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Issues in Risk Assessment
4
Key Scientific Problems Limiting Application of Ecological Risk Assessment
EXTRAPOLATION ACROSS SCALES
The most common scientific limitation exemplified in the case studies is the problem of extrapolating across scales of space, time, and ecological organization. For the most part, scientific data related to a specific stressor are limited to what can be obtained in a controlled laboratory setting or in a limited field study. Observations of environmental contamination and ecological effects of tributyltin were limited to a few marinas. Testing of pesticides even in the best of circumstances is limited to small field plots and carefully controlled applications. Table 4-1 shows, for all the case studies, the scales at which the data used in the assessments were collected and the scales of interest in decision-making. In most cases, the scales of interest in decision-making are substantially larger in space and of longer duration than could be accommodated in any practical assessment effort. Some form of extrapolation, either with explicit mathematical models or with judgment-based decision rules, is necessary to make the risk assessments useful for decision-making. The PCB study discussed by Di Toro (Appendix E) clearly illustrated the value of explicit models for estimating recovery times in response to hypothetical management actions. In the pesticide registration process described by Kendall (Appendix E), extrapolation is based primarily on qualitative evaluation of test data and information on expected use patterns. Kendall argued that models of ecological effects of pesticides are needed to reduce uncertainty and to account for effects
OCR for page 260
Issues in Risk Assessment
TABLE 4-1 Scales of Observation and Management in Case Studies Evaluated by the Committee
Case Study
Observational Scale
Management Scale
Spatial
Temporal
Spatial
Temporal
Tributyltin
< 1m3
< 1 yr
Chesapeake Bay
> 5 yr
~ 1 ha
(laboratory)
< 5 yr (field)
Agricultural chemicals
~ 1 ha
< 1 yr (field test)
Agricultural region
> 5 yr
PCB and TCDD
< 1 L
~ 1-month (laboratory)
Lakes or rivers
> 10 yr
Spotted owl
~ 300 km2
< 6 yr
Pacific northwest
> 100 yr
Species introduction
< 100 m2
~ 1 yr (greenhouse)
Agricultural region
>> 1 yr
Georges Bank
~ 104 km2
last 30 yr
~ 104 km2
next 5 yr
