Plant-soil feedback, invasion, and consequences for community assembly dynamics

Background/Question/Methods

While it is generally accepted that plant–soil feedback can affect invasion dynamics – and while the number of studies exploring these dynamics have tripled over the last five years – there is little consensus about whether negative or positive feedbacks predominate, whether these feedbacks are strong relative to other factors, and how these feedbacks change over time. One possible integrating approach is to expand on the seminal work of Bever and colleagues in 1997, viewing feedbacks during invasion in a coexistence theory framework. We focus on how niche and fitness differences between natives and invaders can be expressed in terms of feedbacks through the soil community. Using data from plant-soil feedback studies, we compare the strength and direction of conspecific and heterospecific feedbacks for both invader and native species. We synthesize these results, parameterize models of spread under different scenarios that incorporate the range of strengths and directions of feedbacks found in these studies, and compare with actual abundance patterns.

Results/Conclusions

Invasive plant species can influence the composition of the soil community in a way that results in a positive feedback and enhances their spread. However, in other cases, even the rarest native species did better in the soils of dominant invaders than in its own soil. We find that this range of feedback processes can result in several different assembly processes: rapid exotic dominance, native resistance to the potential invader, local coexistence, or a mosaic of both native- and invader-dominated patches (alternative states) with regional coexistence. One key unknown is the importance of frequency dependency in relation to other factors that affect invader establishment when rare (e.g., propagule pressure). The translation of plant-soil feedbacks measured in monocultures to effects in multi-species communities is also a key unknown.  Viewing plant-soil feedback and invasion within coexistence niche theory leads to a more general understanding of how plant-soil feedback may affect community assembly.