Conspecific negative density dependence in a temperate forest
Conspecific negative density dependence (CNDD) is a mechanism which works to maintain high plant biodiversity and can be described as a reduction in establishment, survival, and/or growth rates when conspecific densities are high. This is often due to host-specific plant enemies, such as insect herbivores and fungal pathogens, occurring in high densities at their host plant species. The Janzen-Connell hypothesis proposes that enemy-driven CNDD is what maintains the hyperdiversity of tree species in tropical forests.
Enemy-driven CNDD has been studied extensively in tropical forests, but does this also happen in temperate forests, and to what extent? With Sofia Gripenberg, I am establishing a new study system in Wytham Woods, Oxfordshire, which will parallel our work in Panama. We are particularly interested in the role of pre-dispersal seed enemies in causing CNDD and have been monitoring premature fruit drop in Crataegus monogyna in relation to the spatial distribution of individual trees. We are also rearing insects from fruits and seeds collected at Wytham to build a food web of internally feeding seed predators, which will enable us to determine the host-specificity of these plant enemies at Wytham.