The spread rate of introduced plant species is not associated with the presence or number of close relatives in the invaded area
Alien species can damage ecosystems and the ecological services we derive from them; futhermore the invasions of natural communities is a cause of biodiversity loss. To explain why some species are successful invaders while others are not, we tested a prediction based on Darwin’s naturalization hypothesis; which poses that introduced species with no close native relatives at genus level will spread to a greater number of localities than species with close relatives in the introduced area. We compared the spread rate of introduced species with and without relatives and their biological attributes using herbarium records of 315 weed species from the ‘Introduced weeds in Mexico’ database (CONABIO). We modelled the number of locations as a linear function of residence time. Residuals of this model were used to classify species in two groups, those having more locations than expected and those having less. These sets were again subclassified in two, species with and species without relatives. We compared the number of relatives between the latter subgroups with a chi squared test. Correlation between residuals and number of relatives was assesed with a Spearman test. We used a regression tree to explain residence time as function of biological attributes.
Residence time explains 44% of the variation in the occupied localities (y=–2.093 + 1.5964*x, R²= 0.448 and p <0.001), the more time has passed since the first record of a species, the more locations it has been collected. However, some species are found in more or less locations than expected according to their residence time. We selected those species with more than 103 years of residence, 88 species appeared in more localities than expected while 77 were less frequent than expected. No difference in the spread rate between introduced species with and without native relatives in each group (χ2= 0.1818, df 1, p= 0.6608; χ2= 0.3521, df 1, p= 0.5529). Correlations between residuals and number of native relatives were not significant for these two groups (R=-0.80, p= 0.45; R= 0.05, p= 0.69). A regression tree showed that number of localities occupied by species is dependent on dispersal syndrome, life form, growth form and life cycle. The spread rate of the introduced species is not associated with the presence or number of close relatives in the invaded area, but appears to be associated with their dispersion syndrome, life form, growth form and life cycle.