In a study published in Journal of Environmental Management, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences showed that water availability combined with herbivory or phosphorus limitation created an advantage for alien plant species over natives.

The researchers conducted a large-scale common garden experiment in XTBG, using two invasive plant species against five co-occurring native species. They grew the plants alone, with intraspecific competition or with interspecific competition, and exposed the plants to three stressors (drought, phosphorous limitation and herbivory).

They found that water availability was the key factor. The invasive plants significantly outperformed native species in competitive interactions under well-watered conditions. Under well-watered conditions, invasive plants significantly outperformed native species in competitive interactions.

The competitive advantages of invasive plants was dramatically boosted  under conditions of high water availability. The advantages disappeared under drought stress. Crucially, common stressors like nutrient limitations or being eaten by herbivores didn't necessarily hinder them. When combined with ample water, these stressors actually widened the performance gap between invaders and natives

The researchers also found evidence that the presence of non-specific herbivores (snails that ate both native and invasive plants) reduced the invaders' advantage linked to escaping natural enemies, but this was overridden by the strong positive effect of water combined with herbivory pressure.

“Our results suggest that non-limiting water conditions, especially in combination with phosphorus limitation or herbivore presence, may allow the successful invasion of alien species. Dry season might be an effective period for removing invasive species” said ZHENG Yulong of XTBG.