: past and present

Bio 415/615 March 5, 2013 Questions

• What makes some more vulnerable to than others? • What are 5 modes of extinction? • What has caused mass extinctions in the geologic record? • How do recent rates of extinction compare to the historic record? • Why might extinction rates continue to increase in the near future?

Background extinctions

• Almost every species that ever existed is now extinct

• Under “normal” conditions extinction rates are only slightly behind speciation rates Background extinctions

• Estimates of paleo-extinctions derived from wide-ranging marine

• Average species may survive for 1-10 million years

• record is extremely fragmentary

• Last true occurrence of a species is rarely ever known (Signor-Lipps Effect)

Why do species go extinct?

• Darwin thought species went extinct entirely because of natural selection – some species eventually become less fit relative to others and therefore go extinct.

• Is extinction caused solely by reduced fitness (i.e. genetic factors) as Darwin thought?

• What other factors may contribute to extinction? Species vulnerability to extinction

• What makes some species more vulnerable to extinction than others? • Species with small geographic ranges

• Narrowly distributed endemic species e.g. Joshua Tree (Yucca brevifolia) in the American southwest • Species with small number of populations

• Low population density/size e.g. Devils Hole pup fish in Nevada – may be the worlds rarest fish, only 1 natural population with < 100 individuals

• As population size increases risk of extinction decreases

Kerr 2013 *assuming no density dependence and b=d

• Genetic Variability

• Linked to population size

• As population size decreases genetic variability also decreases

• Deleterious alleles/traits may become fixed in the population

• Less likely that adaptation to changing conditions/threats will be possible, increases extinction risk

Species vulnerability to extinction

Gonzalez-Suarez and Revilla, Ecology Letters 2013 Species on islands generally more vulnerable to extinction - isolated habitat - small geographic range - relatively small population sizes (decreased genetic variation) e.g. - ground nesting - “naïve” species (megafauna) Classic example: Dodo (Raphus cucullatus) extinct mid/late 1600’s

Tambalacoque

Mauritius 42% (57) of ’s bird species extinct - 38 following Polynesian arrival - 19 following European Factors leading to extinction: - Hunting - Introduction of predators - Habitat loss Laughing

Haast’s Eagle Chatham Poipoi Stephen’s Island Wren

Modes of extinction

• 5 modes of extinction 1) Extinction via reticulation e.g. Grey ducks and Mallards Modes of extinction

• 5 modes of extinction 1) Extinction via reticulation 2) Extinction via competition//pests e.g. introduced rats & house decimated pacific island birds Stephen’s Island Wren Modes of extinction

• 5 modes of extinction 1) Extinction via reticulation 2) Extinction via competition/predation 3) Extinction via over-exploitation e.g. - human arrivals in New Zealand - human arrivals in Hawaii – extinctions of at least 39 endemic land birds - Passenger Pigeon – population of over 3 billion in 19th century to extinct by early 20th century - Pleistocene megafauna? Modes of extinction

• 5 modes of extinction 1) Extinction via reticulation 2) Extinction via competition/predation 3) Extinction via over-exploitation 4) Extinction via e.g. - Passenger Pigeon – population of over 3 billion in 19th century to extinct by early 20th century - Ivory-billed woodpecker - Dusky Seaside sparrow Modes of extinction

• 5 modes of extinction 1) Extinction via reticulation 2) Extinction via competition/predation 3) Extinction via over-exploitation 4) Extinction via habitat destruction 5) Extinction via changing environmental conditions e.g. Golden Toad (Cranopsis periglenes), formerly of Costa Rica Mass extinctions

• A major episode of extinction for many taxa, occurring relatively quickly

• 5 mass extinction events observed in the fossil record

What causes mass extinctions?

1) Flood basalts and volcanism • Causes ocean acidification and anoxia

• Evidence of massive flood basalts and volcanism coincide with Triassic and Permian mass extinction events

• Also evidence for massive volcanic activity around Cretaceous extinction event Flood basalts and volcanism

Timing of Jurrasic extinction Timing of Permian extinction Central Atlantic group (180 mya) Siberian traps (250 mya)

Columbia River group (17-14 mya) Deccan traps (65 mya) Timing of Cretaceous extinction What causes mass extinctions?

1) Flood basalts and volcanism 2) Glaciation and cooling

• Large-scale global cooling hypothesized as cause for Devonian extinction

• Short consecutive glaciation events hypothesized as cause for Ordovician extinction

What causes mass extinctions?

1) Flood basalts and volcanism 2) Glaciation and cooling 3) Extra-terrestrial impacts (asteroids, comets, bolides) • Alvarez et al. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1108. • Believed that a giant asteroid had hit the earth, that this had caused a dense cover of particulate matter in the atmosphere that blocked the sun, causing global cooling, and the death of many plants • They predicted that an iridium layer should separate cretaceous from tertiary strata

• Swisher et al. 1992, also published in Science, found the asteroid crater on the ocean covered continental shelf of the Yucatan ScienceShot: Big Smash, Dead Dinos

Recent findings show that the asteroid impact may have occurred at the same time as the mass extinction of the dinosaurs (± 32,000 years)

Still much debate about timing and relative roles of impact versus volcanism and possible interactions of the two Pleistocene extinction • The sixth mass extinction? • Almost all North-American mega-fauna went extinction • Climate change? • Overkill hypothesis - Human arrival and overexploitation? • Comet impact? Mass extinction redirects the course of evolution down pathways defined by the survivors

• Before Permian extinction Brachiopods were the dominant mollusc. Afterwards only a few remained

• Mammals and birds come to dominance following extinction of dinosaurs 65 million years ago Recent extinctions

Is there another mass extinction occurring right now?

Recent extinctions

• Background extinction rate has been calculated at 0.0001 – 0.00001% of species per year • Current rate of bird and mammal loss is 0.01% per year • 100-1000 times greater than historic background levels However… • Other estimates suggest current levels are only 36-78 times higher than historic levels (Regan 2001) • Remember fossil record is incomplete at best, so may lead to erroneous conclusions • Values calculated from short-term record may also lead to erroneous conclusions • Work by Pimm et al. (2006)

• Approx. 10,000 known bird species • 129 recorded extinctions since 1500 = 26 extinctions/million species/year (compare to background rate of ~1 E/MSY)

However, also need to include “cryptic” extinction estimates: • Many species are already extinct, or on verge, but we don’t yet know it • Remains of extinct species keep being found This may increase extinction rates to ~150E/MSY And could reach ~1500E/MSY by 2100 given predicted rates of habitat loss

However, Conservation efforts have prevented many extinctions

Do we expect rates of extinction to accelerate in the near future?

Reasons we might expect this to be the case:

1)Increased habitat destruction 2)Increased introduction of pests/predators 3)Rapid climate change