Ch 6: Mass Extinction & Global Change

Extinction• Inevitable• 99.9% of species that ever lived have gone extinct

• Some species changed enough to be considered a new species• Others were evolutionary dead ends

• Living fossils• persist, unchanged for millions of years

• Rate of extinction not constant• At least 5 mass extinctions• Are we in the middle of the 6th?

History of Life on Earth• Prokaryotes• 1.9 BYA: Eukaryotes• 600 MYA: Cambrian Explosion• macroscopic organisms!

• 5 mass extinctions since then• Overall, biodiversity increasing

Cretaceous-Tertiary Extinction• 315 mya: the amniotic egg• 280 mya: reptiles dominate• 65 mya: dinosaurs extinct (except birds), mammals dominate• This marks the end of the Cretaceous and the beginning of the

Tertiary.• Meteorite strike thought to be root cause• dust blocks solar radiation• tidal waves• fires• acid rain

Permian Extinction• 251 mya• Four times as many extinctions as the other mass extinction

events• only known mass extinction of insects

• Root cause unknown• Meteorite• Volcanic activity• Continental drift

• Pangea, unstable climate• Drop in sea level dries out shallow marine areas

• Low oxygen limits terrestrial life to low elevations• Hypoxic oceans

Recovery After Extinctions• Takes tens of millions of years• So while we won’t destroy the planet, we will be extinct before it

recovers

Estimate the Rate of Extinction• Challenges• Hard to be sure a species is extinct• Most of the world’s species are undescribed

• Attempts• World Conservation Monitoring Centre

• Most comprehensive list, ~ 90 species of plant, ~ 726 species of animal

• Coendangered species ~ those we know little about, but are closely associated with endangered species

• Species-Area relationship

Species Area Relationship• S = CAz

• S, species; C & z, constants depending on taxa and island set; A, area• Combine with an estimate of tropical forest decline• Use that to estimate yearly extinction rate

Background Extinction Rate• Typical lifespan of a species is one million years, each year 1 of every

million species should go extinct• if 10 million species, 10 species per year

• Extinction rate from species-area curve is 27,000 species each year• Assuming human population stabilizes between 10-15 billion

• We may lose between 10-25% of species

Global Climate Change• What makes glacial and interglacial periods?• Tilt of Earth on its axis 22-25o

• Shape of Earth’s orbit• precession of equinoxes, where in orbit solstices and equinoxes

occur • Together, a 100,000 year cycle of glacier and inter-glacial periods

• Other factors• solar flares/variation in solar output• ocean currents and jet streams• presence of glaciers, CO2, greenhouse gases that affect solar

radiation (absorbed) and radiant energy (reflected)• Variations in global and local trends in temperature

Historic Responses to Climate Change• Range shifts• toward poles or equator• well-documented in fossil record

• Contract range to refugia, remnant habitat• Species tend to respond individually, not by whole-community shifts

Future Climate• Currently in an interglacial period• enter an extended, super-interglacial period?

• The role of CO2

• greenhouse gas• concentration rising, 30% over pre-industrial levels

• methane has doubled• corresponding rise in temperatures• even if CO2 isn’t the culprit, not a bad idea to curtail use of fossil fuels

Adapting to Future Changes• Cycles repeat every 100,000 years, biota survived those changes evidence of adaptability

• Limits to adaptability• populations already stressed by habitat loss, overexploitation…• amount of available habitat limited• barriers to dispersal including roads, urban areas, agricultural lands

• Unprecedented changes?• Greater increase in temperature

• geographic bottleneck, can only change range so much• mountain-top species• Between the devil and the deep blue sea

• Rate of change too fast• mobility, dispersing and sedentary stages, philopatric spp.

• Range and phenology changes already detected

Between Generational Mobility KeyMobile Between Generations Sedentary Between Generations

Mobile as Individuals

Migratory birdsInsects in ephemeral pondsPelagic fishes

Philopatric migrantsInsects in deep lakesAnadramous fishes

Sedentary as Individuals

Territorial fishes with planktonic larvaeEarly-successional plants; self-incompatible annualsIntertidal molluscs

Desert-spring fishesLate-successional plants; self-compatible perennialsTerrestrial molluscs