Biology Reference
In-Depth Information
eration type to study. Little is yet known, however, about other vertebrate and plant species
living in the jack pine forests that do require fire episodes.
A footnote to the Kirtland's story made me hopeful about the species' long-term survival
prospects. Until 1996, all known nests had occurred within sixty miles of Grayling. But in
2007, three Kirtland's warbler nests were discovered in central Wisconsin and one nest was
found at a Canadian Forces base in Petawawa, Ontario. By 2010, the presence of 25 Kirt-
land's males in four Wisconsin counties had been recorded, and fifteen nesting attempts were
monitored that summer. And one of our guides remarked that the number of singing males in
Michigan's Upper Peninsula had increased to 34 in 2010. Sarah thinks these developments
are a sign that the population of nearly 2,000 singing males has grown to near saturation of
their habitat in the Grayling area and has started expanding into new areas.
An ability to disperse from a breeding or feeding area that has somehow changed in quality
or become overpopulated to an equal or superior but more distant new site makes you what
biologists call a successful fugitive species: when conditions shift over time, you escape and
find new ground.
Unfortunately, most other extreme habitat specialists, especially plants on unusual soil
types and many terrestrial invertebrates, are relatively poor dispersers or have nowhere to go.
Unlike the Kirtland's warbler, they are incapable of dispersing when their habitat begins to
change. Often they have resided in habitats that have been stable for a long period of time,
long enough for them to be seen as distinct endemic species and perhaps intolerant of dif-
ferent habitats nearby. Many of the plants in the Cape flora of South Africa, in southwestern
Australia, and in New Caledonia are in this situation. There is an old German expression,
“Never move an old tree,” a metaphor for the unintended consequences of shifting the aged
from their longtime domiciles to strange locales. The literal meaning of this phrase also holds
true: most trees, shrubs, and herbaceous plants can't move far from their home base, nor can
the flightless or weak-flying insects that live on them. In this way, lack of dispersal ability
both contributes to isolation and sets the stage for the emergence of new species, many of
them rare.
On the drive back to Detroit, not far from Grayling I pulled over by a large marsh where, in
late morning, the songbirds were still active. The imperative of establishing and holding ter-
ritories in mid-May leaves the abundant yellow warblers and yellowthroats with little time for
rest. “
Sweet-so-sweet, I'm so sweet
” and all its variations for the yellow warbler, “
Witchity-
witchity-witch
” for the common yellowthroat: an orchestral piece for two warblers, with en-
core after encore. There were no Kirtland's warblers here; they shun such swampy locales.
But it was a happy scene, a wetland brimming with song and color.
