What Causes Tropical Biodiversity?

Ecologists have always thought that tropical biodiversity came from increased species interaction. Find out why they may have been wrong all along.

I’m infatuated with IMAX movies. I can barely walk past an IMAX theatre without buying a ticket. 

Something is intoxicating about the massive, curved screen, the 3-dimensional effects and the exquisite sound systems. One of my favourite IMAX films is called Tropical Rain Forest.

The film’s creators structured it around the fact that we see far more biodiversity in tropical rain forests than in our planet’s temperate zones. They make a passionate case that these habitats are precious and must be conserved. 


One of the items on my bucket list is a trip to Costa Rica to see the tropical forests there for myself. I just don’t want to do it in a way that harms the habitat.

I’m not the only one in awe of tropical biodiversity. In 1849, British naturalist Alfred Russell Wallace wrote about Brazil. He said, “There is, however, one natural feature of this country, the interest and grandeur of which may be fully appreciated in a single walk: it is the ‘virgin forest.’ Here no one who has any feeling of the magnificent and the sublime can be disappointed. 

He went on to write, “These, and many other novel features – the parasitic plants growing on the trunks and branches, the wonderful variety of the foliage, the strange fruits and seeds that lie rotting on the ground – taken altogether surpass description, and produce feelings in the beholder of admiration and awe.”


For some reason, we see far more varied and intriguing rare species between the Tropic of Cancer and the Tropic of Capricorn. The traditional explanation for tropical biodiversity goes all the way back to Charles Darwin. He thought that the reason we see such a large number of species in the tropics is that there is more interaction between species.

The idea is called the “biotic interactions hypothesis.” Other terms for it include biological interactions or symbiosis. 

Interactions include anything from bees pollinating flowers to parasites living off hosts to predators eating their prey to insects invading tree species. These interactions are thought to enhance the natural selection process.


Plant diversity increases when flowers develop more vivid colours to attract pollinators or toxins to ward off plant enemies, for example. Predators and their prey may each become faster and more agile in a Darwinian arms race.

The more interactions there are in a population, the hypothesis tells us, the more demanding natural selection will be, and the faster species will adapt. More adaptation leads to species diversity, resulting in greater overall tropical biodiversity.

The most common and essential biological interaction all over the world is predators eating their prey, or “predation.” The web of life in the animal kingdom is mainly about who eats who.


Ecologists have made several studies over the years to verify the biotic interaction hypothesis. The results are mixed.

Some studies did see more animals eating eggs, baby birds, shellfish or seeds in the tropics. Others didn’t find any relationship between predation and latitude. A few found that there’s more interaction outside the tropics.

Some scientists have looked at fossils for evidence, but this has also been inconclusive. There does seem to be more predation near the equator for certain kinds of shellfish. Still, there’s no evidence that this has led to faster evolution.


A new study published last week in the journal Nature Communications suggests that the tidy, intuitive explanation of tropical biodiversity may be wrong. The researchers looked at the predation relationships between tuna and large sharks and their prey.

They took advantage of four massive databases that track longline fishing in the open oceans. The ecologists analyzed figures from the East and West Pacific, Atlantic and Indian Ocean basins.

Their results showed more biotic interaction among fish outside the tropics than near the equator. Then, closer to the poles, the interactions drop off again.

MOST INTERactions were in between tropics and arctic

Their comprehensive analysis contradicts the idea that there are more symbiotic relationships between species in tropical regions. At least among fish in the open ocean, most interactions were in between the tropics and the Arctic or the Antarctic.

More importantly, all these biotic interactions in temperate zones should have caused more biodiversity there. The team found the opposite.

Professor Marius Roesti began this research at the University of British Columbia. He’s now a member of the Institute of Ecology and Evolution at the University of Bern. He explained how today’s big data systems enabled the work.

“investigation possible due to extraordinary data set”

“This investigation was only possible due to this extraordinary data set. The full data set spans the entire planet and records over 900 million catches of fish predators by longline fisheries over the past 55 years.”

They then compared the data on fish catches with measurements of species richness in a database called Aquamaps. This allowed them to find the correlation between predation and tropical biodiversity.

Professor Roesti described his findings this way, “The latitudes with the relatively highest number of captured predators are in or near the temperate zone, and not near the equator. This result is generally true for all ocean basins and the entire period under investigation.”

the story of our place in nature has a hole in it

This is a dilemma for ecologists. It’s an evolutionary gap. The story of our place in nature has a hole in it.

The challenge now is to look for other examples of species interactions and whether they vary according to latitudes. There may be another explanation for tropical biodiversity or even multiple answers.

As often happens, the more scientists have learned about this phenomenon, the more they realize that it’ more complicated than they imagined. The team has called for further study.

they realize that IT’s more complicated than imagined

As the researchers declare in their paper, “Evolutionary ecologists are now charged with explaining why the eco-evolutionary processes thought to generate and maintain diversity are not always strongest in the most diverse regions on Earth.”

We always have more to learn if we dare to know.
Learn more:
University of Bern
Pelagic fish predation is stronger at temperate latitudes than near the equator
Origin of Life Before the Origin of Species – 4 Theories
Is it a Bird? Is it a Bee? No, it’s a Hummingbird Moth
Evolution: Is It Always Genetic?

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