Rate of extinctions

The condition of rarity generally precedes extinction of a species, even though passenger pigeons numbered in the millions less than a century before they died out entirely. Endangered species tend to have the smallest populations and are hence the most likely to die out in the short term. While extinctions can occur for a host of reasons, they can be broadly categorized into two types: systematic pressures and random events. The former include such human activities as habitat destruction, overharvesting, and behaviors that affect the rate of climate change—all of these are factors that can systematically push a species toward extinction. Random pressures, however, are stochastic and may be less obvious. They include such catastrophic events as fires and floods; disease; or demographic fluctuations caused by genetic drift, bottlenecking or inbreeding depression, often acting in combination.

Of course, extinctions are not new events. Fossils provide a clear record of many terrestrial and marine species that once lived on earth and do so no longer. The naturalist Alfred Russell Wallace wrote over a hundred years ago that "...we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared " For Wallace, "recent" meant the Quaternary period, about 12,500 to 11,000 years ago; and the animals he referred to varied according to the continent on which they lived. In the global ocean and in Africa and Asia, there were relatively few Quaternary extinctions; in the Americas, Madagascar, and Australia, however, extinction was nothing short of cataclysmic in its reach. Almost three-quarters of the genera that weighed over 97 lb (44 kg) died out from North America. Australia fared even worse; the continent lost every terrestrial vertebrate species larger than a human being. These included carnivorous kangaroos, a horned tortoise the size of a small automobile, and a monitor lizard almost 22.9 ft (7 m) long. Many small mammals, reptiles, and flightless birds also died out. In fact, eggshells from the giant flightless bird Geny-ornis newtoni indicate that it went extinct simultaneously in three disparate locations. These extinctions occurred during a time in the Quaternary when climate change was relatively mild. They also happened to coincide with the coming of humans to Australia.

Of course, species extinctions also predate human influence and have occurred throughout history as the consequence of climate change, natural selection, and evolution—a fact erroneously used as evidence that conservation is unnecessary. Unfortunately, a consensus exists among many scientists that the earth's species are vanishing at an alarmingly fast rate when compared to background levels of extinction. Sobering warnings come from esteemed scientists like Paul Dayton, Paul Ehrlich, Jane Lubchenco, Stuart Pimm, Michael Soule, and E. O. Wilson, who have each worked for decades in their respective fields. We dismiss their opinions at grave risk. E. O. Wilson, professor and curator of entomology at Harvard University's Museum of Comparative Zoology, predicts that as many as 20% of the species alive today will be extinct by the year 2030 if conservation measures are not implemented.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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