Measuring the demographic impact of conspecific negative density dependence
Rare plant species often suffer stronger conspecific negative density dependence (CNDD) in studies that assess the impact of local conspecific density on individual survival. All else equal, this causes a relative disadvantage among rare species that appears inconsistent with the role of CNDD in coexistence. The resolution to this apparent paradox is for lower species abundance to decrease the frequency of conspecific interactions sufficiently to outweigh the disadvantage of stronger CNDD. Whether this occurs in natural systems is untested because existing metrics do not isolate demographic impacts of CNDD, and it is also uncertain for tropical forest trees because the greater spatial aggregation observed in rare species could cause higher frequency of conspecific interactions despite lower abundance on the landscape. We develop a new metric, effective density-dependent mortality (EDDM), to quantify the proportion of individuals that are killed by density-dependent effects. We apply EDDM to a long-term study of seed fall and recruitment at Barro Colorado Island, Panama. Rare species had stronger CNDD but lower conspecific densities, and EDDM increased with abundance. Lower abundance, thus, reduces the frequency of conspecific interaction and, consequently, mortality associated with CNDD. This mechanism allows rare species to avoid a disadvantage-when-rare that would, all else equal, result from stronger CNDD in rare species. Our work provides empirical support for a resolution to the apparently paradoxical findings that rare species experience stronger CNDD and may help reconcile contrasting findings for the relationship between the CNDD strength and abundance.
KeywordsAdvantage-when-rare Coexistence Community compensatory trend Janzen–Connell hypothesis Seed dispersal
We thank Haldre Rogers, Joshua Tewksbury, Janneke Hille Ris Lambers, and Stefan Schnitzer for helpful discussion and comments on the manuscript. ECF was supported by a National Science Foundation Graduate Research Fellowship and a University of Washington Program on Climate Change Fellowship. Seed production and seedling censuses were funded by the Environmental Sciences Program of the Smithsonian Institution. The forest dynamics plot was founded by SP Hubbell and RB Foster and is now managed by R Condit, S Lao and R Perez under the Center for Tropical Forest Science and the Smithsonian Tropical Research in Panama. Numerous organizations provided funding, principally the U.S. National Science Foundation, and hundreds of field workers have contributed.
Author contribution statement
ECF conceived of the study. SJW collected field data. ECF and SJW designed the analysis and wrote the manuscript.
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