Ecological limitations of SSDs
The University of Sheffield, UK
Species sensitivity distributions are generally derived using data from single-species toxicity tests. The species used in these tests are often from a limited geographic and/or habitat range and toxicity is measured in the absence of interspecific interactions. SSDs are used to assess the risk of chemicals to ecological assemblages containing many interacting species, often in a range of habitats (e.g. rivers, ditches, ponds) in different geographic regions. An extensive and detailed analysis of toxicity data for 67 pesticides (16 insecticides, 9 herbicides, 42 fungicides) has considered the implications of generating SSDs using data sets that contain toxicity data for species from different broad taxonomic groups (e.g. primary producers, fish, arthropods etc), from different habitats (e.g. fresh water, sea water, lotic, lentic) and from different parts of the world (e.g. temperate, tropical, UK). Moreover, this analysis considered how toxicity thresholds derived from SSDs compared to toxicity thresholds derived from multispecies, semi-field studies. The key findings of the analysis reported by Maltby et al, 2005, 2009; van den Brink et al, 2006, are:
- the species sensitivity profiles for 30 fungicides could be described by a single SSD, but separate SSDs for different taxonomic groups were required for herbicides, insecticides and the remaining 12 fungicides. Herbicides were most toxic to primary producers, insecticides were most toxic to arthropods but most fungicides were general biocides.
- Toxicity data for species from different geographical areas can be combined as long as the SSD is based on the sensitive taxonomic group(s). Similarly, toxicity data for species from different habitats can be combined as long as taxonomic differences are accounted for. The potential effects of test conditions on exposure, and hence sensitivity, should be considered whenever data are collated across different studies, irrespective of the geographical region in which the data were generated or the habitat from which the species were obtained.
- Threshold values derived from SSDs can be regarded as protective when compared with threshold values in multispecies studies. SSD-derived values (HC1, HC5, LLHC5 [Lower limit hazardous concentration for 5% of species]) were compared with the NOECeco values derived from the most sensitive structural or functional endpoint in each of 32 mesocosm studies. For the majority of pesticides, the HC1 or LLHC5 were lower than the NOECeco, as was the HC5/3.
The main conclusion of this analysis was that, if based on the most sensitive taxonomic group (determined by mode of action), SSDs derived from a collection of species from different habitats and geographies tested in the absence of interspecific interactions, can be used to derive threshold values that are protective of effects in more ecological complex systems. However, a limitation of this analysis is that the toxicity data sets used to derive SSDs generally do not contain information on all taxonomic groups and information on heterotrophic microorganisms, which are known to play key roles in many ecosystems, is generally absent. The very limited information available indicates that microbially-mediated functions (e.g. decomposition) may be protected by threshold values based on non-microbial toxicity data, but this is an area that requires more investigation.