Quantifying population-level risks using an individual-based model: Sea otters, Harlequin Ducks, and the Exxon Valdez oil spill
Authored by Mark A Harwell, John H Gentile, Keith R Parker
Date Published: 2012
DOI: 10.1002/ieam.1277
Sponsors:
ExxonMobil Corporation
Platforms:
No platforms listed
Model Documentation:
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Model Code URLs:
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Abstract
Ecological risk assessments need to advance beyond evaluating risks to
individuals that are largely based on toxicity studies conducted on a
few species under laboratory conditions, to assessing population-level
risks to the environment, including considerations of variability and
uncertainty. Two individual-based models (IBMs), recently developed to
assess current risks to sea otters and seaducks in Prince William Sound
more than 2 decades after the Exxon Valdez oil spill (EVOS), are used to
explore population-level risks. In each case, the models had previously
shown that there were essentially no remaining risks to individuals from
polycyclic aromatic hydrocarbons (PAHs) derived from the EVOS. New
sensitivity analyses are reported here in which hypothetical
environmental exposures to PAHs were heuristically increased until
assimilated doses reached toxicity reference values (TRVs) derived at
the no-observed-adverse-effects and lowest-observed-adverse-effects
levels (NOAEL and LOAEL, respectively). For the sea otters, this was
accomplished by artificially increasing the number of sea otter pits
that would intersect remaining patches of subsurface oil residues by
orders of magnitude over actual estimated rates. Similarly, in the
seaduck assessment, the PAH concentrations in the constituents of diet, sediments, and seawater were increased in proportion to their relative
contributions to the assimilated doses by orders of magnitude over
measured environmental concentrations, to reach the NOAEL and LOAEL
thresholds. The stochastic IBMs simulated millions of individuals. From
these outputs, frequency distributions were derived of assimilated doses
for populations of 500?000 sea otters or seaducks in each of 7 or 8
classes, respectively. Doses to several selected quantiles were
analyzed, ranging from the 1-in-1000th most-exposed individuals (99.9\%
quantile) to the median-exposed individuals (50\% quantile). The
resulting families of quantile curves provide the basis for
characterizing the environmental thresholds below which no
population-level effects could be detected and above which
population-level effects would be expected to become manifest. This
approach provides risk managers an enhanced understanding of the risks
to populations under various conditions and assumptions, whether under
hypothetically increased exposure regimes, as demonstrated here, or in
situations in which actual exposures are near toxic effects levels. This
study shows that individual-based models are especially amenable and
appropriate for conducting population-level risk assessments, and that
they can readily be used to answer questions about the risks to
individuals and populations across a variety of exposure conditions.
Integr Environ Assess Manag 2012; 8: 503522. (c) 2012 SETAC
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