Causes of Extinction

Lecture © Nikolai Tatarnic
BIOL 606 Session, University of Alberta, March 14, 2001

Extinction is the natural end point for all species; of all the estimated half-billion species that have lived at one time or another, those that are alive today represent only about 2% of the total. Of all these organisms that have gone extinct, about 90% contribute to background extinctions. The remaining 10% have been extinguished during mass extinctions events; events characterized by substantial losses in biodiversity from a wide range of taxa on a global scale (Jablonski 1994).

Of these mass extinction events, the extinction of the dinosaurs 65 million years ago is probably the most widely popularised, in part due to the charismatic nature of the organisms destroyed, but also because of the wealth of corroborating evidence- both marine and terrestrial- suggesting a rapid event of global proportions. It is estimated that 15% of all marine families and about 25 % of all terrestrial families were lost. The 2 major theories behind the cause of this event are the Impact Theory of mass extinction (Alvarez et al. 1980)- which contends that a large extra-terrestrial impact led to the demise of these organisms, and the Volcanic Theory of mass extinction (McLean 1979), which invokes a major volcanic eruption leading to rapid and drastic climate change via the Greenhouse effect. Both models are supported by geological evidence, and debate as to the ultimate cause of the K-T event continues to this day.

The K-T extinction was not the only major extinction event, nor was it the largest. There were in fact 4 other major extinction events (and several other minor events) prior to this: Ordovician, 440 m.y.a.; Devonian, 365 m.y.a.; Permian 245 m.y.a. and Triassic, 210 m.y.a. By far our best records of these extinctions come form marine organisms, especially invertebrates. In most cases, extinctions at the family level were between about 15 and 30 percent. The one exception was the Permian extinction, which saw the demise of over 50% of families. This translates into the loss of about 95% of all marine species, making it the largest known extinction event in the Earth's history. Rather than succumbing to a massive volcanic event or extra-terrestrial impact, it appears that these organisms were killed off by long-term climatic change. There is evidence that global cooling compressed tropical organisms towards the equator during these crises, and this may in part be attributable to continental drift. As bodies of land moved into different climatic regions, collided with one another or grew more and more spread out, ocean currents, weather patterns and water levels were drastically altered.

We are now facing the 6th Mass Extinction, brought upon by human impact. In light of this, it is prudent for us to understand the underlying principles behind extinctions. Is extinction random with respect to evolutionary history, or are some taxonomic groups more prone to extinction? A recent study by Purvis et al. (2000b) on species currently at risk of extinction, suggests that small geographic range, low population density and slow reproductive rates are the best predictors of risk. However, this comes as no surprise, and in the case of the first 2 factors, does not differentiate between evolved traits and results of human impact. In our focal paper (Purvis et al. 2000a) the authors go one step further and relate extinction risk to phylogenetic patterns. In order to compare random extinction models (e.g. Nee and May 1997) to those that are taxonomically biased, they looked at phylogenetic clumping of species at risk, and measured the amount of phylogenetic diversity (P.D., measured by branch length) lost under each model. Using categories of risk derived from the IUCN Red List of endangered species (Baillie and Groombridge 1996), they show that while random extinction models tend to spread species losses throughout genera, models based on actual levels of risk result in clustering of extinction events within genera. This translates into the potential loss of a greater amount of evolutionary history, as long branches of P.D. are lost once all their terminal taxa are removed.

This study is in no way conclusive- increased losses of P.D. under non-random extinction were only significant in simulations of primate extinction, which are not necessarily representative of other organisms. Furthermore, data on carnivores, though not significant, appears under one simulation to show the opposite pattern; as more species at risk are "removed", overall P.D. conserved increases. Clearly this model has weaknesses which must be addressed before one can consider applying these methods in a conservation context. However, it does serve to remind us that not all species are equal, nor are all clades equal with respect to both risk and evolutionary history. A propensity for extinction is in many cases an inherited trait that puts some groups of species at greater risk than others. When whole clades are lost to extinction, entire branches representing vast amounts of evolutionary history are lost forever.

________________

Alvarez, L.W., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction: Experimental results and theoretical interpretation. Science 208:1095-1108.

Baillie, J.E.M. and B. Groombridge. 1996. IUCN Red List of threatened animals. Gland, Switzerland: International Union for the Conservation of Nature and Natural Resources.

Jablonski, D. Extinctions in the fossil record. 1994. Phil. Trans. R. Soc. Lond. B. 344:11-17.

McLean, D. M. 1978. A terminal Mesozoic "greenhouse": lessons from the past. Science 201: 401-406.

Nee, S. & May, R.M. 1997. Extinction and the loss of evolutionary history. Science 278: 692-694.

Purvis, A., P-M. Agapow, J.L. Gittleman, and G.M. Mace, 2000a. Nonrandom Extinction and the Loss of Evolutionary History Science 288: 328-330.

Purvis, A., J.L. Gittleman, G. Cowlishaw, and G.M. Mace, 2000b. Predicting extinction risk in declining species. Proc. R. Soc. Lond. B. 267: 1947-1952.