Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 599

ecosystem. It does not, for example, account for the impact on species that are
very important for a whole chain of other species. Otherwise, species in soil might
be redundant (e.g., Buckley and Schmidt 2001 ). The many processes (Ecosystem
Services) are especially strongly interlinked. A healthy ecosystem has a complex
organisation, with a less dominant role for opportunistic species. From this per-
spective, it is complicated to define relevant endpoints. The performance of relevant
indicator species , however, could serve as an endpoint.
13.5.5 Other Stress Factors
13.5.5.1 Ecological Impact
It must be noted that the vitality of the soil ecosystem is not only impacted by
soil contamination, but also depends on other environmental factors. Some exam-
ples are an unfavourable pH regime (usually a low pH, acidification), drought,
soil compression, sealing, soil organic matter loss and, last but not least, human
presence.
At any contaminated site, the risk assessor must be aware of these other stress
factors for at least two reasons. Firstly, when ecological effects are observed, not all
of the effects can be attributed to exposure to contaminants in soil. Secondly, in most
cases it is not effective to reduce the contaminant load or bioavailability by means
of Risk Management, when other stress factors are dominating ecological effects.
This is, however, a policy decision.
A multivariate approach can be applied to filter the effects of contaminants
from the many interacting factors at the ecosystem level (e.g., Van Straalen and
Va n Ge s t e l ( 2008 ), with the purpose of assessing ecological risks for diffusively
contaminated sites in the Netherlands).
13.5.5.2 Soil Type, Properties and Structure
Independent of soil contamination, soil life depends on soil type and the prevailing
conditions in the soil. First of all, soil structure must allow for residence possibil-
ities for organisms (the habitat function). Although many organisms create their
own holes, cracks and tunnels, one soil is better suited for this purpose than another
soil, depending on particle size and format (and hence pore structure) and aggregate
building properties. It is interesting to note that the activities of one group of organ-
isms create physical living conditions for other organisms, for example, earthworms
dig tunnels which facilitate root penetration in soil.
Many man-made soils ( Technosols ) provide poor living conditions for organisms
because of the presence of soil-foreign materials such as debris and tar. These mate-
rials create physical and chemical obstacles for organisms (e.g., Rutgers ( 2008 ),
who showed several ecological impacts on the soil ecosystem in the Demmerikse
polder in the Netherlands, an area with an anthropogenic layer of soil on top of peat
soil, including soil-foreign materials).