76
economic and social costs (Born et al. 2005 ). Zavaleta et al. ( 2001 ) describe two
basic mechanisms in which food-web interactions can be distorted by IAS introduction, mainly due to the fact that ecosystems usually do not have self-protecting
mechanisms against harmful effects of invaders. Identifi ed top-down mechanisms
(Fig. 4.1 ) are linked to the presence of higher level consumers (i.e. grazers or predators) regulating the size of population they feed on, while bottom-up mechanisms
regulate the population size of these higher level consumers through the availability
of food resources (amount of primary producers or lower level consumers (Zavaleta
et al. 2001 ). The complexity of relations is intensifi ed by the fact that organisms
often feed at more than one trophic level. Horizontal mechanisms are mainly linked
to competition, for example, for resources, light, physical space and food. However,
IAS can affect native species in a number of other ways too, namely, through crossbreeding and the introduction of pathogens. In multiple-invaded ecosystems, therefore, both the horizontal and vertical relationships between different trophic levels
become much more complex and uncertain (Bull and Courchamp 2009 ).
Worldwide IAS-related environmental impacts infl ict massive economic costs on
fi sheries, industry and other human activities (Shine et al. 2010 ), although the economic evaluation of biological invasions is a diffi cult task. According to Born et al.
( 2005 ), most current studies have methodological shortcomings, mainly due to the
fact that they are ex-post impact assessments and insuffi ciently address uncertainty
issues. Furthermore, calculated expenses include mostly direct costs like the damage of harbour infrastructure caused by fouling organisms and usually exclude indirect economic losses such as biodiversity and habitat change, impacts on endemic
species and decreasing genetic diversity/identity of local populations. Some introductions, however, can be regarded as benefi cial for ecosystems, which adds to the
Fig. 4.1 Simplifi ed food-web regulation via three main mechanisms: top-down by population at
next trophic level ( dark arrows ), bottom-up by the presence of organisms they feed on ( white
arrows ) and horizontal, for example, competition ( double arrows )
K. Smolarz et al.
economic and social costs (Born et al. 2005 ). Zavaleta et al. ( 2001 ) describe two
basic mechanisms in which food-web interactions can be distorted by IAS introduction, mainly due to the fact that ecosystems usually do not have self-protecting
mechanisms against harmful effects of invaders. Identifi ed top-down mechanisms
(Fig. 4.1 ) are linked to the presence of higher level consumers (i.e. grazers or predators) regulating the size of population they feed on, while bottom-up mechanisms
regulate the population size of these higher level consumers through the availability
of food resources (amount of primary producers or lower level consumers (Zavaleta
et al. 2001 ). The complexity of relations is intensifi ed by the fact that organisms
often feed at more than one trophic level. Horizontal mechanisms are mainly linked
to competition, for example, for resources, light, physical space and food. However,
IAS can affect native species in a number of other ways too, namely, through crossbreeding and the introduction of pathogens. In multiple-invaded ecosystems, therefore, both the horizontal and vertical relationships between different trophic levels
become much more complex and uncertain (Bull and Courchamp 2009 ).
Worldwide IAS-related environmental impacts infl ict massive economic costs on
fi sheries, industry and other human activities (Shine et al. 2010 ), although the economic evaluation of biological invasions is a diffi cult task. According to Born et al.
( 2005 ), most current studies have methodological shortcomings, mainly due to the
fact that they are ex-post impact assessments and insuffi ciently address uncertainty
issues. Furthermore, calculated expenses include mostly direct costs like the damage of harbour infrastructure caused by fouling organisms and usually exclude indirect economic losses such as biodiversity and habitat change, impacts on endemic
species and decreasing genetic diversity/identity of local populations. Some introductions, however, can be regarded as benefi cial for ecosystems, which adds to the
Fig. 4.1 Simplifi ed food-web regulation via three main mechanisms: top-down by population at
next trophic level ( dark arrows ), bottom-up by the presence of organisms they feed on ( white
arrows ) and horizontal, for example, competition ( double arrows )
K. Smolarz et al.