Biology Reference
In-Depth Information
today may find a vastly shrinking range tomorrow. The narrowrange Kirtland's warbler may
thus be a bellwether species to help us better understand how species cope in a more restric-
ted area.
Extreme habitat fidelity: is it a cause of rarity itself, or a condition of it? Actually, it can
be either or both, depending on the circumstances. The linked questions have been central to
the debate on what lies behind patterns of rarity and abundance in nature. Our understand-
ing of extreme habitat fidelity as a condition of rarity owes much to the work of the pop-
ulation biologist Deborah Rabinowitz. In 1981, she developed a way to think about rarity
that remains influential today: to a species' range size and its population density, she added a
third quality—its loyalty to a particular habitat—as a condition of rarity. One thing is clear:
if highly selective breeding habitat alone explained a major piece of the rarity puzzle, Dr.
Seuss's classic could replace bookshelves of ecology texts. But there is much more to the
story. For example, if today you must go to Grayling to see Kirtland's warblers with ease and
in appreciable numbers on their breeding grounds, time travel back before the last ice age
might well have offered more viewing options. Some scientists believe that this species was
probably more common prior to the last glacial period, when the jack pine forests were more
widespread. Grayling sits on the edge of the bird's historical range. Some biologists predict
that under several climate change scenarios projected for northern Michigan, the future for
this species could be greatly curtailed, but that still remains conjecture.
At its low point, in 1971, the global population of Kirtland's warblers dropped to around
400 birds. At that time, only about eighteen square kilometers of suitable immature jack pine
habitat remained, and that was in Michigan. The warbler joined the first US endangered spe-
cies list in March 1967 as one of seventy-eight native animal and plant species then under
threat of extinction, and it remains America's most imperiled breeding songbird.
How frequent is the condition of extreme habitat specialization in nature, and how might
interventions on behalf of the Kirtland's warbler inform efforts to save species with similar
distributions? Viewed through our magnifying lens of rarity, maybe the seemingly boring
Christmas tree farm held a lot more interest after all.
Standing in the midst of the jack pine forest on that May day, I still couldn't get over the
sameness of it all. Having lived and worked in species-rich tropical rain forests, I have a nat-
ural aversion to monocultures. If the view from the spotting scope brought rarity into sharp
focus, the naked eye swept a panorama that seemed like a biological wasteland. Why did the
Kirtland's insist on nesting
here
? If I pretended to be a northern Seuss, I might ponder:
What's this business about jack pines, do tell!
Why so picky where you pick to dwell?
And why nest on the ground
where skunks abound,
