What is the main Factor that cause wildlife extinct?

In this section, we are going to determine the main factor that causes wildlife extinct. According to my study in the book of Environmental Science, A Global Concern, 8th Edition by Cunningham and Saigo, 2005, the authors determined two main factors which are Natural Causes of Extinction and Human- Caused Reductions in Biodiversity. Let us look into it one by one.
First, let us look into the aspect of Natural causes of extinction briefly, research on fossil suggest that more than 99 per cent of all species that ever existed are now extinct. Most of those species were gone long before humans came on the earth. Why kind of natural event have caused they aren’t longer exist in this earth?
There are some theories suggested that species change gradually through competition for scarce resources and natural selection, a process in which those members of population that are best suited for a particular set of environment conditions will survive and produce offspring more successfully than ill-suited competitors. This theory is known as theory of evolution, developed by Charles Darwin and Alfred Wallace. Cunningham& Saigo, 2005. Species arise through processes of mutation and natural selection and disappear the same way. The tiny Hypohippus, for instance, has been replaced by the much larger modern horse, but most of its genes probably still survive in its distant offspring. To take a close look on the image of Hypohippus, you may refer to this link: http://upload.wikimedia.org/wikipedia/commons/a/a7/Hypohippus.jpg
Mass extinctions have wiped out vast numbers of species and even whole families. The best event to understand these events is the end of Cretaceous period when dinosaurs disappeared, along with at least 50 per cent of existing genera and 15 per cent of marine animal families. Current theories suggest that these catastrophes were caused by climate changes, perhaps triggered when large asteroids struck the earth.
After having a brief discussion about Natural Causes of Extinction, we are going to address the second main factor which is Human-Caused Reduction in Biodiversity. Between A.D. 1600 and 1850, human activities appear to have been responsible for the extermination of two or three species per decade. Some estimates, we are now losing species at thousands of times natural rates. If these trends continue, the United Nations Environment Program warns, between 22 and 47 per cent of all known plant species-and the animals dependent on them- could be extinct in the next 50 years. Cunningham& Saigo, 2005. After this we are going to look into this Human-Caused Reduction in Biodiversity deeply by introducing its sub-factor. So we are looking forward to our next discussion specifically on these sub-factors of Human-Caused Reduction in Biodiversity.
Before we leave our discussion, we would like to invite you to think the sub-factor that causes the reduction in Biodiversity. :-)