Is variation in conspecific negative density dependence driving tree diversity patterns at large scales?
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)
Does the strength of conspecific negative density dependence vary enough across species and regions to actually explain why tree diversity is higher in the tropics? This review finds solid evidence that CNDD operates within local populations almost everywhere, but concludes the methods available at the time couldn't yet test whether it scales up to shape diversity gradients at landscape or continental scale — the exact claim much of the field had been assuming rather than testing. The gap it identified was closed three years later by the authors' own follow-up Nature paper, which tested the scaling question directly across 23 forest sites.