Florian Hartig

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Is Variation in Conspecific Negative Density Dependence Driving Tree Diversity Patterns at Large Scales?

Lisa Hülsmann, Ryan A. Chisholm & Florian Hartig

Trends in Ecology & Evolution, 36(2), 151–163 (2021)
Cite this
@article{hulsmann2021variation,
  author = {Lisa Hülsmann and Ryan A. Chisholm and Florian Hartig},
  title = {Is Variation in Conspecific Negative Density Dependence Driving Tree Diversity Patterns at Large Scales?},
  journal = {Trends in Ecology \& Evolution},
  volume = {36},
  number = {2},
  pages = {151--163},
  year = {2021},
  doi = {10.1016/j.tree.2020.10.003},
}

DOI: 10.1016/j.tree.2020.10.003
Citation statistics: 100 (Google Scholar) · 79 (OpenAlex), as of 29 October 2026

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Funding

This study was supported through funding by James S. McDonnell Foundation.

View article (DOI)

Abstract

Half a century ago, Janzen and Connell hypothesized that the high tree species diversity in tropical forests is maintained by specialized natural enemies. Along with other mechanisms, these can cause conspecific negative density dependence (CNDD) and thus maintain species diversity. Numerous studies have measured proxies of CNDD worldwide, but doubt about its relative importance remains. We find ample evidence for CNDD in local populations, but methodological limitations make it difficult to assess if CNDD scales up to control community diversity and thereby local and global biodiversity patterns. A combination of more robust statistical methods, new study designs, and eco-evolutionary models are needed to provide a more definite evaluation of the importance of CNDD for geographic variation in plant species diversity.

What the paper shows and why it matters

(AI-generated)

Janzen and Connell proposed that enemies keep tree species from becoming too common, maintaining tropical forest diversity. The hypothesis is tied to conspecific negative density dependence (CNDD): a species does worse where its own kind is dense, which is stabilising if stronger than the effect of other species’ density. It implies that geographic variation in CNDD helps explain patterns such as the latitudinal diversity gradient. Most studies measure local density effects on one life stage, yet much of the literature equates these effects, the original enemy-based mechanism and community stabilisation. The paper asks whether locally measured CNDD scales up to community diversity and geographic patterns.

The paper examines the Janzen–Connell hypothesis and argues that the evidence supports local conspecific negative density dependence (CNDD), but not yet its large-scale consequences. It splits the hypothesis into four claims: CNDD exists; it exceeds density dependence between species; it stabilises the community; and it varies along gradients and drives diversity patterns. Evidence is ample for the first claim, weaker but convincing for the second, and much weaker for the last two: the paper finds no compelling evidence that CNDD causally shapes large-scale tree diversity. A main reason is regression dilution: studies that compare young with adult trees and take proportionality as the no-CNDD null count fewer recruits (young trees) per adult as CNDD, but adult density measured with error flattens the fitted slope and mimics this signature, as a simulation adapted from Detto et al. (2019) shows. Simulations show such biases, with confounding by environment and dispersal, can produce regional and global patterns of CNDD strength reported in the criticised studies of Johnson et al. (2012) and LaManna et al. (2017), even without CNDD.

The paper regards the Janzen–Connell hypothesis as unresolved, although seedlings and saplings of many tree species likely experience stabilising CNDD (stronger than between-species density dependence). Its roadmap asks for unbiased CNDD estimates tested against density dependence between species as the null; standardised observational cross-site studies rather than meta-analyses, or, more ambitiously, a coordinated global experiment; and models linking local CNDD to regional diversity. A later study by a larger group including the paper’s authors (Hülsmann et al. 2024) estimated CNDD at 23 forest sites from mortality data, a design in which regression dilution can only bias estimates conservatively.

References

  • Detto et al. (2019). Bias in the detection of negative density dependence in plant communities. Ecology Letters 22, 1923–1939. doi:10.1111/ele.13372
  • Johnson et al. (2012). Conspecific Negative Density Dependence and Forest Diversity. Science 336, 904–907. doi:10.1126/science.1220269
  • LaManna et al. (2017). Plant diversity increases with the strength of negative density dependence at the global scale. Science 356, 1389–1392. doi:10.1126/science.aam5678
  • Hülsmann et al. (2024). Latitudinal patterns in stabilizing density dependence of forest communities. Nature 627, 564–571. doi:10.1038/s41586-024-07118-4