Ecological applications of niche theory - Ecological Applications: Toward a Sustainable World - page 41

2.2 Unwanted aliens
- lessons from niche
theory

The arrival on the scene of new species, dispersing from elsewhere, has always
been a fact of ecological life. However, human transport has greatly enhanced the
opportunities for species to travel long distances and to bridge ancient barriers,
such as deserts, mountain ranges and oceans. The trickle of naturally invading
species has become a fl ood of human-assisted aliens. Many newcomers fail to launch
self-sustaining populations and others establish but without negative effect. But
a large number have adverse ecological or economic impacts, as pointed out in
Chapter 1.
The alien plants of the British Isles illustrate a number of points about invaders
and the niches they fi ll (Godfray & Crawley, 1998). Specie s whose niches encompass
areas where people live and work are more likely to be transported to new regions,
where they will tend to be deposited in habitats like those where they originated;
thus, more invaders are found in disturbed habitats close to transport centers and
fewer in remote mountain areas (Figure 2.3a). Moreover, more invaders arrive from

(a) Waste ground

Hedges and shrub
Arable and gardens
Rocks and walls
Woodland
Coasts
Streamsides
Marsh and fen
Grass
Heath
Mountains

0 0.2 0.4 0.6 0.8 1.0

Proportion of alien species in total flora

(b) Europe

North America
Mediterranean
Asia
South America
China
Turkey and Middle East
South Africa
New Zealand
Japan
Australia
Central America
Atlantic Islands
Tropics
India

0 100 200 300 400 500

Number of alien species

Fig. 2.3 Alien plants of the British Isles: (a) according to community type (note the large number of aliens in open,
disturbed habitats close to where people live); (b) in terms of geographical origin (refl ecting proximity, trade and climatic
similarity). (From Begon et al., 2006, after Godfray & Crawley, 1998.)