Potential effects of life-history evolution on ecological risk assessment

We investigated theoretically how the sensitivity of organisms to the toxicity of chemicals varies depending on their life-history traits, which are subject to evolution. We used a resource-allocation model in which organisms allocate their resources to reproduction, maintenance of life (reduction of death), and reduction of the toxicities of chemicals. First we investigated the optimal allocation rates in the absence of chemicals. We found that when evolution occurred in low-density populations, the allocation rate for reproduction was larger than that for maintenance of life, and hence an r-strategy evolved. The r-strategists had lower sensitivity (higher resistance) against the toxicity than K-strategists, which was the optimal strategy in high-density populations. Second, we examined the optimal allocation rates in the presence of chemicals. The allocation rate for the reduction of toxicity varied depending on the shape of functions for the reduction of toxicity. When the efficiency for the reduction was low, organisms did not allocate resources to reduce toxicity, and they remained sensitive to chemicals (sensitive type). When the toxicity was efficiently reduced, the organisms allocated resources to reduce the toxicity and became insensitive to the chemicals (resistant type). When the function for the reduction had a sigmoidal shape, evolutionary bistability appeared, and the organisms eventually evolved either to allocate resources for chemical reduction or not to do so depending on the initial conditions of evolution. This result explains the large variation in the sensitivities to chemicals in organisms collected from polluted areas. We also found that the toxicity required to reduce the population growth rate by 10% (EC10) was higher for the resistant type than for the sensitive type in general; however, when the toxicity tests were conducted under a resource-poor condition, EC10 was even smaller in the resistant type than in the sensitive type (i.e., resistant organisms are more sensitive than sensitive organisms). This counterintuitive result occurred because the allocation of resources for toxicity reduction was larger than needed, and was thus an overinvestment under the resource-poor condition. Together with the results, we conclude that lacking an understanding of the evolutionary aspect may lead to insufficient risk assessment and management.