Scientists have many theories about what caused the extinction of dinosaurs. Below are thoughts on some of the most common theories:

• Since about 90% of dinosaurs were herbivores (plant eaters), it was thought that when an asteroid hit earth and many tons of dust was thrown into the atmosphere that the result was a "long night" due to the lack of sunlight that was able to reach earth, the plants died out and without a food source, so did the dinosaurs.

• It could also have been the combination of the Chicxulub asteroid impact event (causing worldwide fires and environmental catastrophe) in association with the Deccan Traps volcanic event, which occurred at around the same time, or possibly as a result of, the asteroid impact event.

• There are many theories that are written throughout history of why the dinosaurs exactly were driven into extinction. Most are meteor impact related. By looking at craters all over the world, scientists can estimate the meteors size, speed and its impacts effect on the local environment. See the related link for more information.

• Though the large meteorite impact (Chicxulub) figures prominently in a lot of the theories of how the dinosaurs became extinct, the idea of a gamma ray burst has also come up now that we are aware of them and the threat they pose. Additionally, there was an "outburst" of volcanic activity during this period. This extinction event is at the Cretaceous-Tertiary (K-T) boundary.

• It should also be added that several other large impact events occurred right about the time of Chicxulub, including Silverpit and Boltysh craters, among others. It seems the deck was "stacked against" the dinosaurs, regardless of how you look at it. It is possible that things are "found" to "corroborate" extinction by space rock, but the facts are pretty convincing, even for the skeptic.

• It was noted but is worth repeating: many of the ideas that represent credible ways that could have triggered the dinosaurs' extinction could have arisen separately and acted in concert to kill off all these creatures. One thing is certain - they're gone. And the demise of the terrible lizards was caused by something dramatic given the relatively narrow window in which they disappeared.

• The hypothesis that states an asteroid-impact was the sole cause of the extinction is just that - an hypothesis, not a fact. Most paleontologists feel that the asteroid impact was just one of a series of catastrophic events that ultimately wiped out the dinosaurs. There is evidence of a huge meteor strike just off the Yucatan Peninsula, true. And a meteor may well have put a final period to the extinction pulse. But there is also evidence that other factors may have been involved. For one thing, the dinosaurs didn't become extinct all at once. The pulse seems to have taken more than 10-thousand years to complete which makes the asteroid theory implausible as a complete explanation. There is evidence that there was massive volcanic activity also, and a drastic climatic change. The dinosaurs were in decline in number of species long before the K/T extinction was over. Furthermore, some dinosaurs did survive into Laramide time - the birds are still around (admittedly that's begging the issue, but it's not unreasonable to ask how they managed to survive when so many their brethren were killed).

• Dinosaurs were already fast becoming endangered species before the Chicxulub impact. Increased volcanic activity such as the Deccan Traps in modern day India was slowly killing many species. Between 68 to 60 million years ago 150,000 km2 of igneous rock was deposited in places more than 2 km thick. The bulk of the eruptions occurred about 66 million years ago near modern day Mumbai. These eruptions lasted as little as 33,000 years but the effect on the environment would have been staggering. Dust and debris thrown into the atmosphere along with sulphur dioxide would have cooled the earth. Also many poisonous gases would also have been released. Studies on modern day birds suggest that these poisonous gases not only directly killed dinosaurs but also caused thinning of the shells of their eggs which meant that many would fail to hatch.

• It has also been suggested that disease killed off the dinosaurs. A very deadly and contagious disease may have circulated among the dinosaurs forcing them to become extinct. Still yet another theory is that the Earth just gradually changed in climate over a long time period and the dinosaurs were not able to adapt to the cooler, dryer climate.
• When the continents broke up, they couldn't get to new feeding grounds when theirs were exhausted

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It mst also be pointed out that the avian type dinosaurs (the birds), did not become extinct and are living today.

This answer has been fully researched, and is as complete and detailed as is practical in this format. If you'd prefer a shorter, more condensed version, please see the question listed below this answer in the "Related Question" field above the "Related Links". The answer has been done in concert with this answer, and follows the progression of this information, if you find you need more details.

An Endangered Species is any species, plant or animal that is "in danger" of dying out, or suffering population reductions to such an extent that the species can no longer recover and faces extinction in the wild. According to the "International Union for Conservation of Nature" (IUCN) Red List, a species would be considered endangered when the best available evidence indicates that the species is facing a very high risk of extinction in the wild. Extinction is usually referring to the species in the wild. Although there may be some specimens in zoos, or controlled environments, if there is really no reasonable doubt that the last specimen has died, then after surveys have been conducted, over the lifespan of the species, in its native, or known habitat, then a species should be declared extinct. But this is a long drawn out process, taking at least the length of the average lifespan of the species suspected of extinction.

There are many different things that cause species to be endangered, and/or become extinct. Depending on the individual species involved, there can be specific causes directly related to that species. Also much information may depend on the adaptability, and what type of species. But generally speaking, there are certainly some things that can be applied to almost all species on average, across the board. Most involve humans to some degree, and are linked to their behavior, or their lack of proper behavior.

Commercial hunting or fishing without regulations or restrictions can, and has driven species to extinction. Even when it is apparent that the species population can no longer support extensive hunting or fishing losses, many times the immediate profits seem more important than the long range health, or even just sustaining the life of the population of the very animals which are making those profits possible. They "harvested" the Passenger Pigeon until its extinction in 1900. The term is used in the Fishing and Birding Industries today, but to call it "harvesting" a wild animal, is putting a pretty picture on it. To harvest something, when you have no hand in the seeding of it, the nurturing, or the growing of it, is not really harvesting. Then when a species has been recognized as in need of protection, and as such is granted a protective status, many will ignore those regulations and laws. Sometimes even whole countries will ignore the rulings and laws passed to protect species, and continue to hunt or fish the protected animals. This places an unbearable strain on the animals, unnecessary tension between countries, and makes the task of catching the illegal black market sale of protected animals, and the products produced with them nearly impossible.

Poaching is widespread, again with profits being considered before the health of a protected population of animals. These are animals, that are making the profits for the poachers. The poacher is ignoring the fact that their daily efforts at killing, and/or capturing the protected species could be the very actions that drive the animal's population past the point of no return. When the population collapses, they put themselves out of a job. This includes the animals that are taken alive, when tiny babies, so they will bond with their human owner. While others are captured in mass, no matter their age, or health, to be sold as exotic pets. The number that die in transport is staggering, but then, they do it all for the profit, and to meet a demand. The poacher does not care about the species, or any parasites, diseases, or viruses it might spread to other people, or animals it may come in contact with. The poacher's only concern is the money they can make " literally" off the animal, in whatever way brings them the most profit. In many areas of the world poachers are willing to kill guards posted to protect lands and the species that inhabit them, just for the chance to hunt the animals. This ability to look without seeing the impact of one's actions seems especially strong in those who poach animals.

The buyers demand these black market items. Some of the poached items are pelts, organs and bones, which are often used in Traditional Chinese Medicines; the Rhino's horn, often mistakenly thought to be an aphrodisiac in the powdered form, but really used in Traditional Chinese Medicine also, as a fever reducer and prescribed for convulsions; the Elephants' tusks, and Gorilla paws (hands). These are other examples of what will be taken for profit, and set for sale in small back street markets. Many things bought from these types of markets are merely trophies. The person buying these products is no better than the poacher, who sneaks through the woods or jungle, trapping or shooting a species that is protected. The buyers need to be held in same contempt as the poacher is! Without the demand, the value of these items would not make the risk of hunting and selling endangered species profitable, and the poacher might not be hunting the animal in the first place. The "Traditional Chinese Medicines" originated thousands of years ago, and is still in use today in much the same form. It is still using now endangered species as ingredients, rather than trying more modern approaches to replace these ingredients, and continues to fuel the illegal sale of endangered species from all over the world. This inaction, and sometimes ignorance of the ramifications, just continues to drive endangered species closer to extinction.

Then you have the loss of habitat. This can be caused by the loss of complete access to all of their habitat, due to the fragmentation of the land, or water; isolation or disruption of breeding grounds; or altered migration routes; and/or the habitat itself is changed to such a degree that it can no longer support the species, with the necessary food sources, water supplies, and shelter requirements. Small isolated species groups become increasingly stressed due to the lack of biodiversity, making the chances of a long term growth and birth rate increase highly unlikely. Changing land for whatever purpose, such as logging, mining, urbanization, roads or rails; changes to rivers; and draining wet lands to build up and on, all effect the animals living in the surrounding or general area, so the entire ecosystems need to be taken into account before these actions are taken. But most often, especially in countries on the fast track of growth, this is done without consideration for the overall impact to the species of the area. Migrating species and those with specific breeding grounds, or other specialized habitats need to be considered carefully before starting the work. Major changes can create havoc in breeding cycles, disrupt the migration routes, or end whole life patterns, and may start the species down the path of endangerment and extinction.

Also, with all the growth of humans and their encroachment into animals' habitat, comes the trash and debris that seems to be a constant around humans. Something that seems perfectly harmless to us, can be deadly to a hungry, or curious animal. Just touching some of the by-products we leave can be dangerous to different species, but most often the damage is from eating something that is poisonous, or being exposed long-term, to something that we use daily. That can destroy their finely balanced systems, injuring them, and have long ranging effects on them and their offspring. Just the normal curious, playful nature of many species can get them into trouble. Finding something they are not familiar with, and investigating, or playing with it, they can become entangled, trapped, or get it snagged on their body, and not be able to get it off. This can strangle a limb or their life off. Or, they simply see something that looks, and behaves like food, so they grab it, and gobble, only what they have eaten wasn't food. Their intestines become blocked, and their life span will be shortened. The shortened lifespan may mean that species couldn't produce its offspring, and that decreases the number of young produced for the next generation. Thus a small piece of trash, that seemed of no importance, may help a species step on the first rung of the endangered species ladder.

All the pollutants that are part and parcel of our industrial age, are disposed of in all manner of ways, with some in the correct, and approved methods. While we hope most people use the correct and legal methods, we know that others are just handling them in the easiest, and quickest way, and these pollutants will end up in the soil, water, air, plants, and eventually in small animals, only to work their way up through the food chain. This will apply pressure on all species it touches along the way, and create another obstacle for the healthy growth of a species population. Many times the most profound effect of these chemicals can be seen in the top of the food chain, the hunters, or predators. The predators are exposed to all the different kind of toxins from all the different animals they prey upon. Each carrying traces of the chemicals it was exposed to, and that trace accumulates in the upper levels of the food chain, and certainly in the top predator, increasing their exposure enormously. All these toxic chemicals, oil spills, acid rain, and other man made pollutants can kill off plants, algae, insects, etc, causing a domino effect. This is the food or water supply of any number of species, where some may already be under strain from other factors, and with the introduction of the pollutants another harmful factor has been added to their environment. This factor can be passed down through generations of a species, through habitats to different species, and even leach into new ecosystems. Sometimes, it is estimated that it will take decades for an area to recover from an accident, or intentional dumping, even after an official clean up.

To add to the list of overall causes of endangerment to species is the introduction of uninvited species. These party crashers of the critter world are the nonnative species, an exotic species released either intentionally, to help control some "pest" only to become a bigger pest themselves or accidentally. The escapees from pet shops, or many times, those released to avoid detection of trafficking in protected animals, and from personal homes once the animal becomes too big, or is more trouble than expected. These "foreign species", once established in an area, can be invasive, and replace, undercut, and push out the native plants, and animals. The native species can be completely defenseless against the invading species and any hijacking diseases, or parasites. The native species can suffer great losses to their populations. If the exotic species flourishes, it will probably have no natural enemies or predators in the new ecosystem to help control its growth, allowing it free rein to overrun the species that naturally inhabit that range. They can have long ranging effects, replacing food sources for other species, and spreading through many ecosystems, destroying the balance that many species rely on for breeding and survival. So, an uninvited guest can ruin habitats, and put many other species at risk just by showing up.

Also, you can't leave out diseases, not only exposure to new diseases from exotic invasive species, but mutated viruses, and diseases. The constant growth, and mutation of some diseases has even stunned some of the doctors working with the primates, and other mammals. This seems to be a field that is growing by leaps and bounds. As with the rabies virus and distemper, at one time, thought to be some what under control, but now out of control in parts of Africa. It is infecting wild carnivores because of exposure to domestic animals, coming into contact with the wild carnivores, and other wild animals, that have no natural protection or immunity. Include, the transference of viruses, and other infectious disease between humans, and wild animals. As humans move ever closer to wild species, and into their habitats, the exposure increases, and with that the risk.

Probably the most debated, and widely contested cause of endangerment for species is climate change. Whatever your beliefs about the causes or the extent of this event, it needs to be listed as one of the overall factors that may have widespread and far-reaching effects on species. The US Fish & Wildlife Service Endangered Species List listed the Polar Bear as "threatened" due to the resulting effects of "climate change" on their habitat, the sea ice. According to IUCN Red List, who have already completed studies regarding the effects of "climate change" on Amphibians, Corals, and Birds, the reported evidence indicates that climate change will impact all species, especially those with "certain traits" (IUCN has identified five groups of traits believed to increase susceptibility to climate change) throughout the coming century. This group of researchers has developed criteria for the classification of "Climate Change Susceptible", and are continuing to expand their studies to other species threatened and non threatened to determine which species are most likely to be adversely impacted by climate change, and what, if any, actions might be taken to ensure their survival, and preservation in the years to come.

Some animals are already such specialized species, that they are at a disadvantage, right out of the gate. The fact that they must have specific circumstances either for normal life, to trigger breeding, or for some other growth stage of life, gives them a risk that many others don't have to contend with. These finely balanced needs, if not met, can cause populations to suffer a collapse. While others clearly have a broader spectrum of habitats, climates, and food sources within their acceptable ranges to adjust within, giving them a more flexible chance for survival. So, the nature of the species also makes a difference in the fate of the animal itself.

All of these factors combine in one way or another to determine which species' populations will be stressed beyond the breaking point and if that population will suffer extinction or recover and flourish. All of these things play a role in the life of a species' population.

With the variety of species in the world today, and the working estimate between 8 to 14 million species planet wide just a portion, around 1.8 million have been "described" or studied. Only 2.5% of the described species has been assessed for their threatened status by the IUCN Red List. The debates rage on about the numbers of species on the planet, the causes of declining populations, and who or what is the root of those causes, for the few species we are aware of. The world spins as the debates do, and time passes quickly. The debate needs to be raging about what we can do to stop species endangerment, and extinction, or about reversing any long term damage done by human activity, and methods to discover other species, record them so they too, can be tracked and protected if need be. We might learn from them, and their circumstances, ways to better protect other species, and maybe ourselves.

Answer

Although this might be meant as the goal of most zoos, it is unfortunate that some, especially smaller zoos, care more for money than they do for their animals. By keeping them confined to small cages with little room to move, while taking the profits made from saving space, for more animals. On the other hand there are wildlife rehabilitation centres which save injured wildlife, then return them when they are healthy again.

Answer

The San Diego Zoo has a Center for Conservation and Research for Endangered Species. It is called "Conservation and Research for Endangered Species", or CRES. It is said to be the the largest (zoo based) research team, or "multidisciplinary research team" in the world. The zoologists and scientists working there are committed to generating, and applying scientific knowledge that is vital to the lives of endangered animals. Also, furthering the world wide education, about endangered animals and the advances in technology for conservation and recovery of these species. They have five different research divisions, each with a specific focus and have been growing this program for over 30 years, and now includes international field studies with over 20 countries There are many zoos with similar programs, this happened to be a long standing, well established one.

Be kind and polite to others. Don't make snap judgments. Also, try to put yourself in the other person's shoes (circumstances).