History of Mass Extinctions and Modern Extinction Processes
Introduction to Conservation Biology and Notable Extinction Examples
The lecture for BIO516 Conservation Biology focusing on topic number four, mass extinctions and extinction processes, begins with an exploration of specific avian and mammalian extinctions. The dodo serves as a primary example; it was a flightless bird endemic to Mauritius Island, located near Madagascar. While the last confirmed sighting occurred in 1662, researchers David Roberts and Andrew Solow used statistical modeling in 2003 to estimate that the actual extinction date likely occurred around 1690. Another significant case is the Thylacine, also known as the Tasmanian tiger, from Australia. The last known individual to be killed in the wild was shot in 1930 by Wulf Butte. These examples highlight the tragic loss of biodiversity and provide a foundation for understanding the mechanics of species loss.
Defining Extinction as Event and Process
Extinction is characterized as both a discrete event and a continuous process. As an event, it is defined as the death of the very last individual of a species worldwide. Effectively, a species is also considered extinct if the remaining individuals are no longer capable of reproduction, rendering the lineage non-viable. As a process, extinction encompasses all the cumulative threats that cause severe population declines. This process is often analyzed through two main scientific frameworks: the small population paradigm and the threatening or declining population paradigm. An illustrative example of this process is the crescent nail tail wallaby, a small mammal the size of a hare, approximately tall, which once inhabited the woodlands and scrubs of Western and Central Australia. Recognized for its silky fur and the unique horny spur at the tip of its tail, it was common even in agricultural districts until 1900. However, it underwent a steep decline by 1908. The last live specimen was caught in a dingo trap on the Nullarbor Plain in 1927 or 1928, sent to Tauranga Zoo in Sydney, and subsequently held in the Australian Museum. The species persisted in arid regions until approximately 1956, when the spread of the red fox likely caused its final disappearance.
Historical Trends in Biodiversity and the Fossil Record
Global species diversity has shown an overall increase since the origin of life, but this growth is characterized by fluctuations including periods of rapid speciation, intervals of minimal change with stagnant numbers, and catastrophic mass extinctions. Research by Wilson in 1989, published in Scientific American, analyzed the diversity of marine animal families over geological time. Marine animals first emerged roughly years ago during the Paleozoic era. The initial years saw a rapid appearance of new families, followed by a year period where numbers remained relatively constant before dropping sharply to around families. Over the last years during the Mesozoic and Cenozoic eras, diversity has steadily increased to the present level of over marine families. This history is punctuated by five major natural mass extinction events before the current human-induced crisis.
The First Four Natural Mass Extinction Events
The first major event was the Ordovician extinction approximately years ago, resulting in the loss of of all animal families, including many species of trilobites. The second was the Devonian mass extinction years ago, where of animal families perished. This event severely impacted Agnathans (jawless fishes), Placoderms (armored jawless fishes), and additional trilobite groups. A notable group affected were the Ostracoderms, which were innovative for using gills exclusively for respiration rather than feeding. While most died out, lampreys survived as modern descendants. Large placoderms like the armored Dunkleosteus terrelli, which reached about two-thirds the size of a modern great white shark, also disappeared. The third event, the Permian-Triassic extinction years ago, is known as "The Great Dying." It was the most severe extinction in Earth's history, eradicating over of marine species and between and of terrestrial species. Scientific evidence published in PNAS in 2014 suggests intense volcanic activity on Pangaea released massive quantities of , greenhouse gases, and chlorofluorocarbons. This led to ocean acidification and cyanobacteria blooms that depleted dissolved oxygen. This event saw the total extinction of Rugose and Tabulate corals, externally shelled cephalopods, and the final disappearance of trilobites. The fourth event, the Triassic-Jurassic extinction years ago, saw a loss of of organisms, including Plesiosaurs and various marine invertebrates. On land, most Archosauromorph reptiles vanished, leaving only crocodilomorphs, dinosaurs, and pterosaurs.
The Cretaceous Extinction and the KT Impact Crater
The fifth and most famous natural extinction was the Cretaceous event years ago, also known as the KT extinction. The "K" refers to "Kreide," the German word for chalk. This event was caused by a large meteorite or asteroid impact. The impact site, known as the Chicxulub crater, is located under the sediments of the Yucatan Peninsula in Mexico, with half of the crater lying in the Gulf of Mexico. This catastrophe resulted in the extinction of all non-avian dinosaurs, including Tyrannosaurus rex, Triceratops, and Pachycephalosaurus. While dinosaurs were the dominant terrestrial life forms at the time, mammals did exist as small, nocturnal creatures. Additionally, Pterosaurs, which were flying reptiles and not technically dinosaurs (such as Pteranodon), went completely extinct. The rise and fall of these groups is detailed in the literature, specifically the book "The Rise and Fall of the Dinosaurs."
The Sixth Extinction: The Anthropocene
We are currently in a sixth mass extinction event, distinct from previous ones because it is caused by human activity, leading to the designation of the Anthropocene era. While insects, invertebrates, and flowering plants reached peak diversity about years ago, species richness has declined since then due to human dominance. The first notable human-induced effects were the widespread megafauna extinctions. In Africa, where humans and megafauna co-evolved, the decline was less severe because animals developed defenses alongside human hunters. However, in other regions, human arrival coincided with massive losses: Australia (settled to years ago), North and South America, and finally small islands like New Zealand, which was settled roughly years ago. In New Zealand, large flightless birds like the Moas went extinct shortly after the arrival of Polynesian and Maori peoples. Recent data from 1984 to 2006 shows an ongoing amphibian extinction crisis, as well as rapid losses among birds and Australian mammals.
Vulnerability of Island Species and Endemism
Island species are disproportionately vulnerable to extinction, exhibiting the highest historic extinction rates. These species often have small population sizes, are limited to single islands (endemism), and rely on specialized habitats. Data on bird extinctions since 1500 reveals a heavy concentration on islands such as Hawaii, Mauritius, the Galapagos, and those throughout Polynesia. These extinctions often coincide with European seafaring, which introduced predators like rats and pigs. Pigs, for instance, are known to consume the chicks of albatrosses. In Australia, a large island with high levels of endemism (including of the world's plant species), mammal and bird taxa have suffered significant losses over the last two centuries. Island species also exhibit "prey naivety," meaning they evolved without certain competitors or predators and thus lack defensive behaviors. This makes them remarkably tame, like the South Island robin in New Zealand, but defenseless against introduced species like cats, foxes, and dogs.
Comparing Background and Current Extinction Rates
To understand the gravity of the current situation, scientists calculate the natural background extinction rate using the fossil record. Under normal circumstances, a species typically survives for to years. This translates to a natural loss of approximately one to ten species per year globally, or a rate between and per year. In contrast, the current observed rate for birds and mammals is approximately per century, or per year. This is to times higher than the predicted background rate. For example, between 1850 and 1950, species of birds and mammals went extinct; natural rates would account for only one of those, while the remaining are attributed to human activity. These rates are currently accelerating due to continuing human-driven changes on the planet.