4 April 2002
Asymmetrical Birds At Risk Of Extinction
by Kate Melville
If conservationists could foretell the future, they'd want to know which animal populations are about to decline. New research shows that monitoring for subtle asymmetries -- such as differences in bone length in the right and left feet -- may do the trick in birds. This work suggests that "asymmetry can serve as an early warning system in conservation," say Luc Lens of the University of Antwerp in Wilrijk, Belgium, and two co-authors in the April issue of Conservation Biology.
The idea is that environmental stress during development can cause small left-right asymmetries that are evident before a population actually begins to decline. While this would be a great conservation tool, previous studies have been ambiguous.
To determine whether there is a correlation between asymmetry and vulnerability, Lens and his colleagues both measured the right and left footbones of 260 Taita thrushes, and estimated the birds' survival probability.
These shy orange-brown-and-white birds are down to about 1,400 individuals and are found only in three forest fragments (Mbololo, Ngangao and Chawia) in southeastern Kenya's Taita Hills. Forest degradation is lowest in Mbololo, moderate in Ngangao and highest in Chawia. The Taita Hills are part of the Eastern Arc Mountain, which is considered among the 17 most endangered biodiversity hotspots worldwide.
The results suggest that asymmetry is evident in Taita thrush populations before they begin to decline, suggesting that it can be used to predict vulnerability to extinction. Lens and his colleagues found that thrushes in the more disturbed fragments had more asymmetry. Specifically, asymmetry was 2.5 times higher in the moderately disturbed fragment and 8 times higher in the most disturbed fragment than in the least disturbed fragment. However, only thrushes in the most disturbed fragment also had a lower survival rate; the survival rate of thrushes in the moderately disturbed fragment was the same as that in the least disturbed fragment.
"Higher levels of habitat disturbance were reflected by increased population levels of asymmetry before a decrease in survival became apparent," conclude the researchers.