Biology Reference
In-Depth Information
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 repres-
ent what biologists call a conservation-dependent species.
Can the recovery efforts for the Kirtland's warbler be applied to other species that are ex-
treme habitat specialists? For the Kirtland's, ensuring the existence of sufficiently large land-
scapes 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 ex-
treme breeding habitat requirements, the Kirtland's joins the ranks of other conservation-de-
pendent 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 out-
breaks 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, pre-
