Xinmin Lu, Huazhong Agricultural University, Joshua Lynn, Manchester University, and Evan Siemann, Rice University, discuss their article: Latitudinal patterns in growth and defense against multiple enemies of an invasive species and its native congener
Why are some plant species more successful invaders than others? A long-standing hypothesis is that invasive plants differ from native species in how they balance growth and defense against natural enemies. However, both growth and defense vary across broad geographic gradients, where climate, soil properties, herbivores, pathogens, and mutualists all change simultaneously. Despite decades of research, we still know surprisingly little about how these interacting factors shape plant traits across large spatial scales.
An additional challenge is that plants rarely contend with a single enemy. Instead, they are simultaneously attacked by insect herbivores, plant pathogens, and belowground parasites such as root-knot nematodes, while also interacting with beneficial soil microbes. Most studies, however, have focused on defense against aboveground herbivores, leaving it unclear whether defenses against multiple enemies follow similar or contrasting patterns across biogeographic gradients.
Our study focused on the invasive plant, Alternanthera philoxeroides and its widespread native congener, A. sessilis. Comparing these closely related species across broad latitudinal gradients allowed us to investigate whether native and invasive plants respond similarly to changing environmental conditions.
What did we do?
We combined a large-scale field survey spanning approximately 1,700 km across China with common garden experiments. Across dozens of populations of both species, we quantified aboveground herbivore communities, rhizosphere fungal communities, foliar herbivory, disease incidence, soil properties, and climate. We then grew plants from approximately 50 populations of each species under common conditions to determine whether populations differed genetically in growth and resistance to three major groups of natural enemies: insect herbivores, root-knot nematodes, and foliar fungal pathogens. Finally, we used structural equation modeling (SEM) to distinguish the direct effects of climate from its indirect effects mediated by above- and belowground biotic communities.
What did we find?
In the field, foliar herbivory and pathogen infection peaked at intermediate latitudes, whereas both the abundance and richness of herbivores, and the richness of total soil fungi, fungal pathogens, and mutualistic arbuscular mycorrhizal fungi (AMF), declined towards higher latitudes. These patterns demonstrate that different components of above- and belowground biotic communities can exhibit distinct biogeographic patterns.
Under common garden conditions, growth declined with increasing source population latitude in the native species but showed little latitudinal change in the invasive species. Consequently, native populations from lower latitudes grew faster than the invasive species, whereas populations from higher latitudes grew more slowly. Resistance of the native species to the dominant insect herbivore and root-knot nematode increased with source latitude, whereas the invasive species showed little evidence of latitudinal differentiation in either defense. This divergence resulted in stronger differences in resistance between species at lower latitudes than at higher latitudes. In contrast, resistance to a common foliar fungal pathogen did not vary with source latitude in either species.
Structural equation modeling further showed that climate influenced plant defense both directly and indirectly by altering above- and belowground communities. These findings highlight the importance of considering multiple interacting biotic and abiotic drivers when studying plant defense across broad geographic scales.
Why does this matter?
Our study demonstrates that closely related native and invasive species respond differently to the same environmental gradients. Perhaps more importantly, defenses against different natural enemies did not change in parallel but reflected the combined effects of climate together with above- and belowground biotic interactions.
These findings suggest that understanding geographic variation in plant traits, and its contribution to invasion success, requires considering more than interactions with aboveground herbivores. Our study highlights the importance of considering climate together with the range of above- and belowground organisms that interact with plants to understand biogeographic patterns of growth and defense. This broader perspective may also improve our understanding of why some plant species become successful invaders.
