Why Tree Numbers Are Not What They Seem
How many tree species are there on Earth?
There are at least 10 tree species for every mammal species on the planet, and a third of tree species are at risk of extinction, although no one is sure if this is true.
Imagine counting every tree species on Earth—from towering sequoias to obscure tropical hardwoods hiding in unexplored forests.
Now imagine completing this impossible task and arriving at an exact count…
58,496 species
Not approximately 58,500. Not "around 60,000." Exactly 58,496.
This precise number comes from a recent global assessment, and while impressive, it reveals our tendency to present estimates as certainties. As someone who's dedicated a career to ecological research, I've learned that such precision often masks profound uncertainty.
In this issue, I'll show you how to develop the mindful sceptic's perspective—combining scientific rigour with practical awareness—to evaluate environmental claims without getting lost in technical details.
You'll gain tools to cut through information overload and find what truly matters, whether you're a student navigating research literature or simply trying to make sense of our changing world.
The best part?
These critical thinking skills work far beyond tree species counts.
You can apply them to any statistic or claim, helping you cut through information overload to find what matters.
Ready to become a more mindful sceptic?
Here we go.
How many tree species are there on Earth?
Do you know how many tree species there are in the world?
No, I don't either.
According to a recent report on The State of the World’s Trees produced by the environmental NGO Botanic Gardens Conservation International, there are 58,496 tree species on Earth.
For comparison, 58,496 is an order of magnitude larger than the number of mammal species.
A review of the taxonomic literature reveals that mammal taxonomists have descriptions for 6,495 species of currently recognised mammals (96 recently extinct, 6,399 extant). This amount represents an increase of 1,079 species in about 13 years, thanks to the efforts of taxonomists in making new descriptions, who described ~25 new species per year.
Hold on.
An increase in mammal species?
I thought a biodiversity crisis was happening, and the Sixth Mass Extinction was in our faces and on our hands.
Well, yes, both are true.
Current extinction rates are way above the fossil record and genetic data baseline. The trajectory defined by the rates of loss predicts that soon, a blink in evolutionary time, the number of species lost will reach the threshold that defines a mass extinction.
Although not precise, a widely accepted definition is that a mass extinction occurs when a significant proportion of Earth's biodiversity is lost within a relatively short geological time frame. Typically, this involves the extinction of many species across various ecosystems, and a commonly used threshold value is the loss of at least 75% of species on Earth.
The fossil record identifies several mass extinctions that meet this criterion.
The "Big Five" mass extinctions in Earth's history include the Ordovician-Silurian, Devonian, Permian-Triassic, Triassic-Jurassic, and Cretaceous-Paleogene events. Most of these were due to rapid climate change caused by volcanic activity. Even the Cretaceous-Paleogene extinction, when the impact of a large asteroid led to the extinction of the dinosaurs and many other species around 66 million years ago, would have been a shock, caused by cooling from dust in the atmosphere.
So, why is the number of recorded species going up?
Despite the rich tradition of natural history exploration to remote and undiscovered regions to collect specimens, make detailed observations, and document new species perfected and caricatured by Victorian gentlemen, we still do not have a complete catalogue of the biodiversity on Earth.
In the 1800s, many naturalists established private collections, and these specimens formed the basis for early natural history museums, where the material for taxonomy was curated. No matter that some of these guys—Darwin, Wallace, Gould, Huxley, Owen—are still remembered, they didn't complete the catalogue.
Their extraordinary efforts and the work of the taxonomists who followed them didn’t reach all the habitats, they did not find all the different types of organisms, and they did not formally describe all the specimens they found.
A significant portion of Earth's biodiversity remains undiscovered and undescribed.
Estimates suggest that over three-quarters of marine species and a significant percentage of terrestrial species remain undiscovered and lack formal descriptions. The proportion will vary across different taxonomic groups and ecosystems, but the consequence is essential—we don’t know how many species there are on Earth.
Estimates of species diversity vary widely, with some researchers suggesting numbers exceeding 100 million species while others propose much lower estimates. The most commonly accepted best estimate is somewhere around 8.7 million species. However, the actual number of species remains unknown, and most recent estimates fall within a range of around 5 to 10 million.
Let’s say that the number of species on Earth is 8.7 million.
The complex and dedicated work of generations of taxonomists has described roughly 2.16 million species, a quarter of the total.
The proportion is higher for species in big, colourful, easy-to-see and collect groups like mammals, birds, and reptiles, but the inventory is incomplete in all categories.
It explains why mammal taxonomists can generate ~25 new species descriptions every year.
Back to the trees
Botanic Gardens Conservation International says there are 58,496 tree species on Earth.
They must be confident because they quote the number in five significant figures. In numerical measurements, significant figures are the digits in a value contributing to its precision. They include all the certain digits and one uncertain or estimated digit.
When performing calculations, the result should be reported with the same number of significant figures as the least precise measurement involved in the calculation. The number 58,496 implies precision to the exact integer; you can’t have a fraction of a species in a count.
But this level of precision is impossible to achieve.
Determining the exact number of tree species on Earth is challenging for several reasons…
Taxonomic Complexity: Identifying and classifying species is a complex task, and taxonomy is a dynamic field. New species are continually being discovered, and taxonomic revisions may lead to changes in the classification of existing species.
Cryptic Species: Some species may appear similar but are genetically distinct, leading to the recognition of cryptic species. Advances in molecular techniques have allowed scientists to uncover hidden diversity not apparent through traditional morphological characteristics.
Inaccessible Regions: Dense forests, mountainous terrain, and other inaccessible areas make conducting comprehensive surveys challenging. In these regions, many species may remain undiscovered.
Lack of Documentation: Some regions with high biodiversity may lack comprehensive documentation, either due to political instability, limited resources, or other factors. This hinders the collection of data needed for a complete species inventory.
Extinction and Threats: Human activities, including deforestation, habitat destruction, and climate change, contribute to species extinction before they can be documented. This makes it challenging to obtain a complete record of all tree species.
You can probably guess where this is going.
The NGO's report is entitled ‘State of the World’s Trees,’ and it headlines that at least 30% of the world's tree species face extinction in the wild.
Well-known oaks, magnolias, and tropical timber trees are among the 17,500 tree species at risk of extinction—twice the number of threatened mammals, birds, amphibians, and reptiles combined.
Mass extinction?
How is tree diversity and risk calculated?
The research for the report collated information from the IUCN Red List Categories and Criteria (IUCN Red List, 2021), the most widely used system for assessing the probability of extinction for species.
The IUCN Red List uses standardised assessment procedures to assign species to different extinction risk categories based on five quantitative criteria, including measures of population sizes, restricted geographic distribution and rate of decline. Assessments are also complemented with a map and additional supporting information, including specific threats, uses and ecology.
The information comes from the research literature, where tree taxonomists, ecologists, and foresters report on surveys of trees.
Here is how tree species are split among the IUCN categories.
A couple of things you might notice from this graphic, beyond the high proportion of species categorised as threatened.
A fifth of all tree species are data deficient or not evaluated (21%), meaning that scientists do not know enough about the tree species on the planet to make a precise call on their status.
We are talking about trees—big organisms that do not move around and can only hide by looking like another tree. Humans have used trees for millennia and know a lot about how and where they grow, so it is odd not to know the status of one in five tree species on Earth.
Also noteworthy is that almost half of all species are either doing okay or only possibly at risk (49%).
The Top Threats Putting Trees at Risk
The biggest threats to tree species globally are
forest clearance for crops (impacting 29% of species)
logging (27%)
clearance for livestock grazing or farming (14%)
clearance for development (13%)
fire (13%).
In short, tree species are at risk from people.
Human need for resources and space to grow food is the standard problem for conservation.
Climate change might be on the list, but it is typically a confounding or accelerating factor contributing to human impacts. A tree cannot run and has trouble hiding in the days of GPS systems, chainsaws and transport machinery.
A Mindful Sceptic Take on the Extinction of Tree Species
Whenever a number is an answer to a question, it begs another. How reliable is that evidence?
58,496 is not the number of tree species on Earth. It estimates the actual number because not all tree species are described or all habitats are surveyed. The number is an approximation with unknown uncertainty.
As new species are discovered, the number goes up. As extinction occurs, the number goes down. So, for most purposes, the number itself is far less critical than how reliable that estimate is and whether the number is trending over time.
Our recommendation is always this…
Never leave a number alone
Consider this little anecdote for a number.
Is 90,000 a big number?
It could be.
If 90,000 were the number of…
children who died from preventable diseases in the last month, it would be both significant, tragic and unacceptable
passengers aloft in commercial airliners at any given time, then a whole heap of airlines would go bust overnight, given there are roughly a million people in the air at any one time
potholes per kilometer of road, you would be in Zambia
The problem is that 90,000 on its own has no context or comparison.
I could tell you that 90,000 ha of native vegetation cleared for agriculture triggers a policy review for legislation that regulates what farmers can do on their land in NSW, Australia.
Farmers apply to legally manage or clear native vegetation on their properties, and when the cumulative area of approvals reaches 90,000 ha, civil servants agree in advance to examine the policy closely.
It should be noted that during the policy development, any agreement on the trigger number was hard-fought, for there are advocates for zero hectares and those who believe that no number is big enough.
At least this 90,000 has context. It represents an area of land and has a purpose.
Among other things, it anchors the debate between advocates who claim the economic progress from agricultural development and detractors who lament the loss of native plants from the landscape. On its own, 90,000 ha garners opinions, even as a trigger number.
However, what that 90,000 needs is some company.
To know its place and find meaning in its existence, the ‘90,000’ needs to have some other numbers. For example, the area of native vegetation under conservation, restoration or active management (12,863,450 ha), the area of winter crop for 2018 (3,100,000 ha) or a string of numbers such as the area converted to arable agriculture each year for the last 50 years (curiously this value is difficult to pin down).
Only when 90,000 ha has other numbers can it find itself, make sense and contribute to society.
This is true of almost all reported numbers.
Suppose a Minister announces with great fanfare a further $4 million in funding for schools. That's not bad, you might think. It would take the average Joe a couple of working lifetimes or a lottery win to get that kind of cash.
In Australia, $4 million is enough for the salaries and overheads of roughly 40 teachers for one year. These teachers would be expected to look after 560 students, quite a few it would seem. Only there are 1.52 million high school students in Australia making 560 a tiny proportion of those seeking erudition and selfies.
So the 40 extra teachers recruited from the Minister’s largess would teach 0.036% of the high school population for one year.
After that, the Minister would need to make another announcement.
Now back to the 90,000 ha.
It is certainly a precautionary number in the context of the area treated under a native vegetation policy. As a proportion of the area of NSW (80.9 million ha), it is minuscule, even as a proportion of area under arable production, it is small.
Then there is an unprecedented fire season, and over 3.6 million hectares are burnt by bushfires that rage for months, the largest covering more than 800,000 hectares of continuous forest.
This time, 90,000 ha has a very different context. Fire is not the same as clearing, for most trees will recover from fire, and seedlings will be established in the ash beds when it rains, but the point is that the area needs context.
Is it worth discussing if the trigger is just 10% of the area of a single fire?
Comparison is always critical when dealing with numbers. On its own, a number makes no sense; it's naked, self-conscious, and insecure. It needs some context for clothing and some friends to compare against.
Next time there is an argument over a single number—like there will be over the 3.6 million hectares of bushland burnt in 2019—remember you can’t leave it alone.
Mindful Momentum
The Lonely Number Challenge
Next time you see an environmental statistic in the news, write it down and ask yourself three questions:
What is it being compared to?
What might be missing?
Who measured it and why?
Keep a small notebook of these numbers and their context. After a week, you'll start seeing patterns in how numbers are presented and what they often leave out.
Key Points
The reported figure of 58,496 tree species globally demonstrates how precise numbers in environmental science can mislead without proper context. This case study in mindful scepticism reveals the challenges in cataloguing global biodiversity and shows why seemingly exact figures often mask significant uncertainty.
Understanding biodiversity numbers requires examining both what is counted and what remains unknown. With 21% of known tree species lacking sufficient data for assessment and many regions still poorly surveyed, current species counts represent estimates rather than definitive totals. This uncertainty extends even to relatively conspicuous organisms like trees.
Major threats to tree species stem primarily from human activities, with forest clearance for agriculture, logging, and development affecting the majority of at-risk species. However, evaluating extinction risk requires careful consideration of assessment methods, data quality, and the complex interplay between direct threats and amplifying factors like climate change.
The practice of mindful scepticism - questioning numbers while maintaining objective analysis - proves essential for understanding environmental data. Just as no tree stands alone in a forest, no number should stand alone in analysis. Context, comparisons, and careful evaluation of uncertainty help transform raw statistics into meaningful insights for environmental decision-making.
Curiosity Corner
This issue of the newsletter is all about…
Through the lens of global tree diversity, we discover why mindful skeptics never let a number stand alone, showing how context transforms environmental statistics from misleading precision into meaningful insight.
1. How do we know we've found most tree species? Instead of accepting the precise number, this question challenges our assumptions about completeness and reveals the fascinating challenges of global biodiversity surveys. It opens up stories about unexplored places and undiscovered species.
2. What makes a tree species different from another tree species? Rather than just counting species, this deeper question exposes the complexity of taxonomy and classification, showing how even apparently simple categories like 'tree' involve sophisticated scientific judgment and evolving understanding.
3. Why do we count species at all? This steps back from the numbers to examine our motivations and values, helping us understand that species counts aren't just academic exercises but tools for conservation and understanding environmental change. It connects data to purpose.
4. What happens to our conservation strategies if our species counts are wrong? This practical question moves beyond abstract numbers to real-world consequences, exploring how uncertainty in our knowledge affects our actions and decisions about protecting biodiversity. It makes the abstract concrete.
5. How can we make good decisions with incomplete information? This is perhaps the most powerful question because it acknowledges uncertainty while focusing on action. Instead of being paralyzed by imperfect knowledge, it asks how we can act thoughtfully and effectively anyway - the essence of mindful skepticism.
Question 5 is a ripper because it transforms scepticism from a potential barrier to a practical tool. It's the kind of question that doesn't just challenge assumptions but builds bridges to solutions. While the other questions help us understand the problem, this one helps us move forward despite uncertainty - exactly what we need in tackling environmental challenges.
What’s your better question?
In the next issue
Ever faced an elephant while running a fever in the Okavango Delta?
In next week's issue, I'll share how one of my most uncomfortable moments as a field scientist taught me that real scientific thinking isn't about white coats and sterile labs but finding clarity in chaos.
Join me to discover why the world needs more awkward scientists and how scientific thinking could transform your approach to life's biggest challenges.
Wonderful article, and it really does apply to almost every subject we discuss in public debates, not just ecology issues. Numbers are far too often used to purposely support a pre-decided policy, one they would usually not seem to support if we gave them some friends and a context to play in together. This is crucial for everyone interested in public discourse to understand.