A Univ.
of Florida study demonstrates
extinction’s ripple effect through the animal kingdom, including how the demise
of large mammals 20,000 years ago led to the disappearance of one species of
cowbird.
The study shows the trickle-down effect the loss of
large mammals has on other species, and researchers say it is a lesson from the
past that should be remembered when making conservation, game, and land-use
decisions today.
“There’s nothing worse for a terrestrial ecosystem than
the loss of large mammals—and the loss of apex predators like sharks, tuna, and
other large fish will have the same negative impact on the oceans,” said study
co-author David Steadman, ornithology curator at the Florida Museum of
Natural History
on the UF campus. “We’re seeing it with the loss of lions and elephants in
parts of Africa, as well as in Florida
with the decline of panthers. There’s no question these losses will have a
negative domino effect on our ecosystems.”
The fossil study of eight songbird species from northern
Mexico by Florida Museum
ornithologists is currently available online in Palaeogeography, Palaeoclimatology, Palaeocology.
An extinct cowbird, Pandanaris
convexa, is the most common bird found at the fossil site called Térapa, in
Sonora, Mexico, about 150 miles south of Arizona. This is the first time
fossils of the large bird, a member of the blackbird family, have been found in
Mexico.
Finding the extinct cowbird at the fossil site was
unpredictable and unexpected, according to Jim Mead, chair of the department of
geosciences at East Tennessee State Univ., who has collected a variety of
fossils at the site, including the birds used in the study. Mead described the
findings at Térapa as “bizarre and exciting.”
“The tropical environment is unusual because the site is
so far from the coast,” Mead said. “The fossil record also provides evidence
animals migrated from north to south and, unexpectedly, from south to north.”
The cowbird has previously only been found at the Rancho
La Brea fossil site in California and a site
in Reddick, between Gainesville and Ocala in North Central
Florida. The study expands the bird’s known range and creates new questions
about whether it may have lived across the southern U.S.
“The extinct cowbird needed grasslands and these big
mammals to survive,” said lead author Jessica Oswald, a National Science
Foundation predoctoral fellow at the Florida
Museum. “Those two things
play into each other because mega mammals maintain grasslands. They keep big
trees from coming in and colonizing the areas because they graze, stomp and
trample little saplings.”
Like modern cowbirds, this species probably fed on seeds
and insects large mammals exposed, Oswald said. The mammals included extinct
species of ground sloth, mammoth, horse, tapir, camel, and bison.
About 20,000 years ago, most of these large mammals went
extinct, which lead to the extinction of scavengers like condors and vultures,
as well as cowbirds, Steadman said. Extinctions, especially mass extinctions,
can cause radical species loss and changes in species distribution.
“Big species can’t exist in a vacuum, nor can smaller
species,” Steadman said. “When one piece of the puzzle goes extinct, there is
no good way of predicting what sort of trickle-down effect, what kind of
cascade effect that will have.”
The study also confirms the area was once marshy
grassland, possibly surrounded by a savanna near a river. Fossils of plants,
reptiles and mammals of all sizes, and 31 species of birds other than songbirds
have been recovered from the Térapa site over the past 10 years. Most of these
species are found today in grasslands or wetlands, Steadman said.
Steadman and Oswald used the Florida Museum’s
more than 24,000 skeletal specimens of birds to identify the Mexican fossils.
Songbirds make up more than 50% of the world’s living
bird species, but the fossil record is poorly developed, especially in Central
and South America. Oswald said this study
helps build the fossil record of songbirds in Mexico.
Finding bird fossils, as well as bones of other small
animals, is a time-consuming and labor-intensive process. Sediment is placed in
a fine mesh sieve and water is used to remove dirt and debris from the bones.