Tweet This
A recent study shows California condors had a large, genetically diverse population until invasive humans hunted and poisoned them to the brink of extinction
When Captain Meriwether Lewis and his friend, Second Lieutenant William Clark, first set eyes on a California condor in October 1805 whilst camping on the lower Columbia River, they knew they'd seen something that few other white men had ever seen before. Something extraordinary.
At that time, the California condor occupied a huge range; soaring effortlessly on its nine-and-a-half foot wings from the northern portion of Baja California all the way up to and throughout the Pacific Northwest, whilst scavenging on beached whales and other large sea animals.
But the California condor originally had a much larger range than even Lewis and Clark could have known. During the Pleistocene (2,588,000 to 11,700 years ago), condors ranged from Canada to Mexico, from coast to coast across the southern United States, and north along the East Coast to New York. Their bones, feathers and eggshells found in caves in the Grand Canyon indicate they lived there, too.
The condors' range severely contracted during the late Pleistocene (approximately 14,000 years ago) when most of North and South America's large mammals, such as mastodons, camels, horses, tapirs, giant ground sloths and saber-toothed cats, went extinct. (Although we don't know for certain what drove this particular extinction event, it coincides with the arrival of the first human migrants on these continents.) This severe range contraction led some people to argue that the modern California condor was merely a remnant population that was already tottering its way towards extinction and, for this reason, conservation efforts would be a waste and "we should allow nature to take its course."
After Lewis and Clark returned east, their tales of remarkable abundance captured imaginations. Soon hundreds, then thousands and tens of thousands, of settlers followed the Oregon Trail to exploit the plenitude of the West Coast. After gold was discovered in California, this trickle became a torrent. These new arrivals, most of whom were from the East Coast, signaled the beginning of the end of the condors: habitat destruction and uncontrolled shooting combined with egg collecting, poisoning and persecution by ranchers and farmers severely reduced their numbers. By 1982, only 22 California condors remained. These birds became the last hope for the iconic species when they were removed from the wild and placed into a captive breeding program.
Today, the California condor is one of the rarest birds in the world. It may also be one of the most inbred: all California condors alive today are descendants of just 14 individuals, so their genetic diversity is very low. This raises the question: were California condors more genetically diverse before their recent population crash, or are they really a Pleistocene relict with “one wing in the grave?"
This is where Jesse D’Elia, an endangered species biologist with the U.S. Fish and Wildlife Service, enters the picture. He and his colleagues designed a study meant to answer two questions: how genetically diverse were California condors historically, and was there only one population of these iconic birds, or more than one?
California Condors Formed One Continuous Population
To answer these questions, Dr. D’Elia and his colleagues needed DNA from as many historic specimens as possible. The team obtained tissue samples from 93 California condor museum specimens collected between 1825 and 1984. These specimens represented the historic sample of the population that lived prior to the establishment of the captive flock in 1987. The researchers also obtained genomic DNA samples from the genetic founders of the captive population and two of their progeny from the San Diego Zoo’s Institute for Conservation Research.
Working in co-author Dr. Susan Haig's lab at the U.S. Geological Survey's Forest and Rangeland Ecosystem Science Center, the team took extreme care to avoid both contamination and damage to the fragile ancient DNA that was extracted from the museum specimens. From these samples, Dr. D’Elia and his colleagues amplified a 930-basepair fragment containing domains I and II of the mitochondrial control region. This region is important to studies like this because it is the major noncoding region in animal mitochondrial DNA (mtDNA), but it functions like genetic support staff by quietly performing a critical role in the replication and transcription of essential mtDNA molecules (ref).
To maximize the number of historical samples that they could include in their analysis, Dr. D’Elia and his colleagues limited their sequence analysis to 526-basepairs. They aligned all mtDNA sequences so variable sites and unique mitochondrial regions (haplotypes) could be easily spotted and identified. A haplotype, or haploid genotype, is a group of genes that are inherited together as a single unit from only one parent -- in this case, because they are located on the mitochondrial chromosome, they're inherited only from the mother.
Color-coding and mapping the location of each haplotype revealed that California condors roamed extensively. For example, the haplotypes detected in the Pacific Northwest samples (H1 and H7) were common throughout the historical range, occurring from the Pacific Northwest to the southern end of the bird's historical range in Baja California (Figure 1):
Both haplotypes H1 and H7 survived the genetic bottleneck and are still represented in the California condor population today.
California condors recently suffered a substantial loss of genetic diversity
The most important question this study examined is the overall genetic diversity of the historic population -- did the historic California condor population truly have "one wing in the grave" as often argued?
Careful examination of the sequence alignment data revealed that the historical population of California condors had a minimum of 18 haplotypes with 19 variable sites, whereas the genetic founders of the captive population had only 3 haplotypes with 4 variable sites.
Analysis of the relationships between the mtDNA yielded a starlike haplotype network (Figure 2), which shows low levels of sequence divergence and a high frequency of unique mutations. This analysis reflects an 80% decline in overall genetic diversity (or unique haplotypes) from the historical population through the genetic bottleneck in 1987. Most of this genetic diversity was lost after 1900 (black = proportion of historic haplotypes, grey = proportion of post-genetic bottleneck haplotypes; figure 2), although there were so few museum samples from the 20th century that it is impossible to narrow down this estimate to a more precise date.
Examination of the studbook suggests no net loss of haplotypes has occurred since the 1980s when the birds were brought into captivity. But the haplotype network shows that mtDNA haplotypes are unequally distributed, with ~62% of the population assigned to haplotype H1, ~15% to H4, and ~23% to H7 (ref).
But overall, these findings support the hypothesis that California condors had a surprising amount of genetic diversity prior to their decline due to human-caused mortality, a decline that quickly reduced their numbers in the 1800s and early 1900s.
“These results document a significant decline in mitochondrial DNA diversity over the past century, which also suggests a corresponding reduction in nuclear DNA diversity,” said Jeff Johnson, an Associate Professor of Biology at the University of North Texas, who was not part of this study.
In addition to underscoring the reduction in genetic diversity, this analysis also shows that the condors in California were genetically similar to the now-vanished Pacific Northwest population.
“Additional research we've conducted [...] has shown that environmental conditions in the region appear favorable for supporting a reintroduction effort, and that perhaps there are even more expansive areas of potential habitat than where they currently exist,” said Dr. D’Elia in email.
Thus, there’s no reason not to release some of today’s captive-bred condors into those unoccupied areas of their historical range.
“Efforts are currently underway to evaluate additional reintroduction sites in the Pacific Northwest,” Dr. D’Elia said -- news that will be greeted with great excitement by birders in Oregon and Washington states.
“This should provide some hope that there are additional opportunities on the landscape for condor recovery,” Dr. D’Elia said.
Museum collections are invaluable to conservation research
This study could not have been done without the many museum specimens collected and carefully maintained throughout the last century and a half.
“The value of museum collections for answering important questions when considering population translocations and species’ reintroductions cannot be overstated,” Dr. D’Elia observed.
“They provide a direct window into a population’s history and as new genetic and genomic tools continue to be developed, the value of these specimens only increases,” Dr. D’Elia said.
The modern technologies used to generate these DNA data are important tools that can inform ongoing conservation efforts.
Captive breeding must begin long before a species is critically endangered
“Our study highlights the importance of considering conservation breeding programs earlier, before species are critically endangered,” said Dr. D’Elia.
Intensive management of endangered populations is also vital, particularly early in the recovery process.
“[T]he careful attention made to maintain founder lineages and prevent inbreeding is critical for improving the likelihood of producing an eventual self-sustainable wild population,” agreed Dr. Johnson, who is an expert on incorporating genetic information into conservation efforts.
When working with an endangered population, there is a continued need to increase the total population size combined with comprehensive management to maximize retention of all the remaining genetic diversity.
“Those involved in the California Condor recovery efforts are leading experts in captive propagation and release, and the methods developed and used, particularly those benefiting from whole nuclear genome approaches, acknowledge the importance of maintaining existing genomic diversity both at the individual and population level,” Dr. Johnson explained.
For example, genetic management of the California Condor population is based on a kinship matrix of the complete nuclear genomes from all of the founders, followed by pedigree information on their descendants, instead of being based on mtDNA haplotypes. (Haplotype diversity represents the overall genetic diversity of the population.)
“Now we need to work on recovering it to a point at which ongoing management is minimal. That will take some time, but it is important to take stock of where the species has been and how much success the program has had over a relatively short amount of time.”
“It really is a legendary success story in saving an endangered species from extinction, but the story is not over yet and more work needs to be done,” Dr. D’Elia said.
The relevance of this research is not limited to California condors.
“These methods will also benefit other endangered species captive breeding programs that possess similar demographic histories,” Dr. Johnson said -- particularly other species of vultures.
"Of the 23 vulture species in the world, 16 are listed as Near Threatened, Vulnerable, Endangered, or Critically Endangered by the International Union for the Conservation of Nature,” Dr. D’Elia said.
Already, several other endangered vulture species are in captive breeding programs in other parts of the world.
“Such efforts are underway in Asia, where vultures have suffered catastrophic declines due to diclofenac poisoning -- research suggests that these breeding programs can be successful at retaining significant genetic variation in the captive population,” said Dr. D’Elia.
California condors still aren't safe as long as lead pollutes the environment
Yet despite all the money, research and effort invested to save the California condor, people still pose an ongoing and serious threat to this species's continued recovery and survival because of the widespread use of lead ammunition by hunters.
“More critical than the genetic issue is the existential threat to the species of continued poisoning of their food supply by lead ammunition,” said Dr. D’Elia.
Although lead ammunition is not environmentally friendly, hunters favor it because it is cheaper than ammunition made from other materials. Lead is dense and deeply penetrates flesh, it is soft and thus, it tends to fragment inside an animal's body. This poses problems since condors that feed on carcasses and gut piles left behind by hunters end up accidentally ingesting lead (ref). The condors' powerful stomach acids dissolve the lead fragments which then enters their blood stream, poisoning the birds and dooming them to a painful, lingering death.
"The fate of the condor rests largely on our ability to solve this socially complex, but logistically simple, problem,” said Dr. D’Elia.
Sources:
Jesse D’Elia, Susan M. Haig, Thomas D. Mullins, and Mark P. Miller (2016). Ancient DNA reveals substantial genetic diversity in the California Condor (Gymnogyps californianus) prior to a population bottleneck, The Condor: Ornithological Applications, 118:703–714 doi:10.1650/CONDOR-16-35.1
Also mentioned:
David A. Clayton (1992). Transcription and replication of animal mitochondrial DNAs, International Review of Cytology, 141:217–232 doi:10.1016/S0074-7696(08)62067-7
Mary S. Adams and Francis X. Villablanca (2007). Consequences of a genetic bottleneck in California Condors: A mitochondrial DNA perspective. In California Condors in the 21st Century (A. Mee and L. S. Hall, Editors). Nuttall Ornithological Club, Cambridge, MA, USA, and American Ornithologists’ Union, Washington, DC, USA. pp. 35–55. (free PDF.)
Dawn Starin (2015). Condors or lead ammunition? We can't have both, The Ecologist, retrieved 17 October 2016.
Additional reading:
Jesse D'Elia and Susan M. Haig (2013). California Condors in the Pacific Northwest. (Oregon State University Press/Amazon USA/Amazon UK) ISBN: 978-0-87071-700-0.
California Condors Have Been Endangered Since Humans Arrived In America | @GrrlScientist
