Biology Reference
In-Depth Information
sands 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 sep-
aration 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 dis-
tances 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 under-
story. 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 spe-
cialization 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 judg-
ment—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,
