The Ecology Book

272

by the ecologists Phyllis Coley,

John Bryant, and F. Stuart Chapin,

argues that an invasive species

thrives because it is already well

suited to its new environment

and can take advantage of any

surpluses in resources. The

evolution of increased competitive

ability hypothesis, published by

ecologists Bernd Blossey and

Rolf Nötzold in 1995, suggests

that invasive plants facing fewer

herbivores in their naturalized

environment can allocate more

resources to reproduction and

survival and so out-compete the

native species. The enemy release

hypothesis, set out by ecologists

Ryan M. Keane and Michael J.

Crawley in their 2002 article “Exotic

Plant Invasions and the Enemy

Release Hypothesis,” argues that the

invasive species has fewer enemies

in its naturalized environments, and

so can spread farther. The reality is

that the success of invasive species

is likely due to many mechanisms

working together.

Plant invaders

One plant that appears to support

multiple hypotheses about the

success of invasive species is garlic

INVASIVE SPECIES

mustard (Alliara petiolata). Native

to Europe, western and central

Asia, and northwestern Africa,

it was brought to North America

by early settlers to use in cooking

and medicines, and rapidly spread.

Continued infestation has affected

the growth rate of tree seedlings

and reduced the native plant

diversity, leading to changes in

the forest ecosystems invaded.

In its native range, garlic

mustard is consumed by as many

as 69 insect species, but none of

these is present in North America.

This lack of predation and the

plant’s invasive success provide

They have fewer

enemies than

native species.

They out-compete

native species.

They are toxic

to native species.

They are well

suited to the new

environment.

Why invasive species succeed

The zebra mussel

The case of the zebra mussel

demonstrates the diverse

ways to approach invasive

species control, and the

challenges that result. Zebra

mussels are small, fingernail-

sized mollusks with a dark-

striped shell. The mussel is

native to Eurasia but was

discovered in the Great Lakes

area of North America in 1988,

probably carried there in

ballast water discharged from

ships traveling from Europe.

Since then, zebra mussels

have spread throughout the

midwestern United States,

and have been found as far

west as California.

The zebra mussels attach

themselves to clams and other

mussels, filtering out algae

that the native species need

for food to survive. They also

clog water intake pipes used

for power plants and drinking

water supplies. Current

control mechanisms include

chemicals, hot water, and

filtering systems. While each

has had some success, none

of these solutions has been

capable of safely eradicating

the mussels. As a result, they

continue to spread throughout

the waterways of the US.

Garlic mustard is highly invasive

in North America, inhibiting other

plants. In its native habitat, it is

considered an attractive wildflower,

although it can have a strong smell.

We are seeing one of the great

historical convulsions in the

world’s fauna and flora.

Charles Elton

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