Using Anecdotal Occurrence Data for Rare or Elusive Species: The Illusion of Reality and a Call for Evidentiary Standards

1. What they did

How do we know a rare or elusive animal is really there? The authors, three United States Department of Agriculture (USDA) Forest Service researchers, argue that anecdotal occurrence data (sightings or reports of an animal or its sign that cannot be checked) are often trusted too much, even when they come from trained biologists. Such reports often come with weak physical evidence, such as casts or photos of tracks, fuzzy or distant photographs, or recordings that are not diagnostic, which then gets treated as confirmation.

They looked back at three case histories in the United States where anecdotal records shaped conservation decisions:

• The fisher in the Pacific states. Anecdotal records were compared with records from standardized remote camera and trackplate surveys, backed up by genetic studies.
• The wolverine in California. Decades of reported sightings were compared with camera, bait station, helicopter and DNA surveys, historical records and genetic analyses.
• The ivory-billed woodpecker in the southeastern states. Claims that the bird survives in Arkansas and Florida were compared with the physical evidence offered.

They then proposed a sample set of evidentiary standards, in which the evidence needed to accept a record rises as a species becomes rarer.

2. What they found

The fisher

• Anecdotal records suggested fishers were spread continuously across much of their historical range in Washington, Oregon and California. On that basis, a 1990s petition to list them under the Endangered Species Act was denied.
• Standardized surveys, many placed deliberately where fishers had been reported, found fishers only in restricted parts of southwestern Oregon and in several separate areas of California. This revealed large range losses in Washington and Oregon.
• Genetic studies showed fishers in the southern Oregon Cascades had been introduced from British Columbia and Minnesota, and that the native populations in the Siskiyou Mountains were isolated.
• A third petition in 2000 led to the fisher being declared “warranted but precluded” in 2004. The authors judge that relying on anecdotal records delayed conservation action by at least a decade.

The wolverine

• In 1937, wolverines in California were described as confined to the southern Sierra Nevada and close to extinction. From the 1950s to the 1970s, many sightings were reported, and a 1977 report based entirely on anecdotal data concluded wolverines were present across most of California’s mountains. A 1978 state report suggested numbers were increasing, and the broad range is still the state’s official position.
• From the late 1980s, remote cameras, bait stations, helicopter surveys and DNA analyses of feces and hair collected near reported dens or sightings detected no wolverines. The last verifiable record was from 1922.
• Historical and habitat analyses suggest wolverines probably never lived in mountains without extensive alpine habitat, such as the North Coast region. Genetic work showed California’s historical wolverines were isolated. Most likely they were wiped out early in the 20th century.
• In February 2008, a remote camera set for American martens near Lake Tahoe photographed a wolverine, the first confirmed record since 1922. The photographs were diagnostic. Hair and scat samples were collected, and DNA showed a male of a haplotype (a genetic type) found in the Rocky Mountains, Alaska and Canada. So the animal was not a native California wolverine, although where it came from and how it arrived is still unknown.

The ivory-billed woodpecker

• The last verifiable evidence was from 1944 in northeastern Louisiana. Since then many sightings have been reported, in places where the bird would be plausible.
• A 2005 claim that the species persisted in Arkansas rested on seven sightings and two pieces of inconclusive evidence: sound recordings that could not be told apart from unusual blue jay calls, and a blurred video that may show a pileated woodpecker.
• More than a year of intensive searching found no conclusive evidence, and more than four years later the video was still the only physical evidence. Even so, more than 7400 hectares of swampland were protected for the bird, with about $4.2 million of federal funds and $2 million in grants spent on land.
• A 2006 report of the bird in Florida, based on 14 sightings, many possible calls and large cavities, also offered no conclusive evidence.

Why anecdotal records mislead

• In all three cases, anecdotal records greatly overestimated range and abundance, and for two species they “resurrected” animals that were probably extinct there.
• Such records are often numerous, spread out in time and space, detailed and in plausible places, and the observers are usually well meaning, sometimes experienced biologists.
• Even a small misidentification rate adds up over decades. If 1% of bobcat sightings are wrongly called lynx, every 1000 bobcat sightings give 10 false lynx records. Lynx would keep being reported even if none were left.
• The rarer a species, the higher the share of false records, and the higher the social and economic cost of decisions based on them.

3. What they concluded

• Conclusions about whether rare or elusive species are present must rest on verifiable physical evidence. Anecdotal reports are still useful as early warnings and for deciding where to run proper surveys.
• Evidentiary standards should be agreed in advance and should become stricter as a species becomes rarer. Their sample gradient ranks evidence from least to most reliable: visual observations of the animal’s sign; visual or auditory observations; photographs, video or recordings lacking diagnostic characteristics; diagnostic track impressions; diagnostic recordings; photographs or video showing diagnostic characteristics; diagnostic DNA evidence; and a live or dead specimen. The weakest kinds are adequate only for common species, and the strongest are needed for species believed absent or extinct, or new to science. The authors note that the reliability of each kind of evidence will vary between taxa.
• Professional societies for particular groups of animals could each develop their own standards, for example in instructions to authors and reviewers, and use them to rate the reliability of existing databases.
• Such standards should not get in the way of long-term surveys of common species, such as the Breeding Bird Survey and the Christmas Bird Count.
• Acting on the chance that a species is present (the precautionary principle) is a matter of policy, not science. Scientists can best support policy by setting standards for occurrence data.

4. How tracking was or was not used

The authors did no fieldwork of their own. Tracks and signs appear throughout the paper as kinds of evidence. Casts or pictures of tracks are given as examples of weak evidence that is often wrongly treated as confirming a sighting. In their sample standards, visual observations of an animal’s sign rank as the least reliable evidence, and diagnostic track impressions rank in the middle, adequate for a species that is rare but known to be present. The fisher surveys used trackplates as well as remote cameras, and scats and hair were collected for DNA analysis in the wolverine case.

6. Other technologies, skills or innovations

• Remote, motion-triggered cameras and trackplate surveys with standardized detection protocols.
• Noninvasive genetic sampling of hair and scats: mitochondrial DNA haplotypes to tell native from introduced or immigrant animals, and a genetic sex test.
• Diagnostic DNA markers for the ivory-billed woodpecker developed from museum specimens, so that even a feather or droppings could prove the bird is present.

7. Implications for conservation (from the paper)

• Anecdotal records can hide range losses until it is too late to act, delay protection (the fisher) or reintroductions (the wolverine), and divert scarce funds (the ivory-billed woodpecker).
• Had decisions in these three cases been based only on verifiable records, the authors believe more appropriate management decisions would probably have been made.
• Standards for occurrence data, set in advance and scaled to rarity, are essential for knowing the true distribution and status of rare or elusive species.