Ecology helps us understand the importance of mass extinction in evolution, and it also helps us understand some of the causes of extinction. Ecological niche is one of the most important concepts to understand. An animal or plant lives in a certain place is active at a particular times, and eats certain things; all these factors describe its ecological niche.
Our environment is parted into millions of ecological niches, each that represents a probable home for life. Taking advantage of new opportunities are what animals and plants will always try to do, so they will always try to make a home in an empty niche. To specify, only one animal or plant can have a certain niche. When two different organisms try to take the same niche they will compete for the same resources, and one will always try to out-compete the other. When the species is extinct and there is an empty niche, there will be a race to fill it. Mass extinctions open up a swarm of niches, and there is evolutionary explosion as animals and plants adapt to fill the vacant homes.
As we all know the environment of the Earth is always changing which might have led to some niches being destroyed, making the species that have took it to extinction. A fall of sea level might lead to loss of shallow seas, so the species that made the niche in shallow seas home might become extinct, or evolve into forms adapted to the new environment. On the other hand, if two continents crash to form one, larger continent, then the niches of two continents can merge, no longer separated by a barrier. This leads to competition between the two species for the same niche, and can cause the extinction of one.
The dinosaurs did not survive on Earth. How is it that mammals and many birds did? Of course millions of species and thousands of types lower in the phylogenetic tree survived as well even though millions more went extinct. The basic rule of evolution is that the fittest organisms (species) will differentially survive and produce, thus out competing the less fit. But fitness is in place to the conditions of the environment and niche of the species. The continents have always been shifting. The local ecologies have always been changing, sometimes speedily, more often gradually. The more rapid changes are the ones that stress species and provide the major distinguishing forces that do the selecting. And then some species are more able to operate under the changed conditions and compete against rivals for the niches available more magnificently.
The End-Cretaceous Event appears to have been an equally unexpected one by physical and climatic criteria. The usual current hypothesis holds that the event was activated by an asteroid of enough mass slamming into the Yucatan around 65 mybp. Recent evidence adds strength to this hypothesis. The cataclysm had global effect, what is now called a nuclear winter-like wonder that so brutally changed Earth's climate that the food sources for the dinosaurs died back dramatically, therefore the dinosaurs were no longer fit. Selection did the rest. Punctuated equilibrium has been presented as a ranked theory of evolution. Supporters of punctuated equilibrium see speciation as analogous to mutation and the replacement of one species by another as analogous to natural selection. This is called species selection. Speciation adds new species to the species pool just as mutation adds new alleles to the gene pool. Species selection favors one species over another just as natural selection can favor one allele over another. Evolutionary trends within a group would be the result of selection among species, not natural selection acting within species Many species of mammals and birds also went extinct. But no dinosaurs other than birds survived. Several hypotheses have been progressive to explain this occurrence as well. Between them have been the general average sizes of birds and mammals, which were much smaller than the average dinosaur and that size difference plus metabolic energetics gave smaller birds and mammals an advantage. Other hypotheses involve the fact that most mammals, at the time, were burrowers so might have been protected from the harsher features of the climate changes.
* http://www.talkorigins.org/faqs/faq-intro-to-biology.html * http://paleobiology.si.edu/geotime/main/htmlversion/cretaceous4.html * www.sciencedaily.com/releases/2008/09/080930102631.htm