One if by Land, Two if by Sea
(http://www.ccbbirds.org/2014/06/24/one-land-two-sea/)
By Mike Wilson
The black rail is the most imperiled bird species along the Atlantic Coast.
This species has undergone a range reduction, a loss of historical breeding
sites, and a decline in numbers at their most critical strongholds. Recent
surveys have shown a drastic 80% loss of breeding sites in the Chesapeake Bay
over the short span of 15 years. It is very possible that black rail will
become extirpated in many portions of their range in our lifetime without
emergency management intervention.
Like Paul Revere's historic ride to warn the people of Concord of the impending
British march, conservation biologists are pressed to provide the appropriate
signal in the tower of the Old North Church to prevent the species collapse.
The Center for Conservation Biology (CCB) has taken a lead role in bringing
biologists together and is undertaking specific field studies to help plot a
course to halt and reverse this species' declines.
Unfortunately, recent investigations suggest that all three lanterns are
required to warn the conservation community. Black rails appear to be
threatened by two distinct lines of attack resulting from unprecedented rates
of sea-level rise and high levels of nest predators that emanate from adjacent
upland habitats.
This past year, CCB conducted an experimental approach to investigate the
nesting potential of the high marsh habitats that black rails exclusively rely
upon. The high marsh is a thin margin of habitat that forms on elevated
terraces between the lower marsh and terrestrial upland. Historically, high
marshes were only inundated during extreme tidal events and storms. The
vegetation of this habitat forms a savanna-like groundcover of distinct marsh
grasses and the ground is relatively dry when travelling across. Black rails
successfully evolved nesting in these higher elevation habitats because they
were at a lower risk of being flooded.
Specifically, our study was designed to determine fates of artificial nests
containing quail eggs in the high marsh to pose as surrogates for species like
clapper rails, Virginia rails, and black rails. Sea-level rise ultimately
threatens to alter or destroy high marsh habitats through conversion but more
proximately acts as a silent killer to species like black rails by inundating
nests at a greater frequency. High marshes undergoing higher rates of
inundation may show little sign of habitat change over the short term while the
habitats are essentially being denuded of reproductive potential. Throughout
the course of the study, the high marshes examined were repeatedly flooded with
water levels greater than nesting height. Each of these flooding events poses
significant risks of nesting failure.
The pattern of nest predation also revealed how reproductive potential of high
marshes are severely limited. Sixty-four percent of artificial nests were
depredated within the first 7 days of exposure and nearly 92% were depredated
within the first 20 days of exposure. The average number of exposure days
before depredation (=7.4) of all nests was less than half the time required for
species such as black rails, clapper rails, and Virginia rails to complete a
full incubation period.
Nest losses from flooding and predation are overwhelming threats to black rails
and other ground nesters in high marsh habitats. Results from broad surveys for
black rails in Maryland, Virginia, and North Carolina indicate that there are
large blocks of appropriate habitat that remain unoccupied. This suggests
that disruption of demographic processes may be more important to explaining
population declines than habitat loss. Losses of black rails from historic
strongholds that appear to have experienced little or no vegetation change in
the face of sea-level rise further support the notion that population declines
are tied to demographic threats such as reproductive success or adult survival.
However, the future for black rail habitats also appears dire. Previous work
completed by CCB has shown that greater than 95% of black rail habitats will be
lost over the next 100 years at current rates of sea-level rise
(http://www.ccbbirds.org/2009/05/01/impacts-of-sea-level-rise-on-marsh-birds/).
Management solutions for black rails must provide habitats with high nesting
potential that are also protected over the long term from the negative effects
of sea-level rise. The next challenge for CCB is to begin working on
configuring management recommendations for impounded wetland habitats so they
may be made available for black rail occupation and nesting.
Michael Wilson
Center for Conservation Biology
College of William and Mary & Virginia Commonwealth University
P.O. Box 8795
Williamsburg, VA 23187-8795
phone: 757-221-1649
fax: 757-221-1650
email: mdwils@xxxxxx
web: www.ccbbirds.org
