The prairie chicken is a medium-sized grouse that inhabits the tallgrass prairies of central North America. There are two subspecies: the greater prairie chicken (Tympanuchus cupido pinnatus) and the lesser prairie chicken (Tympanuchus pallidicinctus). The lesser prairie chicken’s range extends across the shortgrass prairies from southeastern Colorado through western Kansas, eastern New Mexico, western Oklahoma, and into the Texas Panhandle. The greater prairie chicken historically occupied the tallgrass prairies from southern Manitoba through the Dakotas, eastern Montana, Nebraska, Kansas, Oklahoma, northern Texas, Iowa, Missouri, Illinois, Indiana, and eastern Kentucky and Tennessee.
However, due to habitat loss and overhunting, the greater prairie chicken’s range has contracted significantly. By the early 1900s, the only remaining viable populations were in Nebraska, Kansas, Oklahoma, Minnesota, Wisconsin, and Illinois. The Illinois population was once widespread throughout the prairies of the northern and central parts of the state. However, by the late 1800s, its range had contracted to a few isolated pockets in the east central counties of Jasper, Marion, Fayette, Clay, Effingham, Richland, and Wayne.
This isolation and reduction of the Illinois birds represents a classic genetic bottleneck scenario. A bottleneck occurs when a population undergoes a sharp reduction in size due to environmental events or human activities. The surviving individuals represent only a fraction of the original genetic diversity of the population. The Illinois prairie chickens went through a prolonged bottleneck starting in the late 1800s, which likely led to reduced genetic variation within the isolated Illinois birds compared to prairie chicken populations elsewhere in their range.
Causes of the Bottleneck
The prairie chicken decline in Illinois was driven by two key factors: habitat loss and overhunting.
Habitat Loss
The tallgrass prairie ecosystem once covered over 60% of Illinois prior to European settlement. However, the rich soils of the prairies attracted agricultural conversion starting in the early 1800s. The development of steel plows enabled settlers to break up the dense root mats that underlay the tallgrass prairies. Farmers rapidly converted millions of acres of Illinois prairie to row crop agricultural production. By the early 1900s, over 99% of the original tallgrass prairie in Illinois had been converted to farmland.
This massive habitat loss decimated prairie chicken numbers in Illinois. The birds require large expanses of prairie for breeding displays on communal grounds called leks. They also need a diverse native prairie plant community to provide food and cover throughout the year. The extreme fragmentation of their preferred habitat left only tiny isolated pockets of prairie where prairie chickens could continue to survive in Illinois.
Overhunting
The prairie chicken was also subjected to heavy hunting pressure in the late 1800s. The birds were prized by market hunters for their meat and were easily harvested on the communal leks during the spring breeding season. Unregulated hunting led to overharvest that further suppressed the declining Illinois population.
The combined effects of habitat loss and overhunting led to the contraction of the greater prairie chicken’s Illinois range to just a few counties by 1900. Populations declined from hundreds of thousands of birds to less than a thousand individuals. This represents a greater than 99% reduction in population size – a classic bottleneck scenario.
Genetic Consequences of the Bottleneck
The isolated Illinois birds likely endured significant genetic consequences in the decades after this prolonged population bottleneck. When an abundant population rapidly declines to just a small number of survivors, those remaining birds represent a tiny fraction of the original genetic diversity.
Three key genetic processes occur after a bottleneck:
Founder Effect
The prairie chicken flocks that survived in isolated Illinois prairie fragments each trace to a small number of founding birds. The founders bring only a portion of the original genetic diversity with them. As result, the isolated flocks likely experienced a founder effect – reduced genetic variation due to small founding population size.
Genetic Drift
In tiny populations, the frequencies of different genes can drift up or down dramatically by chance alone. Rare genetic variants can become more common or be lost entirely. These random fluctuations in gene frequencies were likely accelerated in the tiny Illinois prairie chicken flocks, leading to significant genetic drift.
Inbreeding
Small isolated populations also tend to suffer from increased inbreeding as related individuals end up mating more frequently. This increases homozygosity, or the proportion genes that are inherited from the same ancestor on both chromosomal copies. Excessive homozygosity causes normally rare deleterious recessive mutations to be expressed, lowering fitness – a process called inbreeding depression. The isolated Illinois birds likely experienced substantial inbreeding.
Together, these three genetic mechanisms likely led to the Illinois prairie chickens having much lower genetic diversity than populations elsewhere in the greater prairie chicken’s range. The birds were likely more prone to inbreeding depression and had a depleted gene pool to adapt to future environmental changes.
Evidence of the Genetic Bottleneck
Researchers have used genetic techniques to demonstrate that the Illinois prairie chickens did indeed go through a substantial genetic bottleneck. For example, Bouzat et al. (1998) examined genetic diversity at nine microsatellite loci in the Illinois birds. They found the isolated Illinois flocks had greatly reduced genetic diversity compared to Nebraska prairie chickens from a much larger population. The Illinois flocks showed low proportions of polymorphic loci, low allelic diversity, and high individual inbreeding coefficients.
Westemeier et al. (1998) conducted DNA fingerprinting on Illinois prairie chickens and also documented high homozygosity, low allelic diversity, and low heterozygosity compared to outbred populations. They concluded the Illinois birds when through a significant bottleneck leading to inbreeding depression that has lowered hatching success.
Both of these genetic studies on the Illinois prairie chickens confirm that their isolation and prolonged population decline in the late 1800s resulted in the predicted consequences of a genetic bottleneck. This reduced genetic health likely contributes to their continued struggles to recover their population size today.
Management Implications
The genetic studies clearly demonstrate that the Illinois prairie chickens have reduced genetic variation as a consequence of their population crash over a century ago. This has important implications for their management and conservation going forward.
Habitat Restoration
Continuing to restore and expand prairie habitat in Illinois will be critical. More habitat will allow for larger population sizes that are less prone to inbreeding. Connecting the isolated prairie fragments will also promote gene flow between flocks to increase genetic diversity.
Translocations
Managers may consider strategic translocations to supplement the Illinois gene pool with birds from larger populations elsewhere in the range. Introducing new genetic lineages can help counteract the bottleneck effects.
Captive Breeding
Ex situ captive breeding with careful genetic management may help reduce inbreeding depression in the Illinois birds. Captive-raised chicks can then be released to supplement wild populations.
Understanding the evolutionary history and genetic status of the Illinois prairie chickens will inform management plans to recover this iconic prairie grouse subspecies and restore it to more of its native range in the Prairie State. Ongoing habitat restoration and conservation translocations offer hope for increasing the population size and genetic diversity of these birds over time.
References
Bouzat, J. L., Cheng, H. H., Lewin, H. A., Westemeier, R. L., Brawn, J. D., & Paige, K. N. (1998). Genetic evaluation of a demographic bottleneck in the greater prairie chicken. Conservation Biology, 12(4), 836-843.
Westemeier, R. L., Brawn, J. D., Simpson, S. A., Esker, T. L., Jansen, R. W., Walk, J. W., … & Paige, K. N. (1998). Tracking the long-term decline and recovery of an isolated population. Science, 282(5394), 1695-1698.