Written by: Aashutosh Shrestha - 23004, Grade XI
Posted on: 28 February, 2022
Deep in The Amazon jungle lies the Yasuni national park, Packed within an average hectare of this dense, steamy rainforest there are more species of tree than the US and Canada combined, as well as 150 types of amphibian and an estimated 100000 insect species (According to recent research).There are different life forms in Yasuni than anywhere else in South America. It’s hard to get very far, because every few minutes you see or hear something new, says Matt Finer, staff ecologist at Save America’s Forests. Yasuni may well be the most biodiverse place in the World. If Yasuni does indeed hold this title, its tropical location will come as no surprise to biologists. The tropics boast more than ten times as many species of animal and plant as the Arctic, with diversity steadily as you approach the poles. This gradient holds true for both land and the ocean depths. The big question is why? What is it about the tropics that fosters biodiversity? It is a mystery that has puzzled biologists for decades. Yasuni might help us find an answer.
According to one classic theory, the reason is that there is more habitable space around the Equator than at the poles. On the face of it, this seems to make sense. The tropics encompasses an area nearly five times the size of the Earth’s polar regions, and there is evidence that habitable space is correlated with the number of species on land. Research in the ocean tells a different story, however. David Jablonski of the University of Chicago, and his colleagues are involved in long term study of living and fossil marine bivalves, a group including oysters and mussels. Looking at the present day biodiversity patterns in 4000 bivalve species, they have found no relationship between habitable area and the number of bivalve species. Habitable areas just don't explain marine biodiversity gradients,’Jablonski says. Perhaps biodiversity at sea and land are governed by different rules. The traditional explanation patterns of marine biodiversity is known as Rapoport’s rule. The idea here is that ocean- dwelling species in the tropics are very sensitive to temperature, so are restricted to small ranges where the water is just right, whereas species in cooler waters can tolerate a broader range of temperatures so spread out. This could explain why large numbers of species are packed together in the tropics, but it doesn’t seem to hold consistency. ‘There are more exceptions to this rule than strong examples,’ says Jablonski,’which means it’s not much of a rule.’ So what are the alternatives? Some of the researchers have argued that speciation rates both terrestrial and marine, could be much higher in the tropics, making them a ‘cradle’ of biodiversity. Others have suggested that extinction rates are a decisive factor, with species less likely to become extinct near the equator than at higher latitudes, making the tropic a ‘museum’ of biodiversity.
To tease out the alternatives, Jablonski and his colleagues focused on three key factors: the rate at which species have evolved in any given location, the local extinction rate, and the immigration rate of new species. They found that three quarters of marine bivalve genera existing today evolved in the tropics then spread out towards the poles, while also remaining in their original habitat. So the tropics are a cradle of biodiversity. But that’s not all. There are also a number of old genera in the tropics indicating rates are lower in the tropics than in cooler regions. So the tropics are also a museum of biodiversity. The team concluded that their findings support an ‘out of tropics’ theory to explain and found more evidence that the tropics are both a cradle and a museum of biodiversity.
