Annals of Botany Lecture - Dr. Stephen Hubbell
Hubbell, Stephen .
The Enemy Susceptibility Hypothesis for Tree Species Commonness and Rarity in Tropical Forests.
In tropical forests, a large number of tree species are exceedingly rare. For example, more that half of the 6,000+ species in the global network of plots of the Center for Tropical Forest Science (CTFS) are so rare that collectively they constitute <1% of the 3,000,000+ individuals in these plots. Explaining tropical tree rarity is thus a central issue in explaining alpha diversity—local tree species richness—in tropical forests. I review existing hypotheses and then outline a new hypothesis, the Enemy Susceptibility Hypothesis (ESH), to explain commonness and rarity in tropical tree species. This hypothesis arises from the discovery of coupled population dynamics on landscape scales in tree species in the 50 ha plot on Barro Colorado Island (BCI), Panama. Intraspecific spatial autocorrelations in the intrinsic rates of increase r become strongly negative on spatial scales >500m in many rare species, but not in common species. The negative spatial autocorrelations imply that populations of many BCI tree species, especially rare species, never equilibrate, but continuously wax and wane out of phase on spatial scales on the order of square kilometers or larger. The mechanism driving these large scale anti-phase population fluctuations is currently unknown. The ESH is that they are driven by time-lagged spatial predator-prey dynamics of slow-to-kill, heart- and root-rot fungi that take decades to infect trees and several more decades to kill them. According to the ESH, rare species are rare because they are more susceptible to infection and death from fungal attack than common species. Thus, local uninfected stands of a focal rare species established by seed dispersal at some distance from infected stands do not have as long as common species to build up locally high densities before they are infected and knocked down again. If true, the ESH has important implications for theories of the assembly and maintenance of diversity in tropical forests and strategies for conservation.
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1 - University of California, Los Angeles, Department of Ecology and Evolutionary Biology, 621 Charles E. Young Drive, Los Angeles, CA, 90095-1601, USA
Presentation Type: Special Presentation
Location: Cottonwood C/Snowbird Center
Date: Tuesday, July 28th, 2009
Time: 11:00 AM