title: Testing the Janzen-Connell mechanism: pathogens cause overcompensating density dependence in a tropical tree.
abstract: The Janzen-Connell hypothesis is a leading explanation for plant-species diversity in tropical forests. It suggests that specialized natural enemies decrease offspring survival at high densities beneath parents, giving locally rarer species an advantage. This mechanism, in its original form, assumes that density dependence is overcompensating: mortality must be disproportionately high at the highest densities, with few offspring recruiting below their parents. We tested this assumption using parallel shadehouse and field density-series experiments on seedlings of a tropical tree, Pleradenophora longicuspis. We found strong, overcompensating mortality driven by fungal pathogens, causing 90% (shadehouse) or 100% (field) mortality within 4 weeks of germination, and generating a negative relationship between initial and final seedling densities. Fungicide treatment led to much lower, density-independent, mortality. Overcompensating mortality was extremely rapid, and could be missed without detailed monitoring. Such dynamics may prevent dead trees from being replaced by conspecifics, promoting coexistence as envisioned by the Janzen-Connell hypothesis.
authors: Department of Zoology, University of Oxford, Oxford, OX1 3PS, UK Department of Animal and Plant Science, University of Sheffield, Sheffield S102TN, UK. bagchi.r@gmail.com
authors: Robert
authors: R
authors: Bagchi
authors: Tom
authors: T
authors: Swinfield
authors: Rachel E
authors: RE
authors: Gallery
authors: Owen T
authors: OT
authors: Lewis
authors: Sofia
authors: S
authors: Gripenberg
authors: Lakshmi
authors: L
authors: Narayan
authors: Robert P
authors: RP
authors: Freckleton
mesh_terms: D044822:Biodiversity; D019665:Euphorbiaceae / Q000187:drug effects / Q000254:growth & development* / Q000382:microbiology; D005658:Fungi / Q000502:physiology; D005659:Fungicides, Industrial / Q000494:pharmacology; D011156:Population Density; D036226:Seedlings / Q000254:growth & development / Q000382:microbiology; D014329:Tropical Climate
chemical_list: D005659:Fungicides, Industrial