Abstract
Plants and their environments engage in feedback loops that not only affect individuals, but also scale up to the ecosystem level. Community-level negative feedback facilitates local diversity, while the ability of plants to engineer ecosystem-wide conditions for their own benefit enhances local dominance. Here, we suggest that local and regional processes influencing diversity are inherently correlated: community-level negative feedback predominates among large species pools formed under historically common conditions; ecosystem-level positive feedback is most apparent in historically restricted habitats. Given enough time and space, evolutionary processes should lead to transitions between systems dominated by positive and negative feedbacks: species-poor systems should become richer due to diversification of dominants and adaptation of subordinates; however, new monodominants may emerge due to migration or new adaptations.
Distinguishing local and regional processes that underpin biodiversity is complex because they are inherently correlated: positive ecosystem engineers tend to be members of low-diversity species pools; plants exhibiting small-scale negative feedback tend to be members of diverse pools.Observed communities represent a gradient of transition between diverse, negative feedback-driven and species-poor, positive feedback-driven systems. Over biogeographical timescales, poor systems tend to diversify, while migration or emergence of positive ecosystem engineers produces low-diversity systems.The distinction between diverse, small-scale negative feedback-driven and poor, positive ecosystem engineer-driven communities corresponds to opposing historical perceptions of how communities are organised: the former to Gleason’s individualistic communities, the latter to Clements’s holistic communities.