Native communities are increasingly subjected to multiple non-native plant invaders but interactions among these species are not well understood. One invader may create conditions that facilitate other invaders, which ultimately could lead to a suite of invasive species replacing the native community (i.e., invasional meltdown). In addition, climate change may stress native communities, reduce biotic resistance, and promote establishment of and facilitation among plant invaders. The southeastern US in particular is subjected to multiple invasive plant species and is expected to experience climate change conditions in the form of greater drought frequency and severity in coming years. Here we sought to determine if cogongrass (Imperata cylindrica), a highly problematic invasive grass in the southeastern US, facilitated other invaders under drought conditions. Performance and survival of three secondary invaders, lantana (Lantana camara), Chinese tallow (Triadica sebifera), and Brazilian pepper (Schinus terebinthifolius), was evaluated in common garden plots that were treated with a factorial combination of cogongrass invasion and reduced rainfall compared to controls. We measured seedling survival, plant performance (height, diameter, biomass), and environmental conditions, including soil moisture and light availability, over the 2016 growing season.
Results/Conclusions
For all secondary invader species, plant performance was lowest in uninvaded plots with ambient rainfall, and lantana in control plots did not reach reproductive maturity, suggesting some degree of biotic resistance by native communities. Across all species, rodent herbivory was 130% higher in invaded than uninvaded plots, resulting in 10-40% lower survival among invaders. Lantana was most affected, with herbivory rates over 200% greater in invaded plots. Chinese tallow experienced 40% greater herbivory and 74% lower survival in invaded plots. However, invasion resulted in four times greater average secondary invader biomass of surviving individuals relative to uninvaded controls, regardless of drought treatment. Invaded plots had 31% shorter vegetation due to suppression of taller native herbaceous species. Thus, facilitation of secondary invaders by cogongrass is likely driven by greater light availability for secondary invaders once they breach the cogongrass canopy. Drought resulted in greater secondary invader biomass relative to ambient rainfall. For example, lantana had four times greater biomass under drought and twice as many lantana shrubs flowered in uninvaded drought than controls. Our results demonstrate that climate change and cogongrass invasion each facilitated secondary invaders, reaffirming calls for the removal of invasive species and mitigation of climate change conditions.