Hybridization-prone plant families do not generate more invasive species

Many plant taxa are both hybrid-derived and invasive, suggesting a causal connection. However, given that hybridization is not rare in plants, we should expect some fraction of invasive taxa to be hybrids, even in the absence of an underlying causal relationship. Here, we test the hypothesis that hybridization leads to invasiveness by asking whether the number of hybrids and the numbers of naturalized, weedy, and invasive taxa are correlated across 256 vascular plant families. Data were derived from six regional floras and three global databases listing weeds and invasives. To account for phylogenetic nonindependence, we combined a supertree analysis with phylogenetically independent contrasts. After correcting for family size and phylogeny, we conclude that vascular plant families with a higher propensity for hybridization are not more likely to produce more naturalized, weedy, or invasive species than families less prone to hybridization. Instead, hybridization-prone families were in some cases associated with fewer naturalized species and invaders. We present two hypotheses for these patterns, one based on Levin's (Syst Bot 31:8-12, 2006) ideas on reproductive interference and another based on Darwin's naturalization hypothesis. While these results do not preclude the possibility that hybridization generates weedy and invasive taxa with some frequency, they do suggest that the signal from the hybridization-invasion process may be relatively weak and easily obscured by other processes governing plant invasions.