Biology Reference
In-Depth Information
cipitation, 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, sever-
al 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 predic-
tion 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 gla-
cial 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 his-
torical 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, hab-
itat specialists. A striking contemporary example of this kind of change is provided by a re-
cent 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.
