In this article the authors provide evidence that tetratpod diversity is driven ecologically. So rather due to competition, the diversity was the result of conquering empty niches, a process known as adaptive radiation. Conversely habitat destruction is then likely to be the main element in destroying biodiversity.

 

 

Sahney, Michael J. Benton, and Paul A. Ferry

Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land

Biol. Lett. August 23, 2010 6:544-547; published online before print January 27, 2010, doi:10.1098/rsbl.2009.1024

 

 

Isn't that still natural selection?

Mostly not. It of course during the settling selective pressures can start to act, however the entrance into a new ecological niche is essentially a stochastic effect. It is well known that these and other effects are most likely driving forces of speciation rather than NS. The interesting thing in this paper is that they actually try to quantify it. Of course, once settling in occurs competition may start up again, but the main point is that the driving force is the movement into new niches, rather than having competition on that level.