PARENT SESSION
Oral Session #56: Spatial Ecology: Pattern,movement,dispersal.
Presiding: G. Hartvigsen
Wednesday, August 7. 8:00 AM to 11:30 AM. Grand Ballroom West, Radisson.
Revisiting the Janzen-Connell hypothesis: Localized natural enemies are less effective at maintaining diversity than widely dispersing enemies .
ADLER, FREDERICK*,1, MULLER-LANDAU, HELENE2, 1 University of Utah, Salt Lake City, UT2 National Center for Ecological Analysis and Synthesis, Santa Barbara, CA
ABSTRACT- The Janzen-Connell hypothesis states that local negative density-dependence, mediated through seed or seedling predators that respond either to proximity of adults or to density of seeds or seedlings, can facilitate coexistence of numerous species. It is generally thought that the concentration of natural enemies and their depredations around adults is key to the diversity-enhancing impact. Using spatially explicit simulation models and analytical approximations, we show that natural enemies facilitate coexistence of more species when they act in a less concentrated manner over larger spatial scales, assuming overall predation rates are held constant. The link between localized attack and high diversity thus depends on a stronger functional or numerical response by predators that act over small spatial scales. We develop a "Janzen-Connell index" to summarize the spatial scales of dispersal and density-dependence and predict the degree of coexistence. This index provides the appropriate spatial statistics for comparing spatial patterns of surviving populations and species number across different ecosystems.
KEY WORDS: diversity, janzen-connell hypothesis, spatial statistics