THE MASS EXTINCTIONS OF THE 21ST CENTURY


Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 10: pages 250-259 & 274-276


 We entered the industrial age with the greatest biological diversity the earth has known. Over the past 100 to 200 years, and into the next 100 years, human activity likely will reduce this biodiversity to its lowest level since the K-T boundary, 65 million years ago. This coming extinction spasm is the essence of the biodiversity crisis. It is irreversible -- species that evolved over millions of years will never be seen again.

Extinction Rates

Fossil record: 1 species per year (last 200 my)

Recent/current: 1 species per year (~280 birds and mammals over past 300-400 years)

Current/future: 10,000 species per year, based on habitat loss and species-area curves.

(uncertainties include estimates of actual number of species [including

undescribed taxa], and precise slope of species-area relationship)

Principal Human Causes Species Extinctions

 Over-hunting

Habitat loss/degradation/fragmentation

Invasion of non-native species

Domino effects "evil quartet" + Climate change, Pollution "sinister sextet"

 Prediction: by end of next century, perhaps 25% of species will be lost.

 

OVER-HUNTING

 Most extinctions over past 400 yrs, including familiar examples, are mainly the result of over-harvesting. Hunting for food, fashion and profit has claimed many victims. North American bird species include Snowy Egret, Heath Hen, Passenger Pigeon, Bachman’s warbler, Carolina Parakeet, Ivory-billed Woodpecker. Marine example: Stellar’s sea cow. Many African large mammals are threatened.

 Categories of Hunting:

 Geist gives three categories of hunting: commercial, subsistence and sport. I add a fourth: elimination of human enemies/competitors.

 Sport or recreational hunting causes no endangerment of species where it is well regulated, and may help to bring back a species from the edge of extinction. Many wildlife managers view sport hunting as the principal basis for protection of wildlife.

Subsistence hunting can be blamed for some extinctions, e.g., the eastern variety of Elk in the USA in 19th C

Commercial hunting, both legal and illegal, has caused numerous extinctions and threatens many species today. Snowy egret, passenger pigeon, heath hen are USA examples. At $16,000 per pound, and $40,000 to $100,000 per horn, it is little wonder that some rhino species are down to only a few thousand individuals, with only a slim hope of survival in the wild. The pet and decorative plant trade falls within this commercial hunting category, and includes a mix of legal and illegal activities. The annual trade is estimated to be at least $5 billion, with perhaps 1/4 to 1/3 of it illegal.

 Purposeful extinction - lions and tigers and bears, oh my. Wolves in USA.

The pet trade: (and decorative plants)

We in the developed world may not live on venison and jungle fowl, but we still subsidize the harvesting of wild populations, to provide us with expensive, exotic pets. Examples include tropical birds, tropical fishes (fresh water and marine), corals, and other vertebrates (e.g., snakes, small cats). The convention on international trade in endangered species (CITES) is the main regulatory tool, but is often circumvented (e.g., ship an endangered species to country with weak export laws, then export it under the name of an approved species).

 How an Endangered Species becomes extinct: The Risks of Rarity

 Can human hunting lead to the death of the last individual? Why doesn’t hunting cause a species to become rare, and the end of hunting permit it to recover? In fact, some do (sea otter), some don’t (Passenger pigeon). To understand why in detail, one must examine the particular circumstances of each species. Some examples: heath hen, American Condor, whales.

 Transparencies: none

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