Biology Reference
In-Depth Information
I touched the sharp needles of a young jack pine and inhaled the fragrance of the volatile oils
stored within the green strands. To an ecologist or an aesthete, it was a sensory but most sim-
plified world: sand below, dense groves of even-aged pines, abundant blue sky. To the bio-
logist, the few colors painted a landscape that signified how a narrow breeding habitat serves
as a potential limiting factor in a key stage in the annual cycle.
The host of the warbler, the jack pine (
Pinus banksiana
), is a native of the Far North, a
boreal species adapted to the fierce winter wind and cold at the edge of the tundra. Its native
range in Canada runs east of the Rocky Mountains from the Northwest Territories to Nova
Scotia. The jack pine creeps into the United States from Minnesota to Maine. The southern-
most extent of the range is northwestern Indiana, but distribution of the pine is spotty that far
south. It begins to be more common only in the forests surrounding the northern Great Lakes.
Kirtland's warblers are plucky, but they are no tundra dwellers. Thus, much of the jack
pine range now lies in a belt too far north, where only a few warbler species breed. The south-
ern limit of the jack pine range—the only place thought to be suitable for this diminutive
songbird—represents life on the edge. Some biologists have taken Seuss's “limited niche”
hypothesis a bit further. They suggest that the best niches are located in the center of a spe-
cies' preferred habitat, whereas poor-quality niches line the outskirts because, it is hypothes-
ized, these edge sites blend ecological features from adjacent habitats that offer less of the
right kinds of food, climate, or shelter. But to draw conclusions about this potential contrib-
uting cause of rarity is conjecture in the case of the Kirtland's; the small numbers of breeding
birds preclude testing or arguing that the present distribution is worse because it is on the
limit of its historical distribution.
Still, something doesn't compute. The range of the jack pine is huge, yet the Kirtland's
remaining stronghold is a geographic splinter—about 160 kilometers long by 100 kilometers
wide, illustrating just how range restricted this finicky warbler is. Could it be that the Kirt-
land's, like so many other species, was more common and its range larger during the last ice
age than it is today? Back in the Pleistocene epoch (2.6 million to 12,000 years ago), when
jack pine forests were extensive across the northern United States and southern Canada, Kirt-
land's warblers may have been far more common than they are today. But habitats shift with
climate. For example, about all of the Kirtland's present-day range was covered by solid ice
as late as 14,000 years ago. At that time, the range of the jack pine and its dependent warbler
was probably a contiguous swath between the Appalachian Mountains and the Great Plains,
probably well south of the edge of the ice.
After the retreat of what is known as the Wisconsin ice sheet about 14,000 years ago, the
jack pine began to move north. But the Kirtland's warbler could not follow the cold-hardy
jack pine all the way to the Northwest Territories, reaching only as far north as northern
Michigan. Was the Kirtland's warbler a softie, a cold-intolerant species compared with others
in its family? Both Carol and Sarah doubt it. After all, the Kirtland's has a greater body mass
