ScienceNot yet confirmed elsewhere1 publisher2 min readPublished
Living Planet Index's 73% decline averages monitored populations that lean toward protected areas
WWF's 2026 Living Planet Index puts the average decline in monitored wildlife populations since 1970 at 73%. Researchers who work with the data say many more populations go uncounted and that sampling bias could push the figure either way.
The Scientist · Science desk
What happened
- The index is updated every two years and is one of the metrics used to track progress toward the United Nations' biodiversity goals.
- Counting wildlife is resource intensive and generally cannot be done from space, so systematic monitoring is extremely patchy and many populations go uncounted.
- The index draws on whatever exists: ad hoc academic studies, patchy monitoring programs and management data for harvested species.
Why it matters
- constraint Past populations cannot be counted after the fact, so new monitoring can only improve the record going forward, and the authors expect years before those initiatives produce outcomes.
- contradiction Skew toward undisturbed habitat points to an underestimated decline, while a tilt toward threatened and harvested species points the other way. The net direction is unsettled.
- decision On very short records, sometimes two time points, the maintainers must choose: keep them and carry noisy trends, or drop them and lose data the authors say would worsen the bias.
The denominator is monitored populations, and populations that go uncounted do not figure in the 73% at all [2].
The authors, researchers who handle biodiversity data, say the populations that do get counted are not a random sample. The index is biased toward counts from protected areas [15]. Their team's preliminary analysis indicates that it and BioTIME also hold a disproportionate number of counts from mostly undisturbed habitat, such as remote wilderness or national parks [15].
That bears on what the raw records show: taken at face value, roughly as many populations rising as falling. The authors say the skew might explain that even split, and that failing to account for it risks greatly underestimating how much biodiversity is declining [13]. Habitat change does produce winners, such as farmland birds after forest is converted to agriculture. But conversion of tropical forest to monoculture plantations, or of grasslands to towns and cities, leaves far more populations declining than increasing [14].
The authors do not claim the error runs one way. The index may be more likely to hold data on threatened species, or on harvested species such as fish, and those are species one would expect to be declining [7]. They write that they do not fully understand all the ways the counts could be biased, or how that affects what is known [8]. They do not estimate how large the net correction is or which way it runs, and the article does not say how many populations or species sit behind the 73%.
The index's own adjustment is coarse, by the authors' description. It gives more weight to counts from undersampled regions such as Africa and South America, where the loss of undisturbed habitat is greater and there are far more species [9]. The authors say more sophisticated methods for understanding bias are in their infancy [12].
In our view the 73% supports one claim cleanly: across the populations monitored, the average change since 1970 is a decline of that size [2]. Whether it overstates or understates the picture for wildlife as a whole depends on a representativeness check that is still early. And the direction of that check holds only if the authors' preliminary finding about undisturbed habitat survives fuller analysis [15].
What to watch
- Peer-reviewed results from the authors' preliminary analysis of undisturbed-habitat bias in the index and BioTIME.
- Outputs from the monitoring initiatives the authors say are still years from producing outcomes.
- Bias-correction methods for biodiversity data, which the authors describe as in their infancy.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence50
- Adoption
- Insufficient
- Hype gap
- Insufficient
- Incentives
- Insufficient
- Confidence55
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
The WWF's 2026 Living Planet Index reports that wildlife populations around the world have declined by an average of 73% since 1970.
- [2]
The 73% does not mean that 73% of the world's animals have disappeared; it is the average decline in the size of wildlife populations being monitored.
ReportedSupportedSource: phys.org article by researchers who work with biodiversity dataView cited source - [3]
The Living Planet Index is updated every two years and is one of the metrics used to track progress toward the U.N.'s biodiversity goals.
- [4]
Counting wildlife is highly resource intensive, generally can't be done from space, and systematic monitoring is extremely patchy, so there are far more wildlife populations that no one is counting than the data cover.
ReportedSupportedSource: The authors, researchers who handle biodiversity data, writing in phys.orgView cited source - [5]
Index data come from ad hoc academic studies, patchy monitoring programs and management data for harvested species.
- [6]
Some critiques of the index note that, when records are taken at face value, roughly the same number of populations are increasing as decreasing; a similar pattern appears in BioTIME, another large collection of population change data.
- [7]
The bias could also work in the other direction: the index might be more likely to have data on threatened species or on species harvested by humans, such as fish, which are species one might expect to be declining.
- [8]
The authors say they do not fully understand all the ways in which the wildlife counts we have could be biased, or how this could affect what we know.
- [9]
The index gives greater importance to counts from undersampled regions such as Africa and South America, where loss of undisturbed habitat is greater and there are far more species; the authors call this a coarse way to balance some bias.
- [10]
The index includes data collected over very short periods, sometimes just two time points; this sparseness adds uncertainty about population trends, but excluding those populations would leave the index with even less data and make its underlying biases worse.
- [11]
Initiatives are seeking to address monitoring gaps but it will be years before they produce outcomes, and historical data gaps cannot be filled because past populations cannot be counted.
- [12]
More sophisticated methods to understand bias in biodiversity science are in their infancy.
- [13]
The authors say the skew toward undisturbed habitat might explain the balance of declining and increasing populations in the raw data, and that if it is not accounted for, there is a risk of greatly underestimating how much biodiversity is declining.
ReportedContestedSource: The authors, writing in phys.org2 sources— create a free account to open themView cited source - [14]
Habitat change has some winners, such as farmland birds that thrive after forest is converted to agriculture, but many instances of habitat loss, such as tropical forest converted to monoculture plantations or grasslands to towns or cities, result in far more populations declining than increasing.
ReportedContestedSource: The authors, writing in phys.org2 sources— create a free account to open themView cited source - [15]
The index is biased toward counts from protected areas, and preliminary analysis from the authors' team indicates that both it and BioTIME contain a disproportionate number of counts in areas of mostly undisturbed habitat, such as remote wilderness or national parks.
ReportedInsufficientSource: The authors' team, preliminary analysis, as stated in phys.orgView cited source
Sources
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Topics
- Biodiversity monitoringFollow
- Wildlife population trendsFollow
- Sampling bias in ecological dataFollow