On the proportion of native and nonnative plant species in North American temperate vs Neotropical forests: An intercontinental comparison

Background/Question/Methods

Biological invasions are one of the leading causes of biodiversity loss worldwide. However, not all habitats are equally invaded. Ecological theory would predict that tropical forests are less invaded than temperate forests. Tropical forests are theorized to be less invaded due to their potential greater levels of biotic resistance, lower levels of disturbance, and lower propagule pressure, among other reasons. To our knowledge, the invadedness of temperate and tropical forests has not been rigorously quantified. Additionally, nonnative species may not be equally distributed across growth forms. Temperate and tropical forests are known to vary in how plant species are allocated to growth forms (e.g., trees vs. herbs), but it remains unknown how nonnative species specifically are allocated among growth forms. We hypothesize that Neotropical forests harbor lower levels of nonnative species than North American temperate forests, and that nonnative species will not be equally distributed among growth forms, with herbs and shrubs being overrepresented in both forest types. We used published vascular plant species lists for 9 temperate and 9 tropical forests, paired with the GLONAF database, to calculate the number of invaders at each site. We used the USDA PLANTS database and TROPICOS herbarium database to identify growth form.

Results/Conclusions

Neotropical forests harbored significantly lower numbers of nonnative species than North American temperate forests. Both the raw number and the proportion of nonnative species were found to be lower in Neotropical forests. To our knowledge, we are the first to test the hypothesis that tropical forests are less invaded than temperate forests. We also found herbs to be by far the most common type of nonnative species in both forest types. This is particularly surprising because, although herbs only comprise ~25% of species in tropical forests, they were found to comprise >50% of their nonnative species. Tropical forests are known for their high levels of biodiversity, however, high levels of deforestation and transition into farmland may pave way for nonnative species to more easily invade. Early-successional, light-loving invaders, particularly herbs, may be transported at a greater rate and more easily invade open-canopy, deforested habitats. Habitat preservation efforts must focus on mitigating disturbance levels in tropical forests and countering the spread of nonnative species, particularly herbs. Future research should address which mechanisms (biotic resistance, disturbance, propagule pressure, etc.) are specifically driving these trends.