Double trouble at high density::Cross-level test of ressource-related adaptive plasticity and crowding-related fitness.

Population size is often regulated by negative feedback between population density and individual fitness. At high population densities, animals run into double trouble: they might concurrently suffer from overexploitation of resources and also from negative interference among individuals regardless of resource availability, referred to as crowding. Animals are able to adapt to resource shortages by exhibiting a repertoire of life history and physiological plasticities. In addition to resource-related plasticity, crowding might lead to reduced fitness, with consequences for individual life history. We explored how different mechanisms behind resource-related plasticity and crowding-related fitness act independently or together, using the water flea Daphnia magna as a case study. For testing hypotheses related to mechanisms of plasticity and crowding stress across different biological levels, we used an individual-based population model that is based on dynamic energy budget theory. Each of the hypotheses, represented by a sub-model, is based on specific assumptions on how the uptake and allocation of energy are altered under conditions of resource shortage or crowding. For cross-level testing of different hypotheses, we explored how well the sub-models fit individual level data and also how well they predict population dynamics under different conditions of resource availability. Only operating resource-related and crowding-related hypotheses together enabled accurate model predictions of D. magna population dynamics and size structure. Whereas this study showed that various mechanisms might play a role in the negative feedback between population density and individual life history, it also indicated that different density levels might instigate the onset of the different mechanisms. This study provides an example of how the integration of dynamic energy budget theory and individual-based modelling can facilitate the exploration of mechanisms behind the regulation of population size. Such understanding is important for assessment, management and the conservation of populations and thereby biodiversity in ecosystems

Emmy Magerl's visiting-card

<div><p>The antibacterial properties of nanosilver have led to a versatile application spectrum including medical purposes and personal care products. However, the increasing use of nanosilver has raised concerns about its environmental impacts. Long-term exposure studies with aquatic invertebrates are essential to assess possible adverse effects on aquatic ecosystems. In the present study, acute (48 h), chronic (21 d) and long-term effects of nanosilver (primary size 15 nm) on five successive generations of three <i>Daphnia</i> species (<i>D. magna</i>, <i>D. pulex</i>, and <i>D. galeata</i>) were investigated. Acute EC₅₀ values of nanosilver were 121 µg Ag L⁻¹ for <i>D. magna</i> being the least sensitive species and 8.95 and 13.9 µg Ag L⁻¹ for <i>D. pulex</i> and <i>D. galeata</i>, respectively. Chronic exposure provided EC₁₀ values of 0.92 µg Ag L⁻¹ for <i>D. magna</i> showing the most sensitive chronic reaction and 2.25 and 3.45 µg Ag L⁻¹ for <i>D. pulex</i> and <i>D. galeata</i>, respectively. Comparative exposure to AgNO₃ revealed a generally higher toxicity of the soluble form of silver. The multi-generation experiments resulted in effects on the population level for all tested species. Exposure of <i>D. magna</i> indicated an increased toxicity of nanosilver in the fifth generation of animals exposed to 10 µg Ag L⁻¹. Neonates from pre-exposed parental daphnids did not completely recover when transferred into clean water. Exposure of <i>D. pulex</i> and <i>D. galeata</i> revealed not only increasing toxicity in some generations, but also greater tolerance to nanosilver. This study contributes to the assessment of the risk potential of nanosilver on aquatic ecosystems. It shows that effects of nanosilver vary within one genus and change with exposure duration. Therefore, long-term studies considering different aquatic species are needed to better understand the possible effects of nanosilver on aquatic ecosystems.</p></div