The rationale for the approach

In document The sTaTus of biological invasions and Their managemenT in souTh africa 2017 (Page 34-37)

2. iNdiCatorS

2.2. The rationale for the approach

The phenomenon of biological invasions is caused by a combination of how taxa are moved around by humans (introduction dynamics), the traits of individual taxa (invasiveness), and the susceptibility of the environment to

effecTiveness of responses

A set of four high-level indicators has been developed to track trends in:


the rate of introduction of new unregulated species

to South Africa


PeR yeAR


the number of invasive species that have major


107 SPecIeS


the extent of South Africa that suffers major impacts

from invasions


of THe lAND AReA


the level of success in

managing invasions


The values assigned to these indicators set a baseline against which trends in future can be measured, with the overall goal being to implement control and regulatory measures that will improve the situation as measured by these indicators.

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invasions (invasibility). for example, the current distribution of invasive pines in south africa is a result of how pines have historically been planted for forestry, which species have particular traits that predispose them to invade, and the fact that some areas of the country do not have any indigenous fire-adapted tree species and so are susceptible to woody plant invasions (e.g. the cape floristic region). The explicit consideration of biological invasions in terms of these three aspects [i.e. pathways, species (or more precisely taxa), and areas] is also crucially important for management. focussing on pathways is important to reduce rates of introduction and spread, but does not address current invasions. focussing on species can be highly effective in reducing densities of a single species, but can simply clear the way for other species to invade.

integrated and strategic approaches are needed to deal with suites of co-occurring species in any given area, but if management is to be effective in those areas, pathways of introduction need to be managed and in most cases best practice species-specific control measures will need to be implemented.

The invasion process is commonly categorised in terms of an introduction-naturalisation-invasion continuum (blackburn et al., 2011). There are four major invasion stages – pre-introduction, incursion, expansion, and dominance – that align with four management goals – prevention, eradication, containment, and impact reduction. The combination of the need to look at indicators for pathways, species and areas, as well as the need to look at pre-introduction, incursion, expansion and dominance, gives rise to the 3 × 4 framework. This framework was the basis of the draft national strategy on biological invasions in south africa, and is discussed in detail by Wilson, Panetta & lindgren (2017). however, the development of indicators for all aspects of invasions at all invasion stages still requires some theoretical development. This report concentrates on indicators for the three aspects (pathways, species, and areas), and not on the four stages (pre-introduction, incursion, expansion, and dominance), although a future report may seek to develop the indicators needed to cover all components of the 3 × 4 framework.

There are, of course, many other ways of conceptualising or categorising biological invasions. Taxonomic, disciplinary or functional lines could also be used, e.g. by considering freshwater fish invasions and riparian plant invasions as separate problems. alternatively, a status report could be divided into specific biomes, environments or realms. south africa’s national biodiversity assessment has, to date, taken this approach and is presented as a series of chapters based on ‘realms’ – freshwater, marine, and terrestrial. in terms of biological invasions, there is, however, no neat separation between aquatic and terrestrial environments, nor between fish, frogs, and ferns – the essence of the problem is the same. if propagule pressure can be reduced, will this reduce the likelihood of an invasion? What are the impacts? is a species definitely alien?

management often needs to consider entire systems, e.g. simultaneously managing freshwater fish invasions and riparian plant invasions would lead to a more sustainable outcome than if either group was controlled on its own (impson, van Wilgen & Weyl 2013); and the same pathway (e.g. the pet-trade) can be responsible for introducing marine, terrestrial and freshwater organisms. so while it is important to be able to report along geographical or taxonomic lines, not least as this is frequently the level at which data are collected or management is implemented, it is important that such data can be aggregated to give higher level indicators. in this report, the indicators themselves are not split into geographical or taxonomic lines, but the report will consider groupings within each indicator as per the data sources themselves [e.g. the southern african Plant invaders atlas (saPia) includes data on alien plants across all habitat types except marine and some coastal habitats].

Tus of biological invasions and Their managemenT in souTh africa2017

for the report to be of value it should provide information that can be used to determine how effective interventions have been in reducing the size of current problems. The approach taken in this report is to assess the effectiveness of interventions (which is composed of both the effectiveness of control measures and the effectiveness of regulations) in terms of how they influence aspects of pathways, species or areas. specifically indicators are presented for inputs (e.g. the amount of money spent), outputs (i.e. control measures that are in place), and the outcomes (i.e. how effective the control measures are). in this report, indicators are not, however, developed for: 1) the underlying processes required for those interventions; or 2) the ultimate impact of the interventions. interventions require a suite of enabling processes (specifically: accessibility of data and information; organisational and human capacity; research; and public awareness and engagement), but as these are not directly related to outputs that affect outcomes they are not considered here. developing indicators for these enabling processes might be a priority for future reports. secondly, if the implications of any intervention for the broader south african community are to be assessed, there must be a link made to general environmental and socio-economic indicators, i.e. the impact. in this report, this link is not made explicit nor is an attempt made to develop indicators specifically for this (as it is more appropriate to co-opt existing sector-specific indicators).

it is anticipated that developing the link between what is done in this report (i.e. assessing impact in terms of specific outcome indicators and changes to the indicators of the state of biological invasions) and broader societal indicators for impact will be a major focus of future reports.

in line with international proposals (geo bon, 2015, latombe et al., 2017), the status reported should be modular.

if resources permit, high-level data can be collected without compromising the ability to compare with situations where fewer data or resources are available. for example, accurate distribution data are available for birds, but not for microbes (chapter 4).

sniffer dogs are frequently used to detect illegal imports, including alien species

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In document The sTaTus of biological invasions and Their managemenT in souTh africa 2017 (Page 34-37)