The Kingdom of Rarities (15 page)

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 simplified world: sand below, dense groves of even-aged pines, abundant blue sky. To the biologist, 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 southernmost 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 southern 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 species' preferred habitat, whereas poor-quality niches line the outskirts because, it is hypothesized, 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 contributing 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 Kirtland'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, Kirtland'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 than many of the smaller warblers that nest farther north; in fact, next to the yellow-breasted chat, it is the largest warbler in North America.

Carol and colleagues think they have found the answer to this mystery. In the late 1980s, they explored the Seney National Wildlife Refuge's vast acreage of jack pine forest on Michigan's Upper Peninsula. They found lovely tracts of young jack pines, but the ground
cover and soil moisture were all wrong for Kirtland's homesteading. Thick mats of moss covered the soaking-wet ground. Some high knolls did have the requisite blueberry and other shrubby ground species, but only in isolated pockets. The researchers were not surprised to find the young jack pine stands void of Kirtland's warblers. More detailed surveys on the distribution of very sandy soils farther north could support this conjecture with data showing that although pines are available, the essential sandy, well-drained soils are themselves rare.

At this same time, Burton Barnes and his team of graduate students were studying potential breeding areas of the Kirtland's warbler around Grayling and a few sites on the Upper Peninsula, documenting the specific features that the warbler seemed to prefer. Well-drained soils, both glacial outwash and areas of former glacial cover, seemed to predict Kirtland's warbler occupancy. More important to the bird's reproductive success may be Grayling sand or any similar dry, sandy soil and good drainage under the ground nest. As it turns out, the jack pine is the only tree tough enough to survive centuries of such depleted, dry soil conditions.

This toughness is epitomized even in the jack pine's cones. The next day in the grove, I grabbed a few to examine more closely. The small spheres looked as if they had been glued shut. Nature by design: jack pine cones are serotinous, a term used to describe seedholding packets of plants that open only when triggered by fire. The cones can remain sealed for decades until a forest fire whips through a stand. The hot fire kills the mature trees but guarantees rebirth: when the heat of the flames hits 50°C (122°F), the cones pop open, spill the pine nuts in their protective shells, and reseed the charred earth. Nature's baton cues a new melody, for which the Kirtland's warbler is so precisely tuned, a firebird suite.

Historically, naturally occurring wildfires swept through the region and shaped the distribution and sizes of jack pine forests. In the nineteenth century, loggers came here in search of the coveted old-growth white pine for its fine, knot-free building timber but
ignored the lowly jack. Intensive timbering ended in the 1880s after loggers pillaged the last stands of old-growth white pine and eastern hemlock. Extensive burning continued, which likely increased the range of the fire-adapted jack pine. This interlude, after the loggers cleared out but before settlers came, may have been the golden age in modern times for the Kirtland's warbler. The largest extent of jack pine habitat in Michigan probably existed between 1885 and 1900. After that, extensive fires either no longer occurred or diminished as human settlements spread. As jack pine stands matured under fire-prevention schemes designed to protect public and private property, the warblers, according to local and national surveys, went into a steep decline. Smokey the Bear and other popular symbols of the time may have prevented forest fires, but the suppression of fire was surely a contributing factor to the Kirtland's nosedive.

Visiting the breeding grounds during the Kirtland's spring pilgrimage is permitted only in the mornings, so birders must seek afternoon diversions elsewhere. The nearby Au Sable (pronounced
O
-sible) River offers fine canoeing and top-rated fly-fishing. For searchers of rarity, there is another one-of-a-kind, must-see site that in its own way deserves equal billing with the Kirtland's: about thirteen kilometers northeast of Grayling lies Hartwick Pines State Park, home to the largest remaining stands of 350-year-old eastern white pine and eastern hemlock in North America. And what could be a better ecological complement to a 5-year-old jack pine grove than a forest of centuries-old conifers? My curiosity got the better of me because some biologists speculate that in rare habitats one should naturally find rare species. What treasures did Hartwick Pines hold?

The walkway to the park was full of singing male rose-breasted grosbeaks, perhaps rivaling the scarlet tanager and the painted bunting as the most beautiful songbird in North America. Feeders at the reserve headquarters pulled in evening grosbeaks by the dozen.
A true deep-forest birding adventure awaited those who, like me, had never set foot in an old-growth white pine forest.

Nature writers have compared the feel of wandering into an old-growth stand to that of entering a cathedral. In the park, towering trees of massive girth created a cool environment. The understory layer was empty, as the dense shade prevented much of a shrub layer. The eastern hemlock was once called the “redwood of the east” because it can grow more than 50 meters tall and have a trunk 2 meters in diameter. In Great Smoky Mountains National Park, some 500-year-old trees still survive, but hemlocks elsewhere are not found in the large numbers they once were. Back in the 1920s, a bug from Japan known as the hemlock woolly adelgid appeared in New England and the mid-Atlantic states and began feeding on hemlocks of all sizes. Within eight years after infestation begins, a tree can die. Some ecologists say that the diminution of this widespread and once common hemlock may have had greater secondary effects than the loss of the American chestnut in the 1930s. Hemlocks play a vital role as habitat for species ranging from beetles to trout by providing deep shade along streams and cool refuges in the forest. Remove the hemlocks and the bright sun drives away the shade seekers.

Along the route I heard other warblers singing: pine, blackthroated blue, and black-throated green. The ethereal songs of the wood thrush and hermit thrush made me stop and lie down under a grove of ancient trees to take it all in and reflect on what I had learned so far. I had filled pages of notes, but the lode of natural history insights danced around answers to some central questions: Why do species evolve to become habitat specialists in the first place? How widespread is this condition of extreme habitat specialization in nature, and is it more commonly observed in plants than in animals?

The answer to all three questions invokes evolution by natural selection. Imagine a species with a broad habitat tolerance. Some of its members will be breeding in widely distributed habitat types, while others will be reproducing in rare habitat types. If those individuals
that reproduce in the rare habitat are to evolve into a new species specifically adapted to that habitat, two conditions must be met. First, they must become isolated from the rest of the population so that they breed only with one another, as we saw in the Foja Mountains of New Guinea. Interbreeding with individuals reproducing in other habitats prevents the evolution of habitat specialization. The second condition is that the reproductively isolated population must persist long enough for evolutionary processes to yield a considerably different organism, that is, a new species.

For several reasons, these conditions are much more easily met with plants than with animals. First, among plants whose flowers are animal pollinated and whose seeds also are disseminated by animals, genes are generally transferred over only short distances. In contrast with most animal populations, a plant population does not need to be far from others of its species to achieve the grist of speciation—reproductive isolation. Second, whereas few vertebrates can reproduce asexually, many plants can self-pollinate. This enables them to reproduce successfully in small, isolated areas. Third, because plants are intimately tied to the soil for their nutrients, they more readily evolve adaptations to specific soil types. Few animals have intimate relationships with particular soil types, and for those that do, the relationship is with structure (ease of burrowing, chances of becoming waterlogged), not chemistry. Thus the world abounds with plants that are restricted to specific soil types, often ones found in only a few places. Southwestern Australia, which features rocky outcrops separated from one another by only short distances, for example, has thousands of plant species found on only one or a few outcrops. Even those short distances are too wide for pollen or seeds to be transferred between them.

As I rested, my gaze turned to a cluster of lovely pink lady's slipper orchids growing in the sandy soil. Among the 250,000–425,000 vascular plants on Earth, perhaps as many as 20–30 percent can be considered extreme habitat specialists. To find many of them, though, I was sprawled out in the wrong part of the world. Because
the spot where I was reclining, together with much of the northern forest zone, had been under a glacier 14,000 years ago, the habitats of this region are now broadly similar and the majority of temperate northeastern US forest species are relatively widespread.