mammals, amphibians and reptiles, and for vertebrate extinctions overall (Bellard et
al. 2016a).
In South Africa, a total of 107 alien species are suspected to have major negative
impacts on biodiversity, and most (75%) of these are plants (van Wilgen and Wilson
2018). Despite these concerns, there have been relatively few studies that have
formally quantified the impacts of alien species on different facets of biodiversity
in South Africa. In a review focussing on alien plants, Richardson and van Wilgen
(2004) concluded that information on the ecological impacts of alien plants was
generally poor, and that the consequences of invasions for the delivery of ecosystem
goods and services were, with the notable exception of their influence on water
resources, inadequately studied. Since then, the situation has improved to some
degree.
This chapter evaluates the state of knowledge on the impacts of alien species on
biodiversity in South Africa based on different assessment methods. It focuses
mainly on explicit knowledge of impacts of alien species on compositional and
structural aspects of biodiversity at a species scale, and less at genetic, community,
and ecosystem scales (cf. Noss 1990). Some references are presented that discuss the
cumulative impact of all invasions on biodiversity, but how these impacts interact
with other global change drivers is not covered.
17.2 Assessing the Impact of Alien Species on Biodiversity
Different approaches have been used to quantify, assess and summarise the impacts
caused by alien species on native biodiversity. Here, we focus on three main
methods: impact-scoring schemes, expert opinion assessments, and impacts identified during red-listing processes. Impact scoring schemes such as the Environmental
Impact Classification for Alien Taxa (EICAT; Blackburn et al. 2014; Hawkins et al.
2015; see also Box 17.1) and the Socio-Economic Impact Classification of Alien
Taxa (SEICAT; Bacher et al. 2018), amongst others, can be used to formally assess
and quantify impacts of individual alien species. Impact-scoring schemes are based
on published evidence, and aim to make impacts comparable between taxa and
regions by assigning them to semi-quantitative classes which are clearly defined to
minimise assessor bias. We focus here on the EICAT scheme as it is most relevant
for impacts of a particular alien species on native species. Expert opinion studies (e.
g. Zengeya et al. 2017) also assess impacts of specific alien species, but use experts
rather than published impact evidence to gauge impacts. Expert opinion studies can
be done at a species level (impact of a particular alien species on native species) and
or on the entirety of invasions on an ecosystem (e.g. ecosystem services; van Wilgen
et al. 2008). Lastly, the International Union for the Conservation of Nature (IUCN)
Red List of Threatened Species reports threats from a combination of invasions on a
particular native species (Mace et al. 2008).
17 An Evaluation of the Impacts of Alien Species on Biodiversity
491
al. 2016a).
In South Africa, a total of 107 alien species are suspected to have major negative
impacts on biodiversity, and most (75%) of these are plants (van Wilgen and Wilson
2018). Despite these concerns, there have been relatively few studies that have
formally quantified the impacts of alien species on different facets of biodiversity
in South Africa. In a review focussing on alien plants, Richardson and van Wilgen
(2004) concluded that information on the ecological impacts of alien plants was
generally poor, and that the consequences of invasions for the delivery of ecosystem
goods and services were, with the notable exception of their influence on water
resources, inadequately studied. Since then, the situation has improved to some
degree.
This chapter evaluates the state of knowledge on the impacts of alien species on
biodiversity in South Africa based on different assessment methods. It focuses
mainly on explicit knowledge of impacts of alien species on compositional and
structural aspects of biodiversity at a species scale, and less at genetic, community,
and ecosystem scales (cf. Noss 1990). Some references are presented that discuss the
cumulative impact of all invasions on biodiversity, but how these impacts interact
with other global change drivers is not covered.
17.2 Assessing the Impact of Alien Species on Biodiversity
Different approaches have been used to quantify, assess and summarise the impacts
caused by alien species on native biodiversity. Here, we focus on three main
methods: impact-scoring schemes, expert opinion assessments, and impacts identified during red-listing processes. Impact scoring schemes such as the Environmental
Impact Classification for Alien Taxa (EICAT; Blackburn et al. 2014; Hawkins et al.
2015; see also Box 17.1) and the Socio-Economic Impact Classification of Alien
Taxa (SEICAT; Bacher et al. 2018), amongst others, can be used to formally assess
and quantify impacts of individual alien species. Impact-scoring schemes are based
on published evidence, and aim to make impacts comparable between taxa and
regions by assigning them to semi-quantitative classes which are clearly defined to
minimise assessor bias. We focus here on the EICAT scheme as it is most relevant
for impacts of a particular alien species on native species. Expert opinion studies (e.
g. Zengeya et al. 2017) also assess impacts of specific alien species, but use experts
rather than published impact evidence to gauge impacts. Expert opinion studies can
be done at a species level (impact of a particular alien species on native species) and
or on the entirety of invasions on an ecosystem (e.g. ecosystem services; van Wilgen
et al. 2008). Lastly, the International Union for the Conservation of Nature (IUCN)
Red List of Threatened Species reports threats from a combination of invasions on a
particular native species (Mace et al. 2008).
17 An Evaluation of the Impacts of Alien Species on Biodiversity
491