MarkDo you know how many tree species there are in the world?
58,496 according to a recent report on The State of the Wold’s Trees produced by the eNGO Botanic Gardens Conservation International.
The neotropics covering Central and South America have over a third of these species. Trees are diverse, where the climate has had pockets of warmth for a very long time.
For comparison, 58,496 is an order of magnitude larger than the number of mammal species.
A review of the taxonomic literature counted 6,495 species of currently recognised mammals (96 recently extinct, 6,399 extant). This was 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 there was a biodiversity crisis happening and the sixth mass extinction was in our face and on our hands.
Well yes. Current rates of extinction are way above the baseline recorded from the fossil record and genetic data but the taxonomists are few and have a mountain of classification work to do because not all species are described by science. We do not have a definitive catalogue.
Back to the trees.
At least 30% of the world’s tree species face extinction in the wild.
Well-known oaks and magnolias to tropical timber trees are among the 17,500 tree species at risk – twice the number of threatened mammals, birds, amphibians and reptiles combined.
How is tree diversity and risk calculated?
Research for the report collated information from the IUCN Red List Categories and Criteria (IUCN Red List, 2021), the most widely used system to assess 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 for these assessments comes from the research literature where the tree taxonomists, ecologists and foresters report on surveys of trees.
Here is how tree species split among the IUCN categories
Source: BGCI (2021). State of the World’s Trees. BGCI, Richmond, UK
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 not to know the status of one in five of the tree species on earth is interesting.
Also noteworthy is almost half of all species are either doing ok or only possibly at risk (49%).
Tree 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. The standard problem for conservation.
Climate change might be on the list but it is typically a confounding or accelerating factor added to the human impacts. A tree cannot run and has trouble hiding in the days of GPS systems, chainsaws and transport machinery.
What sustainably FED suggests…
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 true number because not all tree species are described and not all habitats surveyed.
As species are found, the number goes up but down with extinction events. So for most purposes, the number itself is far less critical than how reliable that estimate is and if the number is trending over time.
Our recommendation is always this…
sustainably FED
Science sources for this post
Burgin, C. J., Colella, J. P., Kahn, P. L., & Upham, N. S. (2018). How many species of mammals are there?. Journal of Mammalogy, 99(1), 1-14.
Hero image modified from photo by Cristina Gottardi on Unsplash
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