What determines
the distribution
of life on Earth?
B Y C AR S T E N RA H B E K
Rahbek, the DKK 2,500,000 award will be invested in
research to better understand how the planet’s organisms
move and adapt when their surroundings change.
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W H AT D E T E R M I N E S T H E D I S T R I B U T I O N O F L I F E O N E A RT H ?
Photo: Lars Juul Hauschildt
Carsten Rahbek (born 1965), professor at the Natural History Museum of Denmark, University of Copenhagen 2001, assistant professor at Peking University 2012, and head of the Danish National
Research Foundation’s Center for Macroecology, Evolution and Climate (CMEC) 2010. He is among the 1% most-cited researchers internationally, a member of the Royal Danish Academy of Sciences
and Letters, and the recipient of numerous national and international awards for research excellence.
Biology seeks to find as-yet unknown natural laws governing
the distribution of life on Earth. With the aid of vast databases of species distribution, the latest DNA techniques and bioinformatics tools, researchers are now getting closer to the
answer. Evolutionary processes and variation in the historical climate play a greater role than previously assumed,
where the focus was principally on the current climate.
This research is essential for responding to the global
biodiversity crisis, including climate change. Carsten
Rahbek is the recipient of the Villum Kann Rasmussen
Annual Award for Technical and Scientific Research for,
not least, his research in this field. According to Carsten
T E C H N I C A L A N D N AT U R A L S C I E N C E S
While the search goes on for life on Mars, we inhabit a planet that is largely unknown to us. Of
the assumed 15-20 million species on Earth, only
10% have been described. The geographical distribution and biology is known for only 1% of these
species. This unknown world was revealed to the
Western World more than a century ago when
Humboldt, Wallace and Darwin sailed off on
their famous natural history expeditions. Coming as they did from the relatively low-diversity
of European species, they were fascinated by the
extreme diversity in the Tropics. They had only
to travel another 100 km, and virtually all the local species would be novel and different. “Why
so?” they asked themselves. Today we are still asking: Why is the distribution of life on Earth so
heterogeneous that a mountain forest in Ecuador
smaller than a fair-sized European forest contains
far more diverse bird species than the whole of
Europe as far as the Urals? You can shake more
species of insect from the crowns of a couple of
trees in the Amazon than exist in the whole of
Denmark. The three biologists each proposed a
number of theories on patterns in the Earth’s distribution of life. The most famous of these being
the Theory of Evolution.
The scientific collections in the world’s natural history museums contain unimagined volumes of verified
information that is needed for answering some of
the biggest biological questions today. Here is an image from the bird collections at the National Museum of Natural History, Smithsonian Institution, which
are the world’s third largest with more than 625,000
specimens. The bird collection at the National Natural
History Museum of Denmark, University of Copenhagen, with more than 100,000 specimens, is also among
the largest and most valuable in the world. Photo:
‘Roxie&BirdCollnFinalPrint’ by Chip Clark, Smithsonian
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What we think we know – and how to test it
Over a century of research we have learnt a great
deal. Yet the question of, “what determines the
distribution of life on Earth” persists as biology’s
Holy Grail Question. We know that energy (from
solar radiation, which determines food resources)
is a key factor in determining how many species
and individuals can co-exist. There are almost no
species in the Arctic. We also know that life is dependent on water. Simply visualise a hot and wet
tropical rainforest with millions of species and the
species-poor Tropical desert. Ten years ago, there
was widespread consensus that the present-day
climate (temperature and water) determined the
distribution of life on Earth. Testable hypotheses
were formulated that only had to be confirmed by
global data.
The advent of IT technology made it possible to
test these hypotheses using quantitative data. In
1993, during my PhD studies in the USA, I started
to compare information on the global distribution
of all bird species – in electronic format. Much of
the information had just been lying ‘waiting’ for
decades in the collections of the world’s natural history museums. Ten years later, based on 100 million data units, we were in a position to detail and
analyse the global geographical variation in species
diversity in terms of the number of species coexisting in a given location and zone. By merging
the biological data with climate data (temperature
and precipitation) from 20,000 weather stations all
over the world, we were now able to test the hypothesis that life on Earth was determined by the
present-day climate. That was the good news. The
bad news was – or so many thought at the time
What we don’t know – and new discoveries
The existing hypotheses that the present-day climate alone determines life on Earth failed to account for the diversity of species in mountainous
regions generally, and in no way the extreme diversity found in Tropical mountain regions. It is by
no means insignificant that a presumed 90% of all
species are found in those very locations. As a universal explanation, the theory had now been firmly
disproved. And the act of falsifying a recognised
theory is regarded as a breakthrough in research.
But what then determines the distribution of life
on Earth – aside from energy and water?
Yet another technological breakthrough brought
us closer to the answer. By analysing genetic material – DNA-sequencing – we can describe species’
mutual relationships (phylogenies) and determine
which species in evolutionary terms are old and
which are new. Using the new technology and
new data from the field, we were able to start the
project of documenting evolution. With our geographical data we were able to prove that relatively
few places in the world have served as evolutionary ‘speciation pumps’, from which new species
were ‘produced’, and from there, spread across the
world. Surprisingly, these evolutionary ‘hot spots’
would also appear to be where old species survived
the global changes of previous ages – and there
is even a geographical overlap with the locations
where civilisations arose in the tropics. So, why did
all this happen in the same place?
Perhaps because these places have been climatically
unchanging at local level for millennia in the presence of stable resources – meaning that they were
relatively unaffected by global changes. That, at
– that the empirical data did not confirm hypothesised predictions. An example of the p
­ henomenon
of ’beautiful theories and ugly data’.
least, is our theory.
With our new data on the evolution and distribution of species, we have demonstrated that the dis-
W H AT D E T E R M I N E S T H E D I S T R I B U T I O N O F L I F E O N E A RT H ?
Amphibians (6,000 species)
T E C H N I C A L A N D N AT U R A L S C I E N C E S
Mammals (5,000 species)
Birds (10,000 species)
Global patterns of species diversity and phylogenies (family trees) for the whole world’s species of non-marine mammals
(~5,000 species), amphibians (~6,000 species) and birds (~10,000 species). The colour shading on the maps goes from blue
(fewest species) to green and orange to red (most species). The maps clearly demonstrate that the majority of species are
found in the Tropics, and that the world’s ‘hot spots’ (the orange-red areas) for biodiversity are found primarily in tropical mountain ranges. Graphics: Center for Macroecology, Evolution and Climate, University of Copenhagen
tribution of life, as we see it today, is largely a result
of evolution over hundreds of millennia. The pattern has not been erased or rebalanced in adaptation to our present-day climate. Furthermore, our
latest research suggests that variation in prehistoric
climate is at least as significant as current variation.
How is this type of basic research of relevance
to society?
Our discoveries have significant value in predicting
the effects of man-made global climate change, because previous models have assumed an equilibrium
between life and the present-day climate. This does
not in any way imply that climate change will not
entail radical changes for life on Earth. On the contrary, empirical data from the last 20 years show that
the changes will be immense. We also have an idea
of the direction the changes will take, but cannot
predict that many details. This will require a far better understanding of evolution and historical climate.
As humans, we breathe air, drink water and eat –
typically without giving much thought to complex
processes such as the circulation of matter in nature,
how the functionality of ecosystems is linked to
the species, and that the Earth’s ecosystems supply
‘free’ services corresponding to around half of the
GDP consumed annually by mankind. The global
community is facing major challenges and decisions
which require insights into the more precise impacts
of what is happening, and into the actions we take
and the decisions we make.
Our planet has previously sustained global changes,
including climate change, and life on Earth is likely
to survive. We know from studies of these changes
that there are both winners and losers among the
species. Homo sapiens coped eminently well with the
last Ice Ages – better than the competition. The hypothesis to account for these empirical facts rests on
the ability to think (bigger brain), the application of
knowledge for innovation (fire and tools) and – we
might add – the ability to make the right decisions.
That is why we are here today.
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