Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 598

the soil is in good condition. This means that one relevant endpoint could be 'the
presence of a large, active, diverse microbial population' (e.g., Bloem and Breure
2003 ). More specific endpoints for the effects due to cadmium uptake are used by An
(2004), who measured the seed germination and seedling growth (shoot and root)
for sweet corn, Zea may , wheat, Triticum aestivum , cucumber, Cucumis sativus , and
sorghum, Sorghum bicolor grown on cadmium-amended soils.
A popular endpoint is the number of species, since it is generally accepted that
there is a positive relationship between Biodiversity and Soil Health (Tilman et al.
1996 , Griffiths et al. 2000 ). In addition, the effect on ecological processes (and
therewith Ecosystem Services) is recognized as a meaningful ecological endpoint.
These endpoints are mostly assessed with regard to the percentage of species or
processes (Ecosystem Services) affected (PAF species , Potentially Affected Fraction
of species; or the PAF processes , Potentially Affected Fraction of processes, respec-
tively) (Posthuma et al. 2002 ), or via the inverse criterion, the percentage of species
or Ecosystem processes (services) protected (that is, not affected).
At the more detailed level, however, the criterion 'number of species' or 'effects
on Ecosystem processes (services)' needs specification with regard to the term
'affected'. With regard to the number of species, different possibilities for 'affected'
exist, that is, in ascending order of gravity: reduced activity (although large num-
bers of diverse species of microorganisms can survive in soil in an inactive or resting
state (Conklin 2002 ), diminished reproduction, and death. With regard to Ecosystem
Services, a range of different type of effects can also be distinguished.
An integral approach to ecosystem quality and the effects of stresses on the
ecosystem starts with a proper description of the system. One approach is to focus
on the food web: who eats whom, expressed in the flux of energy and nutrients
between different groups of organisms, used by Hunt et al. ( 1987 ). This approach
requires a detailed data set and, consequently intensive monitoring, with repeated
measurements taken over the course of a year.
Another approach is to derive stable species compositions in ecosystems by using
the allometric approach. Here, the degree of stability is calculated from the numbers
and the mass of individual organisms, and thus from the total biomass of species and
groups of species. The relationship between the place of a species in the web, and
its abundance in numbers and mass, indicates the stability of the web. The approach
has been applied in terrestrial ecosystems (Mulder 2006 ; Mulder et al. 2005 ).
Another approach is to derive criteria for the stability and efficiency of an ecosys-
tem. A system is unstable when insufficient prey is available to feed the predators
and vice versa. A system is inefficient when there is too much prey available,
because then the available prey is not completely converted. This may lead to the
formation of reservoirs of compounds such as storage of organic material because
of the absence of predatory or degrading organisms.
Properly defined, stability and efficiency criteria for ecosystems and food webs
enable the description of a 'good ecological status' (EU-WFD) and 'sustainable use
of ecosystem processes' (EU-SFD) in an operational way (Breure et al. 2008 ).
However, endpoints generally on the same weight for every species and
Ecosystem Service, and do not include information about the organisation of an