R. B. Primack, Essentials of Conservation Biology, Sinauer Associates, Sunderland Massachusetts, 1993.  pp. 75-110

 

Loss of Biological Diversity

 

World’s biodiversity is being rapidly destroyed

Ecosystems being destroyed and degraded

Tropics and temperate zones

Terrestrial and aquatic habitats

Loss of genetic variation as number of individuals in populations is reduced

Populations increasingly isolated from each other

Cause is a variety of human activities that alter and destroy habitats

About 40% of the total NPP of the terrestrial environment is used by humans or wasted

Genetic variation being lost in cultivated species

 

Extinction of species

Communities have the potential to recover … if all the original species are present … even when they’re badly degraded.

That specific combination of genetic material is unlikely to ever recur

Extinct in the wild …. e.g., Franklinia altamaha … known only from botanic gardens

 

Past rates of extinction

Diversity of life has increased since life first originated on the Earth

At the same time, extinction is a normal process

There have been several periods of high extinction in the past

This pattern is visible in the fossil record

 

There is a table of the various geological periods and their dates on page 78 in the reading

 

Flowering plants arose about 110 X 10⁶ years ago

Dinosaurs disappeared about 65 X 10⁶ years ago; mammals achieved dominance

The extinction at 250 X 10⁶ years ago was the most serious of all; it took about 50 X 10⁶ years for the earth to regain the families lost

Major extinction events at 65, 180, 250, 345, and 500 X 10⁶ years ago.

 

New families of marine mammals appear about 1 every million years ….

Probably peaked about 30,000 years ago

Humans have had a major impact since that time

 

Elimination of megafauna in North and South America and Australia about 30,000 years ago or more recently

Possibly due to hunting

Also probably due to human management of landscape … especially burning

Burning for at least 50,000 years in Africa

Grassland and forest converted to pastures and farmland in Europe, Asia, and North America

 

Extinction rates best known for birds and mammals

83 species of mammals and 113 species of birds have gone extinct since 1600

The majority of extinctions in the last 150 years

About 2% of birds and 5% of mammals in serious danger of extinction at the moment

Many species that are not extinct exist only in very small numbers … they may be “ecologically” extinct … they no longer are active participants in communities

 

The current rate of extinction is 100 to 1000 times what we would predict from background rates of extinction

 

Extinctions are concentrated in some groups of organisms – large cats, crocodiles and alligators, orchids

 

Present rate of origin of new species??

 

Highest extinction rates on islands

Endemism

The Hawaiian Islands … probably the best studied example

In Hawaii, about 91% of plant species are endemics

10% are now extinct

40% are threatened or endangered

In Madagascar, about 80% are endemic and many are threatened with extinction

Of 750 species known to have gone extinct, about half were island species

 

Island biogeography model, see page 87 …. Size of island, distance from mainland are the main factors.

 

Habitat islands:  50% of habitat destroyed, 10% of species destroyed

90% of habitat destroyed about 50% lost

99% of habitat destroyed about 75% of species lost

 

see plot page 89

 

Deforestation and plant extinction has occurred rapidly in the past in Madagascar, Africa, and Asia.  In various places between 15-25% of species have disappeared.

Perhaps 15% of Neotropical plant species have gone extinct between 1986 and 2000. 

Wilson (1989) estimated that 0.2-0.3% of all species will go extinct each year (based on a total of 10 X 10⁶ species).  That amounts to 20,000-30,000 per year.

Did 250,000 species of organisms go extinct from 1993-2003?.  Has this occurred?  How can we actually tell?

 

Assumptions:  that the species-area curves are dependable

All endemic species are removed from the area of interest; some adapt and can grow in secondary forest remnants

That areas are destroyed at random … but some are selected for park status or protection for various reasons

The degree of habitat fragmentation affects extinction rates

 

Somewhat different estimates based on different kinds of data ….But these estimates are robust enough that we know:  Hundreds of thousands of species are doom to extinction over the next 50 years or so …

 

This will be the largest extinction since the Cretaceous about 65 X 10⁶ years ago

What kinds of species are least or most likely to go extinct?

How long will it take for a species to go extinct when the habitat is degraded or fragmented?

Reproductive viability

 

Local extinctions

Formerly widespread species are now restricted to a small part of their original range