The Kingdom of Rarities (16 page)

Globally, extreme habitat specialization among plants is more prevalent in Mediterranean-like climate regions than in, say, the temperate forest of Michigan. Aside from one such region mentioned earlier, southwestern Australia south of the city of Perth, there are four others in the world: the California-Mexico coastal region; the Chilean Matorral; the Cape region of South Africa (also called the fynbos); and, of course, the Mediterranean region itself. The Mediterranean region, which offers one of the world's most benevolent climates for humans, is full of plants with narrow ranges because habitat conditions there are often extremely varied. In all five regions, characterized by winter rains and hot, dry summers, fire is an important part of the ecosystem, and the interaction of fire, climate, and different soils, many of them poor, generates a hotbed of habitat extremism.

The soils of Madagascar, New Caledonia, and other ancient islands that formed part of Gondwanaland often contain concentrations of heavy metals. Soils contaminated with heavy metals create a challenge for plants—much more so than for birds or mammals—but also present an extraordinary opportunity. By overcoming nutritional and water constraints of unusual soil environments, an evolving plant genotype escapes the customary tangled mass of vegetation and colonizes what may be virtually free space, with the benefit of reduced competition. For example, in the area around Cape Town, South Africa, the naturally rare plants are not a random bunch biologically. They are for the most part species that are killed by fire but whose seeds germinate in the burned-over soil and, like our jack pine, have short dispersal distances. South African biologists believe that this biology predisposes lineages to produce rare species. George Schatz, an expert on the flora of Madagascar, estimates that a considerable number of that island's
10,000 endemic species are strongly associated with a specific substrate, just as the Kirtland's warbler is to its sandy soils.

A final category of extreme habitat specialization in plants is that made up of climate refugees. These are species with a long history that have become “trapped” in refugia, a term biologists use to indicate a location of an isolated or relict population of a once more widespread plant species that persists today only in pockets where the microclimate still resembles what the plants experienced when they evolved. Worldwide, this group is of huge importance and includes many species. The widely planted dawn redwood (genus
Metasequoia
) is today represented by only 6,000 individuals in nature but was once one of the most common and widespread trees in forests across much of the Northern Hemisphere. The same is true for cycads, a group exemplified by rarities that we know as houseplants such as
Zamia
and those in the genus
Cycas
. Cycads are ancient palmlike seed plants that were extremely widespread in the age of the dinosaurs. More than 20 percent of the nearly 300 cycads are ranked as critically endangered or endangered. Although cycads live a long time, they reproduce infrequently and now have such small populations that further habitat destruction or theft for the plant trade puts some species at great risk of extinction.

The key point, I realized, is that substrates have a huge effect on plants, which typically cannot disperse their seeds far from the parent soil, in contrast with the dispersal capabilities of many animals. Thus globally, either unique or particularly inhospitable substrates, combined with widely varying climatic conditions of rainfall and temperature, yield tens of thousands of range-restricted plants whose numbers and percentages far exceed those of range-restricted vertebrates.

Yet, as we have seen, animals also may evolve to be habitat specialists. Some habitats in which an animal population has found itself have been sufficiently isolated that gene flow was prevented, and the area was large enough for the population to persist. How much separation is needed to prevent gene flow and how large an
area is large enough depends, as we might expect, on the animal. Snails and insects, for example, move only short distances and, being small, often have high population densities. The animals that need the greatest distances and largest habitat patches to become isolated and to persist in isolation long enough to form new species are birds and mammals. That is why there are so few extreme habitat specialists in these groups.

The roster of birds and mammals that share the Kirtland's proclivity for extreme habitat specialization—reliance on a single habitat for some crucial aspect of their life history—is a relatively short one, but it is filled with a lot of exceptional and many famous creatures. Among the warblers, one could list the golden-cheeked warbler of the short oak groves on the Edwards Plateau in Texas and the golden-winged warbler of the eastern broad-leaved forests. Beyond warblers, perhaps no more than 3 percent of the 600 species of birds that breed in North America could be considered extremists. Best known are the northern spotted owl and the southern population of the marbled murrelet, both dependent on old-growth forest, and in the southeastern states, the red-cockaded woodpecker of longleaf pine forests that have the right mix of mature stands and fire to burn out the understory.

Among the roughly 400 species of North American mammals, about the same proportion, 3 percent, could be called extreme habitat specialists. Among the best known are the manatee of warmwater bays, the walrus and polar bear of the ice floes, and the pronghorn antelope and Utah prairie dog of the short grasslands. Globally, the most famous extremist is the giant panda, a species so specialized in diet that it is limited to forests with a dense bamboo understory. Many grassland rodents are extremists but have such wide ranges and are so prolific that they are actually abundant, such as the naked mole rat of southern Ethiopia, Kenya, and Somaliland. Among the more than 5,000 species of mammals globally, extreme habitat specialization is widespread across taxonomic groups and found on every continent.

The focus on rarity may leave the impression that a narrow range, low abundance, or, in the case of the species mentioned here, extreme habitat specialization is equated with a judgment—of inadequacy or evolutionary failure. On the contrary, the Kirtland's warbler and many other extreme specialists are superbly tuned to persist in their preferred environment. The trouble is, those environments may change. Kirtland's warblers do well in their jack pine home but depend on the conditions of their narrow habitat to exist. As the work in the Seney National Wildlife Reserve in Michigan's Upper Peninsula suggests, even where young jacks occur, the soil and undergrowth may have to be just right, too. When conditions change, however, and the forest stands age, Kirtland's warblers are forced to pick up stakes and move to the next patch or perish.

The firebird needs fire, and lots of it, but in the right places and at timely intervals—or, in its absence, carefully managed harvesting in jack pine stands to mimic the effects of fire. Yet, as I soon learned, there is more to the story of the Kirtland's rarity. Even if careless campers or closet arsonists were to set the sandy areas of northern Michigan ablaze—leading to more habitat—the species would still face an even greater threat.

On my second day in the grove, a new birdsong filled the warming air. It wasn't the explosive melody of the Kirtland's but a more metallic and bubbly offering. The members of my birding party noticed that the songs grew louder until we found the source. Hidden behind a natural screen of three-meter-tall jack pines sat a large cage containing six pairs of brown-headed cowbirds. On top was the entrance, accessible to birds wishing to enter and join the cowbirds inside, who were a most sociable lot. These, however, were bait birds, kept alive to attract others of their kind and leave the Kirtland's warbler in peace. For the unsuspecting cowbirds that entered this cage, there was no exit.

More than 100 species of North American songbirds, but especially
breeding pairs of warblers, thrushes, and vireos, suffer from nest parasitism by brown-headed cowbirds. Kirtland's warblers are an especially easy mark for them. The deed is quick, the effects long lasting. When a female Kirtland's begins to lay a clutch of brown-and-white-splotched eggs, she steps briefly off the nest to feed. By the time she returns, a female cowbird, likely having cased the nest from a lookout post, will have visited and deposited an egg or two of her own, typically removing a host egg in the process. The cowbird's chick hatches a day or more before its adopted nest mates and grows more quickly. The adults fail to discriminate against the monster chick in their midst. They keep feeding the imposter, whose larger size means it gets more parental attention than the warblers' own young. Sometimes the more aggressive cowbird chick will push the rival offspring out of the nest.

For the poor Kirtland's warblers, much depends on how many eggs the cowbird lays in their nest; if only one, the pair have a chance to raise some of their own. If there are two cowbirds, none of the Kirtland's chicks survive to fledging. Unfortunately, female cowbirds are prolific egg layers. Over the May to July breeding season, one individual could conceivably put one egg in each of forty nests. This behavior may sound like the handiwork of an exotic species, able to exploit naive native birds, but while the brown-headed cowbird is a relatively recent arrival to Michigan, it is no foreigner. It is a North American species native to the South and West, where it was once known as the “buffalo bird.” When hunters killed off the bison and ranchers replaced them with cattle, the cowbirds followed the domestic livestock too, feasting on the insects churned up by the hooves of the large grazers drifting through the grass. When logging and agricultural development began in earnest in Michigan in the eighteenth and nineteenth centuries, cowbirds followed, later spurred perhaps by the sharp decline of the bison in the 1880s.

How has the Kirtland's warbler held up under this home invasion by cowbirds, whose chicks hatch first and outcompete the
natural nestlings for parental attention? Why has this invasion greatly affected rare species such as the Kirtland's yet abundant species hardly at all? To the second question, the short answer seems to be that some bird species have evolved ways to resist the nest parasites. American robin parents, for example, detect the strange eggs and roll them out. Others may abandon their nest and lay more eggs elsewhere or build another nest on top of the cowbird egg. Kirtland's warblers lack such behaviors, perhaps because, compared with these other birds, they have not yet had the time to evolve better egg recognition. Or the small size of the Kirtland's population may have reduced genetic variation available for selection of the defensive behaviors that have evolved in species with much larger populations and gene pools. Or both. In any case, the impact on Kirtland's reproduction grew to alarming levels. At its nadir in the early 1970s, less than one-third of warbler nests in the Grayling population produced any young, largely attributable to the influence of cowbird parasitism.

In 1972, the US Fish and Wildlife Service, along with other federal and state agencies, began controlling cowbirds with large live traps, such as the one we'd seen, placed in Kirtland's warbler nesting areas during spring and early summer. Wildlife technicians check the cages daily and euthanize any trapped cowbirds—an average of 4,000 per year.

“No one wants to kill cowbirds, but the trapping has worked a small miracle,” Sarah Rockwell related. Nest parasitism rates had dropped sharply, from 69 percent in the late 1960s, before trapping began, to less than 5 percent. Even more inspiring, average clutch size had increased from 2.3 eggs per nest to more than 4, and the average number of young Kirtland's warblers fledged per nest increased from fewer than 1 to almost 3 birds during the same period. Cowbird rustling is an expensive task, and euthanization seems gruesome to some. But for the attending biologists, watching the last few Kirtland's warblers unwittingly feed supersized cowbird
chicks seemed at first surreal and then shameful. Cowbirds have their right to live, too, but at what point do we intervene when their hardwired behavior threatens to drive other species to extinction?

Over coffee with Sarah, I posed a hypothetical question about the Kirtland's future: “If half of northern Michigan were turned into a jack pine preserve and immature stands were increased by a factor of ten, through either fire or management, would we still need cowbird control?”

“Well,” Sarah replied, “what you need to answer your question are data from the last time we had reproductive success information collected before cowbird control.” Larry Walkinshaw, the dean of Kirtland's warbler biologists, had earlier estimated reproductive success to be 0.8 fledglings per pair of adults per year. “So, in the absence of cowbird removal, I guess I wouldn't expect success to go to zero, but the area could certainly become a population sink.” The term Sarah used describes a state in which deaths exceed “recruitment,” or addition of young to the population. And then there is the sad truth: cowbirds are nomadic, so removal during one year has no bearing on the number of cowbirds that will arrive the following year. Nest parasitism would rise again within a year or two and warbler numbers would dwindle if biologists stopped the trapping program.