imports represent one of the main pathways for accidental nonnative insect introductions. Here, we tested the hypothesis that plant invasions precede and promote insect invasions. We found that geographical variation in current nonnative insect flows was best explained by nonnative plant flows dating back to 1900 rather than by more recent plant flows. Interestingly, nonnative plant flows were a better predictor of insect invasions than potentially confounding socioeconomic variables. Based on the observed time lag between plant and insect invasions, we estimated that the global insect invasion debt consists of 3,442 region-level introductions, representing a potential increase of 35% of insect invasions. This debt was most important in the Afrotropics, the Neotropics, and Indomalaya, where we expect a 10 to 20-fold increase in discoveries of new nonnative insect species. Overall, our results highlight the strong link between plant and insect invasions and show that limiting the spread of nonnative plants might be key to preventing future invasions of both plants and insects.

Keywords: invasion debt; nonnative insects; nonnative plants; species flow; time lag.

Fig. 1.

Number of first records (log-transformed) of nonnative insects (A) and plants (B and C) per country.

Fig. 2.

(A) Fit of the GLMMs of current nonnative insect flows between each region as a function of nonnative plant flows (based on cumulative values up to the year shown on the x axis) as a predictor (black) and including general trade flows of 2010 as a second predictor (orange). AIC values were averaged using a 30-y sliding window for visualization. (B) Estimates of trade flows (2010) and nonnative plant flows (1900) as predictors for current (2010) nonnative insect flows in GLMM. Explanatory variables were standardized to a mean of 0 and a variance of 1 to allow coefficient comparison.

Fig. 3.

Observed flows of nonnative insects through 2010 (A) and estimated insect invasion debt (B). The number of species is given in parenthesis for each origin and recipient region.