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Integrated Management of the Greater Prairie Chicken and Livestock on the Sheyenne National Grassland

Minimum Viable Populations


The effects of European settlement on the North American continent, and elsewhere, has been to greatly modify the landscape; improving conditions for some species and diminishing it for others. The greater prairie chicken was initially benefited by the settlement of the prairie but land use intensified to the point where many segments of the population have become isolated as well as deficient in habitat components.

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 1985).

Illinois

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 of translocations.

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).

Heath hens

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 in 1931.

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.

Summary

  1. Small, isolated populations are always at risk due to chance occurrences and reduced genetic variability leading to inbreeding depression.

  2. A hatch rate of 38% was considered the crisis level in Illinois and stimulated actions to transfer eggs between the population segments and translocate birds from Minnesota and Kansas. For now, their population is maintaining, with the aid of translocated birds.

  3. Hatch rates of 85% (in 1983-84) were documented for SNG which is identical to Minnesota (1975-77) where populations are relatively stable. This suggests that the SNG genetic composition was favorable at the time data were collected; however this could change rapidly as occurred in Illionis.

  4. The Attwater's prairie chicken seems to exemplify the problems of small numbers of a population in isolated segments. At present, it is projected to become extinct in the wild without supplementation of captive-reared birds.

  5. Historical evidence suggests that 100 cocks is the critical lower limit of a prairie grouse population in order to have a reasonable chance of survival.

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