Workshop Report 28

Sense, simplicity and successes of SSDs in environmental protection, assessment and management

Leo Posthuma
RIVM, the Netherlands

This contribution presented the versatility of the use of species sensitivity distributions (SSDs) in the contemporary practices of environmental protection, assessment and management in the context of environmental stress. There is sense, simplicity and success – despite various shortcomings of the approach.

Some decades ago, the observation was made that – like many phenomena that exhibit variation – the sensitivities of different species towards a toxic compound were distributed in a way that could be described by a statistical model: this marked the birth of the concept known as the species sensitivity distribution. This was a timely concept, since it helped to solve the questions of that time. Soils and waters were affected by various compounds emitted to- or present in the environment, naturally or from past emissions, and the SSD concept helped to set Environmental Quality Criteria. SSDs were one of the methods used to derive the so-called PNEC for ecosystems. Comparison of a PEC (Predicted environmental concentration) with a PNEC became an important practical approach in environmental policies that aimed at reducing and limiting adverse impacts in ecosystems, since a PEC/PNEC-ratio higher than unity signals a potential for undesired effects. In this context, key issues of concern were and are amongst others quality, number and representativity of input data of SSDs, statistical model choice, and the definition of the PNEC itself, given an SSD. The current workshop focuses, with the science of today, on strengths and weaknesses of this use of the method – to support the best possible decision making.

Holistic goals have been set in environmental regulations, and they need be made operational. The Water Framework Directive (EC, 2000) states that water bodies should reach Good Ecological Status next to Good Chemical Status. When a water body is impacted, this can be the result of a suite of stressors, not necessarily being chemicals or their mixtures. Furthermore, monitoring has revealed that exceedances of the Quality Criteria are observed frequently. These kinds of triggers have prompted attention for the use of SSDs in another way, namely: to derive a local hazard level from an ambient concentration. In combination with mixture modelling, this SSD-use yields an estimated value for the toxic pressure (of single chemicals or mixtures) of an environmental sample. This use has been applied many times, in disciplines and approaches as variable as eco-epidemiological diagnosis of local impacts in aquatic ecosystems on a landscape scale, determination of sanitation urgency in soil management, derivation of impacts of chemical emissions in product Life Cycle Analysis, derivation of the Chemical Footprint of current chemical emissions in an area, and assessment and management of chemical disasters around the globe by UN-field teams.

In total, the sum of the ‘reverse use’ (deriving PNECs) and of the ‘forward use’ (deriving local hazard) of SSDs has grown into a wide field, which encompasses not only single chemicals, but also themes as variable as underwater noise, temperature and radionuclides, and techniques such as field-SSDs. As a response to the high practical importance of SSDs in environmental protection, assessment and management, various studies focused on the validity of model outcomes. What does it mean when environmental concentrations increase? Does the predicted fraction of species affected relate to observed impacts? And if so, is this a straightforward one-to-one relationship, or at least a linear one? Various studies allow for the derivation of confirmation statements on SSD output, and suggest that a nuanced view is needed.

SSDs have sense, in that they apparently help to address contemporary questions for a variety of environmental problem definitions.

SSDs are simple, and they contain not a single bit of ecology – but despite that, confirmation studies highlight a basic relevance of SSD output for those problem definitions. The use of SSDs has resulted in successes, which might best be envisaged by imagining the absence of concepts like the PNEC on the one hand, and the potentially impacted fraction of species on the other.

Reflections on the sense, simplicity and successes of SSDs provide the context within which SSD-oriented improvements can be designed. These reflections, though not necessarily complete, can serve as a basis for the workshop and for further developments and use of SSDs.