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Threats to Biodiversity: Extinction

Brook Milligan

Department of BiologyNew Mexico State University

Las Cruces, New Mexico [email protected]

Fall 2009

Brook Milligan Threats to Biodiversity: Extinction

mailto:[email protected]

Fossil Record and Extinctions


Background Extinction Rates: Fossil Estimates

Species lifetime: 106 – 107 years

Extinctions per year based on lifetime: 10−6 – 10−7

107 species extant

1–10 species extinct per year

Rate: fraction of species extinct per year: 1× 10−6 – 1× 10−7


Large Percentage of Threatened Species


Impact of Human Occupancy on Extinction


Time Course of Observed Extinctions


Estimates of Current Extinction Rates:Pacific Avifauna

Hawai’i60 endemic land birds extinct since human colonization1,500–2,000 B.P.20–25 species extinct in last 200 years

New Zealandat least 44 endemic species extinct in last 1,000 years

Easter Islandat least 25 seabird species originally8–10 species no longer breed on Easter Island13–16 species no longer breed even on offshore islets6 species of land birds originally; all now extinct

Polynesiaranges of most living species much smaller than originallymost species became extinct within the last 3,000 yearsmost islands had 1–4 endemic species of flightless rails; almostall are now extinct2–3 congeners per island typically; now only one


Estimates of Current Extinction Rates:Galapagos Islands

Unlike rest of Pacific, uninhabited before European discoveryin 1535 A.D.

Little human impact until 19th century

0–3 vertebrate extinctions in 4,000–8,000 years prior tohuman contact

21–24 populations lost since human contact

Over 100× increase in extinction rate


Extinction of Mammoths

Climate implicated: end of Pleistocene climate change

Actually, not likely due to climate

Mammoth species on Aleutian Islands until relatively recently

much after end of Pleiostocenemuch after major climate change


Estimates of Current Extinction Rates

Rate is increasing through time for birds: Figure 7.4 (page140)

Declining with age of occupation: Figure 7.6 (page 143)

Large percentage of major taxa threatened with extinction:Table 7.2 (page 142)

1% per hundred years for birds and mammals

Fraction: 10−4 per year

At least 100× background extinction rate


Island Biogeography


Island Biogeography

Species-area relationship

S = CAZ (1)

S = number of species per island

A = island area

C ,Z = parameters describing shape of relationship (often0.15 ≤ Z ≤ 0.35)


Species-Area Relationship

0 2000 4000 6000 8000

05

1015

2025

Area

Spe

cies

0.15

0.25

0.35


Species-Area Relationship: log-log

10 50 500 5000

510

20

Area

Spe

cies

0.15

0.25

0.35


Species-Area Relationship: Carribean

5e+01 5e+02 5e+03 5e+04

12

510

2050

Area

Spe

cies

St. Nevis

Puerto Rico

Cuba

S(A)=0.19 A^0.46


Predicting Extinction Rates


Species-area Predictions of Extinction: Singapore

Last 180 years: 95% original forest removed

Expectation: 30% forest species lost

Actual: 32% lost between 1923 and 1998


Species-area Predictions of Extinction: Neotropics

15% plant species doomed to extinction resulting fromclearing during 1986–2000.

12% Amazon bird species predicted to go extinct


Species-area Predictions of Extinction: Global

0.2–0.3% of all species (i.e., 10,000–15,000 spp.) lost per year(= 34 species per day)

2–11% of all species extinct per decade


Limitations of Species-Area Relationship

Based on “typical” values which may not characterize specificgroups

Assumes complete extinction of endemics, not colonization ofsecondary forest

Assumes random elimination of habitat, even though preservesoften focused on species-rich areas

Ignores habitat fragmentation effects


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