Consequences of Local Conspecific Density Effects for Plant Diversity and Community Dynamics
Abstract
Conspecific density dependence (CDD) in plant populations is widespread, most likely caused by local‐scale biotic interactions, and has potentially important implications for biodiversity, community composition, and ecosystem processes. However, progress in this important area of ecology has been hindered by differing viewpoints on CDD across subfields in ecology, lack of synthesis across CDD‐related frameworks, and misunderstandings about how empirical measurements of local CDD fit within the context of broader ecological theories on community assembly and diversity maintenance. Here, we propose a conceptual synthesis of local‐scale CDD and its causes, including species‐specific antagonistic and mutualistic interactions. First, we compare and clarify different uses of CDD and related concepts across subfields within ecology. We suggest the use of local stabilizing/destabilizing CDD to refer to the scenario where local conspecific density effects are more negative/positive than heterospecific effects. Second, we discuss different mechanisms for local stabilizing and destabilizing CDD, how those mechanisms are interrelated, and how they cut across several fields of study within ecology. Third, we place local stabilizing/destabilizing CDD within the context of broader ecological theories and discuss implications and challenges related to scaling up the effects of local CDD on populations, communities, and metacommunities. The ultimate goal of this synthesis is to provide a conceptual roadmap for researchers studying local CDD and its implications for population and community dynamics.
What the paper shows and why it matters
(AI-generated)
Young plants often survive less well when crowded by their own species than by others. This “conspecific density dependence” (CDD) is read as one way species diversity is maintained: a common species is held back by enemies or competition that act mainly on it, which leaves room for rarer ones. The reading is reasonable. But the research is split across fields that use the term for different things and study different mechanisms, and a negative effect of local crowding on an individual’s survival or growth has often been taken to mean that diversity is stabilised across a whole community. How such local measurements scale up to populations, communities and metacommunities (linked communities) remained unclear.
This synthesis of conspecific density dependence argues that a negative effect of same-species neighbours on a plant’s survival or growth says little about diversity unless it is compared with the effect of neighbours of other species, and that even then a local signal does not show that a population is regulated or that species coexist. It suggests using “stabilising CDD” (the term of Hülsmann et al. 2024) when conspecific effects are more negative than heterospecific ones (those of other species) and “destabilising CDD” when they are more positive. Mechanisms other than host-specific enemies can produce such a signal: mutualists such as mycorrhizal fungi, competition and plants altering their own soil can push it either way. Habitat preferences and dispersal can create patterns that look like CDD. An effect in one life stage can be cancelled by others, so it need not appear in population growth. And even stabilising CDD supports coexistence only if it outweighs the intrinsic fitness differences among species, in line with Broekman et al. (2019). In practice, most predictions tested up to then concerned individual plants rather than populations or communities.
The paper offers a shared vocabulary and a research agenda rather than new data. It asks that studies compare conspecific effects with heterospecific ones before calling a pattern stabilising, combine life stages in population models, weigh stabilising CDD against fitness differences, and test predictions at population and community scales. Scale-transition theory, which links local, nonlinear processes to larger-scale dynamics, is suggested for the last step. Whether local CDD maintains diversity is a question for multiscale analysis, not for a local estimate.
References
- Hülsmann et al. (2024). Latitudinal patterns in stabilizing density dependence of forest communities. Nature 627, 564–571. doi:10.1038/s41586-024-07118-4
- Broekman et al. (2019). Signs of stabilisation and stable coexistence. Ecology Letters 22, 1957–1975. doi:10.1111/ele.13349