Traditionally, a bottom-up approach is adopted in environmental risk assessment (ERA), whereby (eco)toxicity testing results for sensitive ‘model’ organisms are extrapolated using assessment factors in order to protect ‘populations’ representing various trophic levels and taxonomic groups potentially subjected to chemical exposure. Although populations are widely considered to be the ‘operational taxonomic units’ of choice for species protection (IUCN, 2012), they may not always be the most suitable for ecosystem-level protection. This is due to lack of consideration of species interactions (Slocombe, 1993) and other ecological interactions and selective pressures, which promote evolutionary divergence within and between species (Sneath and Sokal, 1973), including their differential susceptibilities to chemicals (Brown et al, 2009; 2014). Consequently, no single ‘model’ species or population will be most susceptible to all chemicals and therefore protective of all other species and populations. Furthermore, the operational taxonomic units of species and populations cannot be applied readily to micro-organisms (Koeppel and Wu, 2013), which provide an enormous pool of biological and genetic diversity and which support / provide numerous ecosystem services (e.g. nutrient cycling, climate regulation, soil formation, retention and remediation, water purification, and waste treatment [Millennium Ecosystem Assessment, 2005b]).