Northern Prairie Wildlife Research Center
At what point does a population reach the point of no return? "The
combined effect of isolation and reduction of numbers is typically a decline
in population viability and an increase in the probability of extinction"
(Brussard 1985:21). Small isolated populations are always at some risk of
extinction from various random (or stochastic) events that affect their numbers
(demography) and genetic composition. These events could include catastrophic
climatic occurrences (floods, hailstorms) or the chance arrival of an efficient
predator. Factors affecting a population's genetic composition include inbreeding
depression (the expression of deleterious genes as a result of the mating
of close relatives) or genetic drift (the random loss of alleles) (Broussard
One of the first clues to a decline in population viability is lowered
fertility rates of eggs and/or adults. In 1985, Westemeier (1985:31) reported
a fertility rate of 95% in Illinois and "no indications of down trends
in either clutch size or egg fertility over the span of 23 years." In
1990, with a spring population of only 76 birds, in 3 populations, Westemeier
(1990) reported a fertility rate of 74% and hatch rate of only 38%,
and expressed concern about the future of the species in that state. He stated,
"There seems to be general agreement among biologists that such declines
in fecundity are classic symptoms of inbreeding depression" (Westemeier
1990:4). The population dropped to 40-50 birds in 1992-93. Their first action
was to transfer eggs between the 3 isolated Illinois flocks and then in 1992
transplanted birds from Kansas and Minnesota to supplement genetically and
demographically (numbers), the local population (Westemeier et al. 1995).
The rescue project can be termed a guarded success for now since the population
in the release area has gone from 7-8 cocks in 1 group in the spring of 1994
to about 60 cocks on 4 well-established grounds in 1995, mostly as the result
The Attwater's prairie chicken in Texas
The Attwater's prairie chicken has declined from a rangewide total
of 1620 in 1984 to 444 in 1993 (Labuda and Morrow 1993) to currently (1996)
only 19-21 males left in the wild (Nova Silvy, personal commun.). A
number of problems have possibly combined to reduce Attwater's chickens to
this perilous level: habitat fragmentation, catastrophic weather events (e.g.
flooding), low insect numbers in brood habitats, disease, and inbreeding depression.
Of these factors, the decline in genetic variability does not seem to be the
major threat, although it has been reduced (Osterndorff et al. 1995).
Efforts are underway to supplement the wild population with captive-reared
birds (Smith 1995).
The demise of the heath hen, the eastern subspecies of prairie chickens,
will never be completely understood but a combination of habitat loss due
to human habitation and fire control, feral cats and dogs, and catastrophic
fire were probably all involved (Hamerstrom 1982). Hamerstrom (1982:39) citing
Gross noted, that in 1906, the number was estimated at 80 then increased "to
about 2,000 in 1915-16 followed by a nasty drop to under 200. On 12 May 1916
a wild fire burned an estimated 10% of the adult birds and all of the nests.
The population increased to the comparative safety of over 500 individuals
in 1950, declined again and never recovered." The last heath hen died
Egg hatchability at the SNG
Newell (1987:28) did not indicate egg fertility at the SNG but "of
212 eggs in 15 initial nests, 15% did not hatch" or a hatch rate
of 85%. This is identical to the hatch rate found by Svedarsky (1979)
in Minnesota where the population is considered "healthy" at least
as far as population stability. Svedarsky found egg fertility to be 92% and,
due to the similarity of hatch rates, might be assumed for the SNG.
Secure numbers at the SNG?
Absolute numbers and trends are both indicators of stability of an
isolated population. A spring census of 144 males at the SNG would not appear
to be a secure population but at least the trend is up. Ken Higgins (personal
commun.) suggests that a stable population at the SNG might be 200 males.
Toepfer et al. (1990:575) noted that, "Historical evidence indicates
that once isolated prairie grouse populations fall below 100 cocks they will
eventually disappear without habitat improvement or acquisition." Perhaps
100 cocks at the SNG could be considered the emergency or "red flag"
level if their long-term survival is considered a priority.