Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

178 E Diversity of landscapes and ecosystems

research programme that is only known sketchily, but

the progress of efforts for the worldwide conservation

of biodiversity depends on its realization.

E 3.6

Introduction of alien species

The term alien organisms is used for those organisms

that have been directly introduced by man or have

indirectly migrated since the start of the modern era

(since 1492). A distinction is made between neozoa

(animals), neophytes (plants) or neomycetes (fungi)

(Barthlott et al, 1999; Geiter, 1999; Kinzelbach, 1998;

Kowarik, 1999; Scholler, 1999). The start of the modern

era signalled a previously unseen worldwide

exchange of fauna and flora elements which led to

the transfer of organisms into regions well beyond

the limits of their natural ranges. If organisms reproduce

in the new habitats, there may be far-reaching

ecological and economic consequences as well as

impacts on human health (Section C 1.3.2) (Loope

and Stone, 1996; WBGU, 2000a). The spread of alien

organisms has accelerated rapidly in the last 20 years

as a result of increasing world trade pathways and

global mobility. In aquatic habitats, for example, only

1.5 per cent of the species present had been introduced

before 1800, during the 19th century 4.3 per

cent and from 1900 to 1939 10.0 per cent. In the following

40 years, the proportion of introductions rose

to 35.5 per cent and in the period from 1980–1998 it

was 19.2 per cent (FAO, 1999d). Some regions of the

Earth, especially islands, have an extremely high proportion

of alien species, which is often over 50 per

cent (Vitousek et al, 1996).

Seen from a world perspective, the introduction of

alien species is the second greatest threat to biological

diversity – after the loss of habitats (Sandlund et

al, 1996). If all biogeographical barriers were to be

dismantled, in theory 70 per cent of all plants, 65 per

cent of all mammals and almost 50 per cent of all bird

species could become extinct (Brown, 1995). In the

past, the ‘services’ provided by ecosystems, such as

the maintenance of biogeochemical cycles, the selfpurification

of waters, local climate compensation or

coastal protection have not been considered in estimates

of the damage caused by alien species

(Costanza et al, 1997; Box D 2.5-1). But the economic

losses are already considerable. Just the direct costs

for the damage to agriculture in the USA by alien

weeds are estimated at US$2–3 million. Together

with the losses that are incurred in animal husbandry,

horticulture and forestry, there are annual costs of

US$3.6–5.4 million, of which herbicides alone

account for US$1.5–2.3 (OTA, 1993).

E 3.6.1

Appearance and impact of alien organisms

Mass developments of introduced species in new

habitats cannot usually be predicted. Many alien

species probably arrive in a range several times

before they can become established there. Sometimes,

alien species need decades in order to adapt to

the new conditions. Only then can there be an explosive

increase (Bright, 1998). Latent periods are probably

widespread among plants (Crooks and Soulé,

1996; Shiva, 1996). However, the population density

of alien species can fall again after a few years or

decades once a new equilibrium has established

itself, as was seen in Europe for the earlier introductions

of the Canadian pondweed (Elodea canadensis),

the mitten crab (Eriocheir sinensis) and the

zebra mussel (Dreissena polymorpha) (Spicer and

Catling, 1988; Walz, 1992). The rise in predators and

self-regulating mechanisms appears to have led to a

fall in the population densities of the above-mentioned

species after a certain period. In the case of

the zebra mussel the position of the mussels in several

layers on top of each other, for example, means

that the animals lower down can no longer open their

shells (Walz, 1992). In recent times Dreissena has also

migrated into the North American Great Lakes and

is undergoing rapid reproduction there (Ludyanski

et al, 1993). Knowledge about previous distribution

histories and the autecology of the species concerned

can serve as a basis for forecasts about the possible

spread of alien species and its consequences. Experience

from well-documented introductions in the past

can be helpful here (eg Carlton, 1996;Williamson and

Fitter, 1996). Especially successful, cosmopolitan

species should be catalogued in a similar way to the

‘Dirty Dozen’ persistent organic pollutants.

Data about the causes and impact of the introduction

of alien organisms are available only in exceptional

cases because science has only been intensively

dealing with this problem since the 1980s (eg

Mooney and Drake, 1984; Drake et al, 1989;Vitousek

et al, 1996). Our knowledge about the impact of

introduced species has therefore been very patchy in

the past and restricted to a few, well investigated case

studies. The introduction of alien species can have a

serious impact on the structure and function of biotic

communities. There may be a redistribution of

species and their population densities and this means

that there may be changes in species spectrums, biomass

and biogeochemical processes (Doyle, 1999).

In detail, the following effects have been identified

so far:

• The introduction of an alien species often initially

increases species diversity. But this is often fol-

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