Planet Earth is filled with life and includes thousands of species of vertebrate animals (mammals, reptiles, fish, and birds); invertebrates (insects, crustaceans, and protozoans); trees, flowers, grasses, and grains; and a bewildering array of bacteria, and algae, plus single-celled organisms, some inhabiting scalding deep-sea thermal vents. And yet, this rich profusion of flora and fauna seems paltry compared to the ecosystems of the deep past. By most reckonings, since the beginning of life on Earth, a whopping 99.9% of all species have gone extinct. 

There are a list of reasons why certain animals and species go extinct: Asteroid strikes, Climate change, Disease, Loss of habitat, Lack of genetic diversity, Better adapted competition, Invasive species, Lack of food, Pollution and Human predation, these are the ten most common reasons for a species to go extinct. 
 
Asteroid strikes: 
The first thing that people relate to extinction was the large asteroid strikes that led to the dinosaur's extinction 65 million years ago. It is likely that many of Earth’s mass extinctions not only the K-T extinction, but also the much more severe Permian-Triassic extinction—were caused by such immense events.   
The K-T extinction was the last asteroid strike that wiped out the dinosaurs at the end of the ‘Cretaceous period’ - the final period that dinosaurs spent on this earth, it was also the period when dinosaurs attained their maximum diversity, as ornithischian and saurischian families branched off into a bewildering array of armored, raptor-clawed, thick-skulled, and/or long-toothed and long-tailed meat- and plant-eaters. It was also during the Cretaceous period that the Earth began to assume something resembling its modern form. At that time, life was dominated not by mammals but by terrestrial, marine and avian reptiles. 
The Perman-Triassic extinction event was the most severe mass extinction in earth's history and was the one that transpired at the end of the Permian period, which annihilated 70% of terrestrial genera and 95% of marine genera. No one knows what caused the Permian-Triassic Extinction, but a series of massive volcanic eruptions resulting in a depletion of atmospheric oxygen which most scientists say was the culprit. It was this "great dying" at the end of the Permian that opened up the earth's ecosystems to new kinds of terrestrial and marine reptiles, and led, in turn, to the evolution of dinosaurs. 

 
Climate change: 
Even in the absence of asteroid strikes which can potentially lower the world's atmosphere by 20 – 30 degrees Fahrenheit – climate change has always been a second reason for certain species extinction. The end of the last Ice age about 11,000 years ago when various megafauna mammals (Giant mammals such as the mammoth and giant land sloth) were unable to adapt to quickly warming temperatures. They also succumbed to a lack of food and predation by early humans. And, the long-term circumstances of the current state climate change are in, in modern times. 

 
Disease: 
Disease isn't a likely cause to be the main cause of extinction to a species, there could be many other variables that could inhabit these species to get extinct from a lethal disease such as starvation, loss of habitat, and/or lack of genetic diversity through evolution. The introduction of any lethal virus or bacterium at an inopportune time can devastate certain species. The current crisis facing the world’s amphibians, which are falling prey to chytridiomycosis, a fungal infection that ravages the skin of frogs, toads, and salamanders, and causes death within a few weeks. 
 
Loss of Habitat: 
Most animals require a certain amount of territory to sustain their species. In these territories they hunt and forge, breed, and raise their young, as well as have room (when/if necessary) to expand their territory. A bird species may just need a branch or a tree to do these things to keep their species alive but large predatory animals like tigers measure their domains in square miles. As human civilization expands relentlessly into the wild these natural habitats diminish – due to the rapid loss of habitat the different species become more susceptible to other extinction pressures. 

 
Lack of Genetic Diversity: 
Once a species starts dwindling in numbers, there’s a smaller pool of available mates and often a corresponding lack of genetic diversity. Because of this small pool there is no room for there to be new mates of the species to develop and evolve genetics, if the small pool of animals mate it will run the risk of the next generation having undesirable genetic traits like susceptibility to fatal diseases. An example of this is: because of extreme habitat loss, today’s dwindling population of African cheetahs suffers from unusually low genetic diversity and because of this may lack the resilience to survive another major environmental disruption. 

 
Better adapted competition: 
There is always a risk of scumming to a dangerous tautology: by definition, ‘better - adapted’ populations always win out over those that lag behind, and we often don't know exactly what the favorable adaptation was until after the event. For instance, no one would have thought that prehistoric mammals were better adapted than dinosaurs until the K-T extinction. Usually, determining which is the "better adapted" species takes thousands, and sometimes millions, of years. 

 
Invasive species:
While most struggles for survival transpire over eons, sometimes the contest is quicker, bloodier, and more one-sided. If a plant or animal from one ecosystem is inadvertently transplanted into another (usually by an unwitting human or an animal host), it can reproduce wildly, resulting in the extermination of the native population – animal or plant. That's why American botanists wince at the mention of kudzu, a weed that was brought to America from Japan in the late 19th century and is now spreading at the rate of 150,000 acres per year, crowding out indigenous vegetation. 

 
Lack of food: 
Mass starvation is the quick, one-way, assured route to extinction—especially as hunger weakened populations are much more prone to disease and predation—and the effect on the food chain can be disastrous. An example of this would be if scientists find a way to permanently eliminate malaria by exterminating every mosquito on Earth. At first, that may seem like good news for humans, but the domino effect of all the creatures that feed on mosquitoes (like bats and frogs) goes extinct, and all the animals that feed on bats and frogs, and so on down the food chain, till eventually the food chain that has the mosquitoes as a primary consumer is extinguished leading all of the animals to either evolve and find different food sources or become extinct. 

 
Pollution: 
Marine life such as fish, seals, coral, and crustaceans can be exquisitely sensitive to traces of toxic chemicals in lakes, oceans, and rivers—and drastic changes in oxygen levels, caused by industrial pollution can suffocate entire populations. While it's nearly unknown for a single environmental disaster (such as an oil spill or fracking project) to render an entire species extinct. Constant exposure to pollution can render plants and animals more susceptible to other dangers, including starvation, loss of habitat, and disease, which can cause species to go extinct. 

 
Human Predation: 
Humans have only occupied the Earth for the last 50,000 or so years, so it's not fair to blame the bulk of the world's extinctions on the human species. There's no denying, though, that humans have wreaked plenty of ecological destruction during our brief time on this earth: hunting the starved, straggling megafauna mammals of the last Ice Age; depleting entire populations of whales and other marine mammals; and eliminating the dodo bird and the passenger pigeon virtually overnight.