Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p12
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 12/21)
Character Range: 1376165–1378924

while Wheeler et al. (2002) estimated a minimum of 10 to 15 species per toxicant were needed. The decision by the regulating agency about the appropriate number of species is often arbitrary (Pennington 2003): US EPA requires at least eight species (US EPA 1999), the Dutch suggest ten (van Vlaardingen & Verbruggen 2007), the OECD between five and eight (OECD 1992, 1994) and Australia and New Zealand—five species (Warne 2001). It is worth remembering that the above estimates are based on available SSDs that tend to include data from only a small fraction of taxonomic and other groups present in nature. If data were available for a larger range of organisms, the number of species for which data is required may increase. If this occurred, then the findings of Newman et al. (2000), Wheeler et al. (2002), and others would have underestimated the number of species required for estimating the SSDs. Reflecting these findings, the EU has required that future WQGs need toxicity data for at least ten species that belong to at least eight taxonomic groups and an additional assessment factor of 15 to the PC95 should be considered (ECB 2003).

3.2.2.2         Strengths and limitations
SSD methods have a number of strengths:
    * they use toxicity data for all species that is available, thus conforming to risk-assessment principles
    * they have a sound statistical basis providing the assumptions of the method are met
    * they are flexible methods, can use any measure of toxicity, and can calculate HC or PC values to protect any chosen percentage of species except 0% and 100%
    * the methods are transparent and allow the level of protection to be chosen. The approach also enables a more informed debate to occur over the level of protection to be offered
    * they can be used in the reverse manner to determine what level of protection (i.e. percentage of species) is offered when a certain concentration of a contaminant occurs in the environment. This should be useful in ERAs and site-specific investigations
    * several aspects of the methodology have been validated.

The limitations of the methods include:
    * the data requirements may limit the number of guideline values that can be derived
    * it is more complex to understand how the guideline values are derived than with the AF or geometric mean methods
    * several of the assumptions made by SSD methods may be compromised. For instance, SSD assumes that the species are representative of the totality of the ecosystem and all species are equally as important to ecosystem functioning (that is, no consideration is given to keystone species).

3.2.3         Assessment factor methods
In AF methods, all available toxicity data for a contaminant is collated. Then