The poleward naturalization of intracontinental alien plants

Abstract

Plant introductions outside their native ranges by humans have led to substantial ecological consequences. While we have gained considerable knowledge about intercontinental introductions, the distribution and determinants of intracontinental aliens remain poorly understood. Here, we studied naturalized (i.e., self-sustaining) intracontinental aliens using native and alien floras of 243 mainland regions in North America, South America, Europe, and Australia. We revealed that 4510 plant species had intracontinental origins, accounting for 3.9% of all plant species and 56.7% of all naturalized species in these continents. In North America and Europe, the numbers of intracontinental aliens peaked at mid-latitudes, while the proportion peaked at high latitudes in Europe. Notably, we found predominant poleward naturalization, primarily due to larger native species pools in low-latitudes. Geographic and climatic distances constrained the naturalization of intracontinental aliens in Australia, Europe, and North America, but not in South America. These findings suggest that poleward naturalizations will accelerate, as high latitudes become suitable for more plant species due to climate change.

Fig. 1.. Conceptual illustration of observed and predicted latitudinal shifts of intracontinental aliens.

(A) Consider a hypothetical continent that contains four regions. Three species (the black symbols) that are native to the three black-rimmed donor regions have naturalized in the red-rimmed recipient region, where they are intracontinental aliens (gray symbols indicate species that have not naturalized beyond their native regions). The median latitudinal shifts of these three species reveal a trend of poleward naturalization. (B) In a null model of random naturalization, species native to the three donor regions are randomly drawn as naturalized species for the recipient region. (C) In an informed model, the probability of a species being drawn is weighted by certain anthropogenic, geographic, biotic, or abiotic predictors or a combination thereof. Here, taking the model informed by geographic distance as an example, the probability of a species being drawn decreases (as indicated by the thinner lines) with distance between its donor region and the recipient region.

Fig. 2.. Naturalized intracontinental alien plants in the four focal continents.

The number of intracontinental aliens (A) and its relationship with latitude (B) across 243 regions. The intracontinental aliens as proportion of all aliens (C) and its relationship with latitude (D). In (A) and (C), dark gray color indicates regions within the four focal continents lacking data, and light gray indicates regions not included in the present study (e.g., Africa). In (B) and (D), black lines represent the general trends across all four focal continents, with the shaded area representing 95% CIs. Colored lines represent trends within each of the four continents. The lines were fitted using GAMMs. In (B), the number of intracontinental aliens was natural log-transformed and then area-corrected to account for variation in size of the regions.

Fig. 3.. Observed and predicted latitudinal shifts of intracontinental alien plants in the four focal continents.

For each of the 243 recipient regions, the median latitudinal shifts of its naturalized intracontinental aliens were calculated. In (A), blue shades indicate regions that receive more intracontinental aliens from lower latitudes than from higher latitudes (i.e., intracontinental aliens naturalized toward higher latitudes), while yellow shades indicate the opposite. (B) Distribution of observed and predicted latitudinal shifts. The prediction is based on a null model where intracontinental aliens in a recipient region were randomly drawn (naturalized) from the other regions or on an “anti-LDG” model that counteracts the latitudinal diversity gradient (LDG) in native species (e.g., low and high latitudes contribute equally to species naturalization despite their difference in native species diversity; see Methods). South American regions that are located north of the equator were not included in the analysis.

Fig. 4.. Accuracies of models used to predict latitudinal shifts of intracontinental alien plants.

The null model assumed that the intracontinental aliens in a recipient region were randomly drawn (naturalized) from the other regions. The anti-LDG model counteracts the LDG in native species (e.g., low and high latitudes contribute equally to species naturalization despite their difference in native species diversity; see Methods). Models informed by a single predictor assumed that the probability of a species naturalizing to a recipient region increased with the anthropogenic factor (GDP per capita) of the donor region or decreased with geographic, climatic or biotic distance between its native region(s) and the recipient region. Models informed by multiple predictors assumed that the probability of a species naturalizing to a recipient region was jointly determined by two or all of the three predictors (see Methods). Gray indicates models that did not outperform the null model (vertical dashed line), blue indicates models that outperformed the null model, and red indicates models that outperformed both the null model and the average of all other informed models. Error bars represent 95% CIs. South American regions that are located north of the equator were not included in the analysis.