Throughout the nearly four billion year history of life on earth, there have been five times in which nearly all life was extinguished. We refer to these episodes as mass extinction events. The first mass extinction event occurred at the end of the Ordovician period, around 444 million years ago, in which approximately 85 percent of all species alive at the time became extinct. The most devastating event was at the end of the Permian period, 252 million years ago, when as many as 96 percent of all living species became extinct. The most recent event took place 66 million years ago and included the extinction of the dinosaurs. We don't know what caused all five of these mass extinction events. While it now seems clear that the dinosaur's extinction was caused by a massive asteroid hitting the earth, the causes of the other mass extinctions remain debated. Although drastic changes in climate, perhaps triggered by massive volcanic eruptions or other disturbances are suspected. Today, many scientists believe that we're on the verge of another major mass extinction event. This time, however, the cause is quite clear. It's people. Humans have been the cause of many extinctions throughout our species history. Shortly after the arrival of the first humans on the continent of Australia, some 60,000 years ago, around 85 percent of all the large mammal species went extinct. When the first people arrived in New Zealand, around the year 1300, two large birds, the Moa and Haast's eagle soon disappeared. Dodos, flightless birds related to pigeons, were endemic to the island of Mauritius in the Indian Ocean, but became extinct soon after the first Europeans arrived. We don't necessarily know how each of these species became extinct, but we can be confident that humans were to blame for many of them. Large animals have often been the first to disappear after the arrival of humans. The most likely explanation is that people hunted them for food. But people can also compete with large animals for resources like food and habitat. We're very good at modifying our environment to suit our needs but often that means the habitat is no longer suitable for other species. Unfortunately, the situation has only gotten worse in recent years. As the human population has grown, we've needed more and more space, as well as resources like food and water. The result has been less space and fewer resources for other species. In many cases, that's led to more extinctions. Of course, extinctions are a natural part of the history of life, but we know from the fossil record that the rate at which species are disappearing today is much higher than normal. Consider the mammals, the group for which we have the best data. Thirty five species went extinct from 1900-2000. Based on the fossil record, we know that the background rate of extinction, meaning the rate at which species have typically become extinct, is about one species every 100 years. We are currently experiencing an extinction rate about 35 times higher than normal. The only comparable rates of extinction can be found during the five major mass extinction events. If we want to avoid another mass extinction, which could take our species down, along with many others, we need to take a careful look at what is causing species to become extinct, what the consequences are of species extinctions, and what can be done to prevent them. Let's take a look at some local examples. This cute little girl is a Houston toad. The Houston toad was once found not only in Houston, but across much of Southeast Texas, but as their native pine forest habitat began being converted into urban, suburban, and agricultural land starting in the 1960s, there were fewer places for these toads to live. They were added to the Endangered Species Act in 1970. But by 1975, the Houston toad could no longer be found in Houston, the few remaining wild populations became vulnerable to any type of disturbance, including droughts and wildfires, which reduced their numbers even further. But efforts are underway to try to save this species from extinction. Here at the Houston Zoo, there's a program that began in the 1980s to breed Houston toads and return them to the wild. I'm standing inside the Houston toad quarantine facility, which has strict rules designed to keep these animals safe from diseases and other threats. The staff that work here are tasked with getting the adult Houston toads being housed here to breed. When they do, the fertilized eggs and young toads are transported out to the wild and the remaining places where there's still suitable habitat. Each year, they release hundreds of thousands of eggs and occasionally also tadpoles and young toads. Thanks to these efforts, the Houston toad is still hanging on despite all the threats it faces. Another Texas species that has suffered from habitat loss caused by humans is the whooping crane. These incredible birds are five feet tall, they are the tallest birds in North America, but they're also one of the rarest. As of this moment, there are only about 800 individuals alive, including about 100 and captivity and about 700 in the wild. But the situation was much worse in the 1940s when there were only 21 individuals alive in total. These were split in two different populations. The species was at the very edge of extinction. The cause of their decline was a combination of being killed by hunters as well as habitat loss. The habitat loss was occurring in both their summer breeding grounds in Canada and the upper Midwest United States, as well as their wintering grounds along the US Gulf Coast. Captive breeding efforts beginning in the 1960s help to protect the species from extinction. Rearing whooping crane chicks in captivity was complicated by the fact that the chicks imprint on their caregivers. Imprinting on humans would make it impossible for the birds to ever survive in the wild. One solution was further human caregivers to dress up in crane costumes, which include a big white cloth that covers the person's entire body, and a puppet like beak on one of the hands, which is used to feed the chicks. But without enough habitat, it was unclear whether the species would be able to survive in the wild. The wild populations winter habitat is an area of coastal wetlands located near the city of Corpus Christi, about 100 miles Southwest of Houston. Luckily, this area is now protected, but the habitat is threatened by decreases in the amount of fresh water that flows through it as the water is diverted for other uses, as well as by sea level rise caused by climate change. Hopefully, having some individuals in captivity, like these here, will not only help keep the species alive, but will also help people to recognize the importance of protecting the natural environment that these birds need to stay alive in the wild. The whooping crane and the Houston toad are examples of how even the most endangered species can be prevented from extinction if people are willing to do the work needed to protect them and their wild environments. These are the types of efforts that would be necessary if we are to avoid being responsible for the sixth major mass extinction event in our planet's history.