Two fundamental concepts within conservation genetics are that i
) genetic diversity is important for population persistence and ii
) that the amount of genetic diversity is mostly determined by the effective population size (Ne
), which is typically much smaller than the census size (Nc
]. Theoretical models predict that severe reductions in population size termed as population bottlenecks, have a significant impact on the Ne
, and thus the genetic diversity of populations, so understandably, species that have undergone bottlenecks are often the focus of genetic studies. Significant bottlenecks occurred in populations of black-footed ferrets (Mustela nigripes,
]) whooping cranes (Grus Americana,
]), and northern elephant seal (Mirounga angustirostris,
]), and all three species have low genetic variation within the contemporary populations. However, the short-tailed albatross Phoebastria albatrus
, which declined to 50–60 individuals, has high levels of genetic variation [5
The genetic consequences of population bottlenecks can be directly assessed when pre-bottleneck levels of genetic variation can be measured and compared to contemporary samples. For example, temporally spaced samples were used to demonstrate that the loss of mtDNA variation in the nēnē (Branta sandvicensis
) occurred during prehistoric times, and not during more recent declines [6
]. Recent declines did however, impact genetic diversity in greater prairie chickens (Tympanuchus cupido)
], but only influenced genetic structure in peregrine falcon (Falco peregrinus
) populations [8
Kirtland’s warbler (Dendroica kirtlandii
) is the rarest songbird in North America, with a history of rarity and population declines. Kirtland’s warblers are Neotropical migrants that during breeding, specialize on early-successional stands of jack pine (Pinus banksiana
) in the lower peninsula of Michigan, and overwinter in the Bahamian archipelago (Figure ) [9
]. Historical records, and estimates of past habitat availability, suggest that Kirtland’s Warblers were not a common species in the 19th
century, perhaps numbering less than 5000 birds [12
]. However, in the 1940s, Kirtland’s Warbler populations began to decline markedly, consisting of only 530 males in the 1950s. The decline is likely because on the breeding grounds, Kirtland’s warbler abundance is closely linked to the incidence of large-scale wildfires that generate the early successional jack pine habitat on which these warblers are specialized [12
]. The absence of large fires during 1946–1980 reduced the amount of early-successional jack pine stands, which, when compounded with brood parasitism by the brown-headed cowbird (Molothrus ater
), had severe demographic consequences for Kirtland’s warblers. Kirtland’s warblers were listed as an endangered species in 1967, and by 1971, only 201 Kirtland’s warbler males were counted, which was down from the 502 males counted in 1961 (Figure ). This alarming decline led to cowbird control measures, which likely prevented extinction, but Kirtland Warbler populations only began to increase after several large fires increased the availability of larger tracts of suitable habitat [10
]. Fortunately, these management efforts have resulted in the Kirtland’s warbler populations increasing to an estimated 1733 males in 2010.
Map of breeding distribution of Kirtland’s warbler (Dendroica kirtlandii) in Michigan. The boundaries of Oscoda County which is the focal sampling locality for this study is highlighted.
Total counts for annual census of male Kirtland’s warblers from 1951–2010 in the breeding season in Michigan.
In this study we have two objectives, we first measure and compare the genetic diversity from samples collected in Oscoda County, Michigan (Figure ), across three time periods: 1903–1912, 1929–1955 and 2008–2009. Secondly, we determine if the scenario inferred from the molecular data is concordant with the historical census data and if any additional insight could be gained into the population structure of the declining Kirtland’s warbler population.