Harwell Gentile & Associates, LC, Port Orange, Florida 32129-1267, USA.
Integr Environ Assess Manag. 2012 Jul;8(3):503-22. doi: 10.1002/ieam.1277. Epub 2012 Mar 12.
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.
生态风险评估需要超越基于实验室条件下对少数物种进行的毒性研究来评估个体风险，进而评估包括变异性和不确定性在内的环境种群风险。最近开发了两种基于个体的模型（IBMs），用于评估 20 多年前埃克森·瓦尔迪兹号溢油事件（EVOS）后威廉王子湾海獭和海鸭的当前风险，用于探索种群风险。在每种情况下，这些模型之前都表明，个体几乎没有来自 EVOS 的多环芳烃（PAHs）的剩余风险。本文报告了新的敏感性分析，其中假设的 PAH 环境暴露被启发式地增加，直到吸收剂量达到毒性参考值（TRV），这些 TRV 是从无观察到不良影响和最低观察到不良影响水平（NOAEL 和 LOAEL）推导出来的。对于海獭，这是通过人为地增加与地下油残留物剩余斑块相交的海獭洞穴数量来实现的，数量级超过实际估计速率。同样，在海鸭评估中，饮食、沉积物和海水中的 PAH 浓度按其对吸收剂量的相对贡献成比例增加了数量级，超过了实测环境浓度，以达到 NOAEL 和 LOAEL 阈值。随机 IBM 模拟了数百万个个体。从这些输出中，分别为每个 7 或 8 类中的 50 万只海獭或海鸭种群衍生出吸收剂量的频率分布。分析了几个选定分位数的剂量，范围从最暴露的个体（99.9%分位数）的千分之一到中位数暴露个体（50%分位数）。由此产生的分位数曲线族为描述在低于环境阈值时无法检测到种群水平效应而高于环境阈值时预计会出现种群水平效应的情况提供了基础。这种方法使风险管理者能够更好地了解各种条件和假设下种群面临的风险，无论是在这里所示的假设增加暴露的情况下，还是在实际暴露接近毒性效应水平的情况下。本研究表明，基于个体的模型特别适合和适合进行种群风险评估，并且可以很容易地用于回答关于个体和种群在各种暴露条件下的风险问题。
