Florian Hartig

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Comment on “Plant diversity increases with the strength of negative density dependence at the global scale”

Lisa Hülsmann & Florian Hartig

Science, 360(6391), eaar2435 (2018)
Cite this
@article{hulsmann2018comment,
  author = {Lisa Hülsmann and Florian Hartig},
  title = {Comment on “Plant diversity increases with the strength of negative density dependence at the global scale”},
  journal = {Science},
  volume = {360},
  number = {6391},
  pages = {eaar2435},
  year = {2018},
  doi = {10.1126/science.aar2435},
}

DOI: 10.1126/science.aar2435
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Abstract

LaManna et al. (Reports, 30 June 2017, p. 1389) claim that subadult trees are proportionally less common at high conspecific adult density (CNDD) and that this effect increases toward the tropics and for rare species. We show that the CNDD-abundance correlation may have arisen from a methodological artifact and that a range of processes can explain the reported latitudinal pattern.

What the paper shows and why it matters

(AI-generated)

Conspecific negative density dependence (CNDD) means that a tree species does worse where its own adults are common. It has long been discussed as a mechanism that keeps many tree species together. LaManna et al. (2017) tested it in 24 forest plots. In small quadrats they related recruits (young trees) to adults of the same species and reported that the recruit/adult ratio falls more steeply with adult density towards the tropics and for rare species. They checked for bias with simulations and null models (simulated data without CNDD), but their method assumes that, without CNDD, recruit and adult numbers are proportional, that is, that young trees stay spatially coupled to adults. The comment examines this assumption.

The comment argues that the stronger negative density dependence reported by LaManna et al. (2017) towards the tropics and for rare species may be an artefact of how it was estimated. In simulations without density dependence, seed dispersal, adult mortality or habitat preferences broke the coupling between recruits and adults and produced density-dependence estimates and a correlation with rarity compatible with those reported. The comment traces part of this to quadrats with recruits but no adults: one model assigned them an artificial background adult density, the other added 1 to every count, and both distort recruit/adult ratios, most strongly for rare species. Allowing each species its own baseline ratio removed the correlation with rarity, in simulated data and in the Barro Colorado Island plot. The original null models were too limited to rule this out: they included only dispersal, at unrealistically low levels.

The comment concludes that the reported patterns are not strong evidence for density dependence that increases towards the tropics or with rarity, although this does not prove such patterns absent. It regards it as far more likely that other processes, possibly together with CNDD but not primarily driven by it, are responsible, for example weak spatial association between adults and recruits in the tropics (which breaks the assumed proportionality) or latitudinal differences in tree size structure (not examined further, as the original data were not fully available). Because a reliable correction of the original analysis seems nearly impossible, it holds that analysing density dependence in the growth and mortality of saplings, less affected by dispersal, seems far more promising.

References

  • 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