Enemy-mediated negative feedbacks trump competitive interactions as stabilizing forces that maintain the diversity of prairie ecosystems
Interactions between plants and their soil-borne enemies are hypothesized to be strong stabilizing forces that maintain species diversity and determine relative species abundance. However, direct competitive interactions may be equally important to the assembly of plant communities. In this study, we tested the relative importance of plant-microbial interactions versus direct competitive interactions as mechanisms for the maintenance of diversity in a prairie ecosystem. Six species of co-occurring prairie grasses and forbs were first grown as monocultures with live or sterilized soil biota for six months. Next, in a fully reciprocal feedback experiment, we assessed the performance of seedlings when invading either conspecific monocultures or each of five heterospecific monocultures, in the presence or absence of live soil biota.
We found strong evidence for system-wide negative feedback, but only when live soil microbes remained in the soil (F = 4.54, P = 0.007). Five out of the six species exhibited significant negative feedback responses; seedlings of these species performed more poorly when invading conspecific monocultures than when invading heterospecific monocultures. When soil organisms were eliminated, the strength of system-wide negative feedback diminished (F = 2.85, P = 0.0387); seedling growth of none of our prairie species were suppressed when grown in their own communities, suggesting that intraspecific competition was weak. Instead, we found strong support that negative interactions between plants and their specialized soil microbes dominated and are likely sufficient to maintain local diversity.