Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

Introduction of alien species E 3.6

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lowed by a reduction in the number of individuals

within native species due to an altered predatorprey

ratio, competition and introduced diseases;

later there is a reduction in the total number of

species. If the eliminated species occur only in the

habitat concerned (endemic species), they can

become extinct as biological species (Section E

2.3). In Lake Baikal with an approx 80 per cent

proportion of endemic plant and animal species,

the introduction of alien species may have an

especially serious impact and is considered to be

at least as serious a threat to this unique ecosystem

as the degradation of environmental conditions

through eutrophication and/or the discharge of

pollutants (WBGU, 1998a). Some neophytes can

be made directly responsible for the population

reduction in some species: non-native organisms

have been largely implicated in the disappearance

of 43 plant species which have become extinct in

Germany or classed as endangered (Korneck and

Sukopp, 1988).

• The uninterrupted exchange of fauna and flora

elements leads to a worldwide homogenization of

ecosystems (Ritzer, 1995). An important factor

here is overcoming previously existing barriers to

spread, which used to cause genetic isolation of

populations. The crossing of populations which

used to be genetically isolated (introgressive

hybridization) can lead to the loss of certain,

genetically specified features. Hybridization with

alien species can lead to the extinction of species

with site-specific adaptation to certain environmental

changes because of the introduction of

new allele (Tautz and Schliewen, 1999). So far, the

consequences that the homogenization of flora

and fauna may have on the emergence of new

species have hardly been studied (Vermeij, 1996).

• The invasion of alien species can influence the

speed, or even the direction of succession, ie the

temporal sequence of various organism groups as

a consequence of changed environmental conditions.

For example, this can happen as a result of

changes in the chemical conditions in the soil, such

as the acidification of the soil in southern Germany

as a consequence of planting Douglas firs

(Pseudotsuga menziesii) (Knörzer et al, 1995) or in

Berlin/Brandenburg as a result of the spread of

the equally alien false acacia (Robinia pseudoacacia)

led to an accumulation of nitrogen in the soil

(Kowarik, 1992).

• Of the interactions of alien species among each

other, the focus of research interest used to be

mainly on negative interactions, such as competition

for resources. So far, there are only a few

direct indications for the assumption that interactions

between various alien organisms actually

prevent further introductions (biotic resistance

hypothesis).A new analysis of the literature (Simberloff

and von Holle, 1999) tends rather to show

that interactions between introduced organisms at

least as frequently have self-reinforcing effects

(facilitation). Positive interactions between introduced

species can thus accelerate the negative

impact on these ecosystems affected (invasional

meltdown hypothesis). Synergistic effects

between alien species could lead to an unexpected,

sudden acceleration of further movements.

In this case, the prevention of introductions

should be given a higher status than previously.

The Council has already shown the negative impacts

of alien organisms on human health, the economy

and further, indirect influences on ecosystems

(WBGU, 2000a).

More comprehensive studies according to more

generic aspects would have to be carried out to draw

up effective strategies to combat undesired invasions

or their impacts (Vermeij, 1996).The many individual

items of information about alien species should be

brought together in central databases and made generally

accessible in order to allow quick analysis of

the problems associated with the introduction of an

alien species. Coordination, standardization and simplification

of the worldwide or national databases

already in existence or at the planning stage could

lead to more consistency and help to avoid duplication

of effort. Concrete proposals already form a

foundation for work with regard to the specifications

for databases of this kind (Simberloff et al, 1999).

Findings from the aquatic environment should

above all illustrate the mechanisms which are generally

important for the problem of alien organisms

(Box E 3.6-1): of the approx 3,000 recorded introductions

of alien species into aquatic systems, almost

half of them have already become permanently

established in open nature (FAO, 1999e). The main

cause of such introductions, at 38 per cent, is aquaculture.

As in agriculture, species introduced into the

aquatic area also account for a considerable proportion

of production. Worldwide, around 10 per cent of

the production generated by aquaculture is by means

of introduced organisms. In Asia, where aquaculture

had its origins with native organisms, the proportion

is low, but in other continents it may be significant.

Thus, 97 per cent of the production of prawns in

Europe, 96 per cent of fish production in South

America and 85 per cent in Oceania comes from

breeding introduced organisms (FAO, 1999f; Section

E 3.4). Worldwide, 10 per cent of all introductions in

the aquatic sphere resulted from government programmes.The

private sector was responsible for 6 per

cent of cases. Other sources, individuals and international

organizations were responsible for around 8

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