Invasive Species

Global

Human mediated introductions of species to different habitats are a serious threat to global biodiversity. The magnitude of this on-going alien invasion is illustrated in the following table. It shows that the percentage of exotic (foreign) plant species in different regions of the Earth ranges from 4 to 44% of the total number of species in a region.

Number and Ratio of Invasive to Native Plant Species in Different Global Regions

Region	Number of Species		Invasive Species/Total Species
	Invasive	Native	(%)
Southern Africa	824	20,573	3.9
Europe	721	11,820	5.7
Russian Arctic	104	1,403	6.9
United States	2,100	17,300	10.8
Australia	1,952	15,638	11.1
Western Australia	547	1,510	26.6
Ontario	805	2,056	28.1
New York	1,083	1,940	35.8
New Zealand	1,623	2,449	39.9
British Isles	945	1,255	42.9
Hawaii	891	1,143	43.8

(from Lvel,1997)

There are similar, and even greater, increases in the percentages of foreign animals in different regions. For example, the percentage of alien fresh water fish species to the total number of species ranges from 13% in Brazil, to 53% in New Zealand, and to 91% in Puerto Rico. As with the plants, these invasions are especially pronounced in previously isolated (e.g., island) regions.

This "Macdonaldization" of the global biosphere is a major environmental problem. For example, "eastern North American forests have suffered larger perturbations by invading pests and pathogens than from pollution and acid rain" (Lvel, 1997). These invasions are of concern because they (1) decrease the distinctiveness of local floras and faunas and (2) reduce global diversity. As a result, the global biosphere is becoming more homogeneous and less resilient.

(from Lvel, G.L. 1997. Global change through invasion. Nature 388: 627)

United States

Many of the invasive plants in the United States were originally imported as ornamentals. (e.g., fox glove, Norway maple). Among those was the melaleuca tree. It was imported from Australia to Florida around 1900.

A century later, that tree has infested more than 1.5 million acres in the United States, and is spreading at a rate of 50 acres per day. It's prolific growth is crowding out native plant species and choking the Everglades. It is also drying up those wetlands and destroying it's aquatic habitats.

Many other invasive species are adversely impacting the environment, agriculture, industry, and human health. Those costs have been quantified by the Congressional Office of Technology. It has estimated that losses during this century approach $100 billion, and that future losses will total over $100 billion.

(from: Congressional Office of Technology)

San Francisco Bay

Its is estimated that a new species invades San Francisco Bay every 12 weeks. This includes microorganisms, plants, and animals. There are numerous routes of introduction for these invasive species, including:

deliberate introductions (e.g., stripped bass),

inadvertent importation with shipments of oysters and bait,

and the discharge of ballast from ships into the bay.

The total number of documented alien species in the bay now totals 212, and there are another 123 species of unknown origin. As a result, there has been a temporary increase in the bay's biodiversity, but it's original flora and fauna are now rapidly being displaced by what James Carlton calls "an accidental zoo". For example, the thicktail chub has become extinct, the delta smelt is endangered, and the Sacramento perch is no longer found in local waters. Consequently, the bay has lost its distinctive habitats and food webs, and is becoming like other impacted estuaries throughout the world.

Examples of Invasive Species in San Francisco Bay:

Atlantic ribbed horse mussels

cover the mudflats around Bay Farm Island

clamp onto the toes of California clapper rails drown or starve

principal food for clapper rails

 

Norway rat

major predator of clapper rails

 

Red fox

major predator of clapper rails

 

Atlantic cordgrass

covers the shoreline around bay farm island

displaced Pacific cordgrass new habitat

Asian clams

densities in bay > 4,000 per square foot (within a decade of introduction)

too small for economic value displaced larger species

filter overlaying water up to 13 x per day displace planktonic food chains

 

Chinese mitten crabs

may have been planted as a food source

first identified 3 years ago (1994)

previously introduced in Germany in 1930s major problems

Eastern mud snail

Channeled whelk

Chinese littleneck clams

Atlantic green crabs

Stripped bass

(references: James T. Carlton and Andrew N. Cohen)