Sahney, S., Benton, M.J. and Paul Ferry 2010. Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters 6:544-547.
Published in Biology Letters today by myself, Michael Benton and Paul Ferry at the University of Bristol. Also an article at the BBC.
New research suggests that biodiversity is closely tied to the niches animals occupy, and the rich biodiversity we see on Earth today has grown out of expansion, not competition. Darwin cited competition among animals, coined ‘survival of the fittest’, as a driver of evolution in his book, On the Origin of Species; since then competition has been considered key to having grown Earth’s biodiversity. But while competition has been observed on a small scale, (eg. between species), there is little evidence of competition guiding large-scale shifts in biodiversity, such as the dominance of mammals and birds over reptiles and amphibians in today’s world. Our new research supports the idea that animals diversified by expanding into empty ecological roles rather than by direct competition with each other.
When vertebrates moved onto land millions of years ago, they filled empty niches further away from the water, and then they continued to invade new habitats evolved by other organisms such as forests, canopies, and grasslands. These animals began to burrow, climb, fly and take advantage of new food sources.
Our research shows that tetrapods (amphibians, reptiles, mammals and birds) have explored only one third of habitable ecological space and that without human influence, biodiversity would continue to increase exponentially.
Examining the biodiversity of tetrapods, we realized that their taxonomic diversity (the number families) closely matched their ecological diversity (the number of niches they occupied) through their 400 million years of evolution, and that there appears to be little evidence for competition as the driving factor for their great diversity.
Diversity was driven by the dominant animals at the time, which expanded into empty niches. Competition did not play a big role in the overall pattern of evolution. For example, even though mammals lived beside dinosaurs for 60 million years, they were not able to out compete the dominant reptiles. But when the dinosaurs went extinct, mammals quickly filled the empty niches they left and today mammals dominate the land.
Growing and shrinking biodiversity is closely tied to the niches animals occupy, so habitat destruction is a key aspect of extinction. In Earth’s past there have been incentives for animals to move into new modes of life, where initially resources may seem unlimited, there are few competitors and possible refuge from danger.
However, if niches are destroyed more often than created because of man’s influence on the environment, animals won’t have the opportunity to adapt and biodiversity won’t continue to grow.