Summary1. Ecological stoichiometry, that is, the study of mass balance of multiple key chemical elements during ecological interactions, has recently been generalized to most ecosystems. Yet, effects of stoichiometric constraints on the life-history traits of detritivores, one of the most important trophic level through its importance in nutrient cycling, are still poorly documented. 2. Forested headwater streams constitute representative detritus-based ecosystems, relying on leaf litter inputs as the main energy and nutrient source for their functioning. Leaf litter is quickly colonized by aquatic hyphomycetes, which are known to improve the quality of detritus, enabling nutrient and energy transfer to higher trophic levels. 3. In this study, we manipulated leaf litter nutrient content of two common riparian tree species, differing in their initial biochemical composition, by short-term phosphorus addition to leaf litter precolonized by aquatic hyphomycetes. Then, we tested the impact of leaf litter phosphorus content on some key life-history traits, homeostasis and growth, of a common detritivorous invertebrate, Gammarus fossarum (Crustacea Amphipoda). 4. G. fossarum fed individually for 5 weeks with P-enriched leaf litter showed higher survival and growth rates than organisms fed with leaf litter colonized by fungi without P addition. Phosphorus content of resources did not alter G. fossarum elemental composition, suggesting that these detritivores are able to maintain their elemental homeostasis. 5. This study sheds new light on the importance of phosphorus content of detritus for detritivores and paves the way for further studies aimed at transferring ecological stoichiometry concepts to detritus-based ecosystems.

Knowledge of characteristics helpful in screening potential invaders and in elaborating strategies to limit their success is highly desirable. We focused on gammarid amphipods from Western Europe and North America to discover biological and/or ecological traits that may explain successful invasion by these species. Two typologies were considered: an analytical one, with groups built on the basis of biological or ecological similarities, and an empirical one, with groups constituted a priori according to a species' invasive status and its fresh or brackish water origin. The results obtained are discussed in the light of three hypotheses that may influence invasiveness: biotic potential, species size and euryoeciousness. The analysis revealed a particular ecological profile for invaders, with a strong influence of salinity tolerance, but no typology was found based on biological characteristics. Invasiveness cannot be predicted from a limited number of criteria, and is the result of a combination of several characteristics. Invasive species therefore exhibit a particular ecological profile rather than a biological one, contrary to most classical explanations.

No abstract