The Kingdom of Rarities (17 page)

The first federal Kirtland's warbler recovery team, established in 1973, realized that for a bird with such a limited range, it was essential to create more areas of suitable jack pine for it to breed in. So, during the mid-1970s, the recovery team designated some 540 square kilometers of jack pine stands, spanning state and national forests, for management as Kirtland's warbler nesting habitat. Additional lands were added during the 1990s and in 2002 to bring the total public land area specifically set aside for the Kirtland's warbler to more than 770 square kilometers. But how could these and even larger tracts of jack pines necessary to underpin a recovery
be maintained without burning huge areas, even some areas close to towns?

Silviculturalists from the USDA Forest Service argued that fire wasn't the only management tool in the box. Logging practices and plantations could create conditions acceptable to breeding warblers, they said. Ecologists were skeptical. In their view, forestry experts always suggested logging and intensive management as the solution to any conservation problem. The critical question was whether Kirtland's warblers would breed and survive as effectively on plantations as in naturally burned forests.

Again, Carol Bocetti's dissertation work provided answers. “I found that plantations had lower jack pine density and fewer openings in the managed forests than in natural wildfire areas,” she said. But, remarkably, she discovered that the critical response by the warblers—measured by male density, clutch size, number of young fledged, and number of parasitized nests—did not differ between plantations and fire-maintained habitat. It was all about where the females were. Still, she recommended increasing both jack pine density and the number of openings in future plantations, and managers have done so since then.

For a dedicated conservation biologist, a published paper is one reward. But the best outcome is seeing one's research findings become management policy and then witnessing that policy's positive effect in helping to save a species. “I think Carol's work shows that the logging and replanting are effective in replicating fire-maintained habitat,” Sarah concluded. At least for the Kirtland's warbler, Carol resolved the controversy through research. Almost all Kirtland's habitat is harvested and replanted now, so there is very little variation in regeneration 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 Kirtland'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 different 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 territories 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 encore 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.

The yellow warbler and the common yellowthroat are unlikely ever to have a national task force and recovery team formed just for them. In many ways, these two warbler species are the opposite of the Kirtland's. Both the yellow and the yellowthroat have massive ranges, among the largest of any North American songbird, and unlike the Kirtland's they venture far north, into Alaska and the Yukon, respectively. In this Michigan marsh, they were singing in the center of their range—the safest place to be, as the rarity theorists tell us. On top of its range security, the yellow warbler has evolved rather rapidly to outmaneuver the cowbirds. Yellow warblers abandon nests or just build a new nest lining over a parasitized clutch and lay more eggs. These birds are poster species for resilience and resistance to extinction. By contrast, in Michigan Kirtland's warblers will likely remain wards of the state. They represent what biologists call a conservation-dependent species.

Can the recovery efforts for the Kirtland's warbler be applied to other species that are extreme habitat specialists? For the Kirtland's, ensuring the existence of sufficiently large landscapes containing the right mix of young jack pines with other forest classes and controlling the numbers of parasitic cowbirds are key. Because of the cowbird infestation and its own extreme breeding habitat requirements, the Kirtland's joins the ranks of other conservation-dependent extremists such as the rhino in Nepal. For this species, habitat protection, restoration, and antipoaching measures are essential. The recovery strategy for more mobile extremists may offer less insight for the sedentary rarities, especially plants and invertebrates. For these extreme habitat specialists, we have to map their ranges and save them where they occur.

From John Terborgh's studies at Lago Guri described in chapter 3, we saw the impact of the loss of even low-density jaguars, the top predator, in areas of Venezuela: the herbivore populations went haywire and defoliated many plants. Other keystone species, too, can play important roles as ecosystem engineers. Of what ecological effect is a bird so small and so few in number as the Kirtland's, or other rarities like it? Kirtland's warblers, or any of the hundreds of other migratory songbirds—warblers, thrushes, vireos, orioles—cannot be viewed in isolation from their native ecosystems, the broadleaf and conifer forests of North America. We know that Kirtland's warblers gobble defoliating insects and their larvae on their breeding grounds. But even in their heyday, this species was never abundant. However, if we total the original number of all migratory songbird species, many of which are now in decline, their ecological impact as consumers of leaf-eating caterpillars must have been enormous. The ornithologist Scott Robinson has noted that for warblers and other songbirds to play their traditional collective ecological role in forest communities of keeping insect outbreaks in check, they need to be abundant. Today, Bachman's warbler is extinct, Kirtland's is down in the danger zone, and birds such as the cerulean warbler are nearly as threatened. Will there be few or no checks and balances in a brave new ecological world?

The shape of that brave new ecological world will be strongly influenced by climate change, including temperature increases, sea level rise, and shifting patterns of storms, precipitation, and drought. A century or two from now, will that narrow nesting habitat preferred by the Kirtland's warbler even be there? If the range of the jack pine extends much farther north, why can't the Kirtland's just shift up a few degrees latitude? Remember that jack pines can grow on different soils, but the sandy soils preferred by this extremist may not map to the jack pine. In the case of the Kirtland's and other species that migrate to the Bahamas, several climate models predict drier conditions in the archipelago that could reduce the supply of fruit in late winter, while predicted sea level rise could put
some wintering areas under water. How these new conditions will affect the range and density of this species and other rarities favoring fire-dependent habitats is unclear.

For the Kirtland's warbler, persisting on the edge of its former range seems like a ticket to becoming a climate refugee. Yet perhaps more catastrophic forest fires—another prediction of some climate change models—might create more breeding habitat for this bird. Then again, global warming might cause the jack pine's range to shrink from the south, because jack pines thrive in cooler areas, and the species so far has retreated north since the last glacial maximum. Now that we know the world's climate is changing even faster than during the last ice age, it also becomes apparent that species that are extreme habitat specialists, such as the Kirtland's in the jack pines, may not be able to adapt fast enough to deal with changing ecological circumstances.

The story of the Kirtland's warbler and other habitat specialists like it offers an important insight into the nature of ecological rarity. The conditions we observe today are unlikely to be similar either to those that existed when our current habitat specialists evolved what seem to us to be strange peculiarities or to those that will obtain fifty or a thousand years hence. Thus, the Kirtland's warbler may have evolved its adaptations to young jack pine stands when that habitat was very common. Today that habitat has greatly shrunk in extent, owing to both historical climate change and human activities, such that the bird now appears to have extreme requirements. In other words, many of today's habitat specialists may have been common and widespread in the not-too-distant past. Conversely, many of today's common species may become rare habitat specialists in the near future. For example, before Europeans colonized the area, clear-cutting forests and starting many fires, old-growth conifer forests dominated the mountains of the Pacific Coast of North America. The favored habitat of the northern spotted owl was, in fact, the dominant vegetation type. Species that preferred scrublands and young forests, such as the chestnut-sided warbler,
in the same genus as the Kirtland's, were likely rare. Had we visited the area then, we would have called them, not spotted owls, habitat specialists. A striking contemporary example of this kind of change is provided by a recent National Audubon Society survey that shows that populations of many common North American birds are rapidly declining. We do not shoot or poison most of them. Rather, we are driving them toward greater rarity by making their preferred habitats increasingly rare. And if we lose polar bears, it will be because climate change made their required hunting habitat—sea ice—too rare.

Many species are in fact, to one degree or another, habitat specialists. Think, for example, about the several species of prairie dogs that once were among the most abundant and widespread vertebrate species in the American West. Prairie dogs can only live in short grasslands, so they are true habitat specialists, but their habitat was once so abundant that they occurred at high densities over a vast range. For most species, the condition that enable them to survive and reproduce at high enough rates to persist over time are quite specific. As habitats change, for whatever reason, species may have difficulty adapting; some may go into steep decline, while a few may thrive. Thus, we may be misled if we think of today's habitat specialists as being different from species we call habitat generalists. All species have some specific habitat requirements, and we need to be alert to changes that may substantially affect their fate. Currently rare species such as the Kirtland's warbler may require special and urgent attention, but in the long run, all species are vulnerable.