Testing the Janzen-Connell mechanism: pathogens cause overcompensating density dependence in a tropical tree.
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.
Bagchi
Robert
R
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
Swinfield
Tom
T
Gallery
Rachel E
RE
Lewis
Owen T
OT
Gripenberg
Sofia
S
Narayan
Lakshmi
L
Freckleton
Robert P
RP
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
D005659:Fungicides, Industrial