Resource partitioning of a Mexican clam in species-poor Baltic Sea sediments indicates the existence of a vacant trophic niche.

Invasive species are often generalists that can take advantage of formerly unexploited resources. The existence of such vacant niches is more likely in species-poor systems like the Baltic Sea. The suspension feeding wedge clam, Rangia cuneata, native to estuarine environments in the Gulf of Mexico, was sighted for the first time in the southeastern Baltic in 2010 and a few years later in the northern Baltic along the Swedish coast. To explore possible competition for food resources between R. cuneata and the three native clams inhabiting Baltic shallow soft bottoms, stable isotope and fatty acid analyses were conducted. There was no overlap between R. cuneata and any of the native species in either stable isotope or fatty acid niches. This suggests efficient partitioning of resources; multivariate analyses indicate that separation was driven mainly by δ13C and by fatty acids reflecting diatoms and cyanobacteria, respectively (e.g. 16:1ω7 and 18:3ω3). R. cuneata reflected seasonal variation in phytoplankton more than other clams reflecting higher trophic plasticity. In conclusion, the addition of R. cuneata to the Baltic shallow soft bottoms suggests the existence of a vacant trophic niche in these sediment habitats, however the long-term effects on other species and nutrient cycling requires further studies focusing on the population dynamics of R. cuneata and its impact on the Baltic Sea ecosystem.

The role of vital dietary biomolecules in eco-evo-devo dynamics.

The physiological dependence of animals on dietary intake of vitamins, amino acids, and fatty acids is ubiquitous. Sharp differences in the availability of these vital dietary biomolecules among different resources mean that consumers must adopt a range of strategies to meet their physiological needs. We review the emerging work on omega-3 long-chain polyunsaturated fatty acids, focusing predominantly on predator-prey interactions, to illustrate that trade-off between capacities to consume resources rich in vital biomolecules and internal synthesis capacity drives differences in phenotype and fitness of consumers. This can then feedback to impact ecosystem functioning. We outline how focus on vital dietary biomolecules in eco-eco-devo dynamics can improve our understanding of anthropogenic changes across multiple levels of biological organization.

Isotopic niche size variability in an ecosystem engineer along a disturbance gradient in a South African lagoon.

A key challenge for natural resource management is how to detect effects of environmental stress on individuals and populations before declines in abundance occur. Variability in carbon and nitrogen isotope composition (δ13C and δ15N) among consumers can provide information on the population trophic niche and how it may change in response to environmental stress. We measured δ13C and δ15N values in primary producers and in an ecosystem engineer, the bioturbating sandprawn Kraussillichirus kraussi, in Langebaan Lagoon, South Africa, along a human disturbance gradient. Diet partitioning mixing models were coupled with isotope niche analyses and individual body condition data to investigate shifts in resource utilisation and diet plasticity from minimally to highly disturbed sites. The δ15N values of seagrass, Zostera capensis indicated a nutrient gradient, with the highest δ15N values at highly disturbed sites indicating either anthropogenic or marine nitrogen inputs. A decreasing δ15N signal with distance from human disturbance/mouth of lagoon was however not evident for sandprawns nor their presumed dietary sources (phytoplankton, microphytobenthos or sediment organic matter), likely because of faster isotope turnover time compared to seagrass and/or differential fractionation for sandprawns among the sites. Sandprawn isotope niche sizes varied among sites, with no trend along the disturbance gradient. The smallest niche coincided with uniform feeding on microphytobenthos according to mixing models. On an individual level, deviating isotope values from population means were correlated to better body condition, suggesting that a divergent feeding strategy is beneficial. Our results support a generalist feeding behavior of the sandprawns with no evidence of reduced physiological status at the site with most human disturbance.