Bouzat, et al. 1997. Genetic evaluation of a demographic bottleneck in the
Greater Prairie Chicken. Conservation Biology 12:836-843.

(Tone Rawlings)

Bouzat et al. studied the association between genetic variability and the
reproductive performance resulting from bottlenecks in a natural population
of Greater Prairie Chickens. Specifically they measured the related genetic
diversity and fitness estimates of current populations with different
ecological histories in terms of demographic bottlenecks.

The Illinois population suffered an extreme demographic contraction and an
associated decline in population fitness. Bouzat et al. compared this
population with three others from Kansas, Nebraska, and Minnesota, which
have had no documented bottleneck history. The Illinois population has been
bombarded with habitat contraction as well as competition from introduced
species scince european settlement. The population has declined from
millions in the 1860s to less than 50 in 1993. The other three populations
have remained relatively populous in comparison.

They measured hatchibility rates from census data taken from prior studies.
The hatchibility rates were estimated as the mean number of hatched eggs per
total number of eggs in succesful nests. They found that hatchibility rates
in Illinois had fallen from 93% reported in 1935-1936 to 53% in 1990, in
contrast to the 83-100% in successful nests from the other populations.

They further assigned genotypes as well as determined allele frequencies
from direct counts of microsatellite data.
Genetic differentiation was estimated by Wrightís Fst and Rst indices.
Whereas gene flow, defined as the number of reproductively successful
migrants per generation (Nm), was estimated by two methods. The first was
based on the relationship of Fst (island model). The second was calculated
as Mr = (ds-1)/4ds [1/Rst ñ1], where Mr is the estimate of Nm, ds is the
number of populations, and Rst is analogous to Wrightís Fst. The above
relationship assumes that individuals disperse among subpopulations
independently of geographic distance and that population sizes have been
constant enough for migration-drift equilibrium to have been reached.

Genetic variability among the four populations was based on 6 polymorphic
microsatellite loci. Non of the six loci analyzed showed deviations from
the genotype frequencies expected according to Hardy-Weinberg equilibrium.
There was also no significant differences found in the mean heterozygosity
per locus among populations. However, the Illinois population showed the
lowest value of heterozygosity. Further, the mean number of alleles per
locus in the Illinois population was significantly lower. These results
thus support the theory that allelic diversity is affected more than
heterozygosity following population bottlenecks.

At each locus there were alleles present that were shared among each
population, suggesting that all populations were originally part of an
ancestral population. However, present estimates of gene differentiation
indicated relatively low levels of gene flow.

Bouzat et al. argued that although the studied populations were
geographically disjunct (some variability may be caused by environmental
factors) the fact that none of the three other populations showed declines
in reproductive success, while the Illinois population did, lends support to
the idea of fitness reduction as a result of a bottleneck. Further they
argued that the introduction of birds from the three other populations has
lead to the increase in hatching rate to pre-bottleneck levels. The
authors felt that this study provided evidence that evaluating genetic
information and translocation as management practices may be important to
the recovery of populations that have suffered from a decline in population
fitness due to demographic contraction.