The editor’s choice for our September issue is “Combining biogeographical approaches to advance invasion ecology and methodology” by Dean Pearson et al. Here, Associate Editor Ayub Odour explains the importance of this research:
Invasions by alien plant species reduce native biodiversity, alter ecosystem processes, and subvert essential ecosystem services. Therefore, understanding the ecological and evolutionary mechanisms that underlie invasion success of alien plant species remains a major goal in ecology. A greater understanding of the ecological and evolutionary mechanisms that underlie invasion success of alien plants requires studies that are conducted in both the native and exotic ranges of invasive plants. Ideally, such studies should be conducted at various levels of organization, ranging from populations to species and communities, to fully understand how biogeographic differences in biotic and abiotic factors in the native and exotic ranges influence invasion success of alien plant species. However, biogeographical comparisons of invasive plants remain relatively few despite numerous studies on plant invasions biology.
Pearson et al. deployed three complementary biogeographical study approaches to test hypotheses on invasion success of alien plants: (1) directed surveys, (2) randomized surveys, and (3) in situ field experiments. They tested hypotheses on release from factors that limit populations of invasive species in their native ranges (e.g., specialist natural enemies such as pathogens and herbivores; see the enemy release hypothesis) or facilitate increases in the abundance of invasive populations in the exotic range (e.g., acquired novel mutualists such as mycorrhizal fungi; see the mutualist facilitation hypothesis). They studied six grassland plant species Bromus tectorum (cheatgrass), Poa bulbosa (bulbous bluegrass), Carduus nutans (musk thistle), Potentilla recta (sulphur cinquefoil), Hypericum perforatum (St. John’s wort), and Rumex acetosella (red sorrel) in their native (Turkey) and exotic ranges (Montana, USA). They measured plant size, fecundity, recruitment, abundance, invader impact, soil properties and root associations with putative fungal mutualists and pathogens. The three biogeographical study approaches produced mixed findings (positive, neutral, and negative) regarding individual plant performance, population dynamics, and community impacts of the six invasive plant species in their native and exotic ranges (Figure 2). Directed surveys generated consistent patterns on plant performance and plant-fungal interactions, while in situ field experiments controlled for bias and confounding factors that were linked to directed surveys and provided rare information on recruitment and disturbance effects. On the other hand, data from randomized surveys provided robust predictive understanding of invasive plant population dynamics and impact, although the method did not produce robust performance metrics for species that were rare or flowered outside the peak sampling window. Overall, these findings show that whereas each of the three methods had its strengths and weaknesses, deploying all of them in the same study system can generate robust data on biogeographic differences in traits, performances, and impacts of invasive plant species. Future biogeographical comparisons of invasive plant species that combine these three study approaches are likely to yield greater understanding of the ecological and evolutionary mechanisms that underlie impacts of invasive plant species.