Threats to Birds
- Golden Crownbeard (Verbesina encelioides) |
|Albatross nesting amid verbesina on MIdway NWR. Photo by Michael Lusk
During the breeding season, over 1 million
birds are densely packed onto Midway Atoll National Wildlife
Refuge. The 1,600-acre refuge is at the heart of the newly
created Northwestern Hawaiian Islands Marine National Monument,
and is home to the world's largest breeding populations of
Albatross, Black Noddy, White Tern, and Red-tailed Tropicbird.
The endangered Short-tailed
Albatross and the endangered Laysan
Duck are also found there.
Among the 225 non-native plants on Midway,
golden crownbeard is the most invasive. Golden crownbeard
seeds were probably brought to Midway in more than 9,000 tons
of soil that was imported to grow shade trees, ornamental
plants, and food for people who once lived on the military
base. Golden crownbeard may also have also been introduced
as an ornamental plant or via seeds on equipment.
On Sand Island, the largest of Midway's
three islands, the area invaded by golden crownbeard increased
from 44 acres in 1991 to 148 acres in 2004 -a 230% increase.
Eastern Island also recorded a significant increase, from
74 acres in 1991 to 131 acres in 2004.
Golden crownbeard, scientific name, Verbesina
encelioides, is in the Asteraceae familly of annual
flowering shrubs. It is native from North America to the tropics,
where it grows in stands from 1 to 5 feet tall from sea level
to 9,000 feet elevation. As an invasive weed on Midway, it
grows aggressively in stands up to 8 feet tall in soils, sand,
and between cracks in asphalt. It shades out native vegetation
and competes for nutrients and water (though it is drought
tolerant, requiring only monthly watering once established).
It also produces chemicals toxic to native plants. Flowers
produce up to 350 seeds by both cross- and self-pollination.
Stands self-seed annually and are dispersed by wind The seeds
exhibit highest rate of germination in open, disturbed areas
with sandy soils.
Golden crownbeard affects birds in the
- Birds do not build nests in existing stands of golden
crownbeard, decreasing available nesting habitat.
- New stands of golden crownbeard grow rapidly around existing
nest sites, preventing parent birds from feeding their chicks.
The chicks become trapped and are unable to reach the ocean
at fledging. These chicks die of starvation.
- Golden crownbeard shelters aphids, scale insects, and
the ants that tend them. These ants may prey on the eggs
and chicks of ground-nesting birds. The scale insects and
ants are also suspected of transmitting a harmful virus
from golden crownbeard to native vegetation.
Golden crownbeard is reducing nesting habitat
for all ground-nesting birds on Midway, but is of particular
concern for nesting Laysan
The U.S. Fish and Wildlife Service's golden
crownbeard management program on Midway began in the late
1990s, and intensified in 2003. This very labour and time
intensive program has included control by hand-pulling, mowing,
and herbicide application. Native plants are propagated in
areas where crownbeard has been eradicated to help restore
a natural ecosystem, and prevent recolonization by crownbeard
or other invasive plants.
In order to reverse the problems caused
by golden crownbeard and prevent re-occurance all of the following
must be done:
- Prevent flowering
- Remove plants, roots, and seeds
- Replant disturbed areas with native plants
- Monitor managed areas for new golden crownbeard growth
- Monitor managed areas for many years