When a species begins to disappear from places where it once lived, its decline may provide an early warning of a more serious problem ahead.
A new study has established that connection more rigorously than previous research, finding that species at greater risk of extinction are consistently more likely to show declining local prevalence across populations.
The finding indicates that large-scale biodiversity monitoring data, which are already being gathered in many parts of the world, could act as an early warning system for species moving towards extinction.
That includes species that have not yet been identified as threatened.
Population trends and extinction risks
Faye Moyes of the University of St Andrews and Laura Antão of the University of Turku in Finland led the research.
Their team analysed more than 60,000 populations of 2,362 species across 978 marine and terrestrial assemblages: groups of species occurring together that were sampled comprehensively for at least 20 years.
The population trends came from BioTIME, a database developed at the University of St Andrews and one of the most comprehensive collections of long-term biodiversity data ever assembled.
Researchers compared each population’s trajectory over time with its species’ extinction risk category on the IUCN Red List, the global benchmark for assessing how close a species is to disappearing entirely.
They wanted to know whether local population trends and extinction risk classifications conveyed the same picture.
In broad terms, they did. Species placed in higher-risk IUCN categories more often showed declining local prevalence across their populations.
A consistent extinction risk signal emerges
The signal was not perfect, and the resulting picture was genuinely complex. Nevertheless, it was consistent and clear enough to carry meaning.
One figure illustrates the scale of change: fewer than 10 per cent of the populations analysed showed a clear increase or decrease in prevalence over time.
Most remained stable or fluctuated without a strong directional trend.
Although declining populations were in the minority, the link with higher extinction risk was pronounced within that group.
“We have assessed for the first time whether there is a consistent signal between population temporal trends and a species extinction risk status using assemblage monitoring data, rather than ‘dedicated’ species-level assessments,” Antão said.
“Finding a clear signal that decreasing prevalence is usually associated with a higher risk of extinction is a strong indication that we can detect impacts even for species that are not currently classified as at risk.”
The complexity underneath
Local population trends and extinction risk do not correspond in a simple one-to-one relationship.
Some populations belonging to species already classified as threatened were stable or even growing. Meanwhile, some populations of species not currently identified as threatened were declining.
This complexity reflects a reality of conservation biology: extinction risk is assessed for a species as a whole, whereas changes on the ground occur population by population and place by place.
Rather than being noise in the data, these apparent contradictions offer useful information.
A threatened species with stable local populations presents a different kind of conservation priority from one that is declining everywhere.
Likewise, a species not classed as threatened that is quietly losing ground at several sites may merit closer attention than its present IUCN classification suggests.
From warning signs to action
The researchers say the practical implication concerns detection. Conservation resources are limited, making it impossible to monitor every species in every location comprehensively enough to spot emerging problems.
If the link between local decline and extinction risk holds up, large-scale assemblage monitoring data could flag species and populations for further investigation before a species moves up the IUCN threat categories.
“These temporal trends could serve as early-warning indicators and help target both new monitoring efforts and conservation actions,” said co-senior author Anne Magurran of the University of St Andrews.
“For instance, stable populations of at-risk species are of key conservation interest, while declining trends might highlight species that are missing from extinction risk assessments.”
That final point is especially important. Although the IUCN Red List is the most authoritative global system for tracking extinction risk, it relies on dedicated species-level assessments that are costly, time-consuming and inevitably incomplete.
Assemblage monitoring data, gathered systematically for many species at the same time, could help reveal gaps by highlighting species that may be more vulnerable than their current classification indicates.
Turning data into early warnings
The study links two major biodiversity databases with only limited overlap: BioTIME, which records changes in assemblages over time, and the IUCN Red List, which assesses threats at species level.
Combining them in this way extends what either database can reveal on its own.
“Because the task of assessing biodiversity change is gigantic, and we cannot travel in time to collect more data in the past, we want to use all the information available,” said study co-author Maria Dornelas of the University of St Andrews.
“These two large biodiversity databases have only limited overlap and this study shows us how we can leverage both to expand our understanding of biodiversity change.”
“Our results highlight the importance of assemblage level monitoring in conservation strategies and show the value of long-term time series such as those in BioTIME,” Moyes added.
The data contain the species that are quietly declining locally across sites most people will never visit.
The question is whether monitoring systems, and the will to act on their findings, can keep pace with the rate of those declines.
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