LINKECOL Review Conference, Ceske Budejovice, Czech Republic, 25-27 April 2002
ABSTRACTS
Predatory impact of the freshwater invader, Dikerogammarus villosus (Crustacea: Amphipoda)
Jaimie T.A. Dick, School of Biology and Biochemistry, Queen's University Belfast, United Kingdom
In order to assess the increasing threats to aquatic ecosystems from invasive species,we need to elucidate the mechanisms of impacts of current and predicted future invaders. Dikerogammarus villosus, a Ponto-Caspian amphipod crustacean, is invading throughout Europe and predicted to invade the North American Great Lakes. European field studies show that populations of macroinvertebrates decline after D. villosus invasion. The mechanism of such impacts has not been addressed empirically, however, D. villosus is known to prey upon and replace other amphipods. Therefore, in this study, micro- and mesocosm laboratory experiments were used, with both single and mixed prey species scenarios, to assess any predatory impact of D. villosus on a range of macroinvertebrate taxa, trophic groups and body sizes. D. villosus predatory behaviour included shredding of prey and infliction of "bite" injuries on multiple victims. D. villosus killed significantly greater numbers of macroinvertebrates than did the native Gammarus duebeni, which is currently being replaced by D. villosus. This invader thus appears to impact on freshwater ecosystems through its exceptional predatory capabilities. It is predicted that future invasions by D. villosus will have serious direct and indirect effects on freshwaters, with its invasion facilitated in a larger "invasional meltdown" in regions such as the North American Great Lakes.
Importance of the initial structure of mesofauna communities on species dynamics and organic matter decomposition - implications for ecotoxocological studies
Jérôme Cortet, Susanne Elmholt, Richard Joffre & Paul Henning Krogh
Laboratory experiments using mesocosms were conducted to assess the impact of five mesofauna biodiversity levels (control without mesofauna : MIC; one collembola species: FF; five collembola species: COL; five collembola species and one enchytraeid: DET; five collembola species, one enchytraeid and one predatory mite: PRED) on fungal biomass (ergosterol content of wheat straw) and wheat straw decomposition parameters (litter mass loss, nitrogen dynamics). These last parameters were studied using Near Infrared Spectrophotometry technique. The impact of phenanthrene (43 mg/kg) was also analysed on these mesofauna combinations.
Various dynamics were observed for the different species involved in the decomposition processes : some considerably increased (like Folsomia fimetaria), some other decreased (like Isotomurus prasinus), some other presented variable dynamics (like Enchytraeus crypticus).
Looking at functional parameters, results indicated that decomposition was faster without mesofauna, as animals fed on bacteria and fungi. One important result suggested that decomposition, particularly nitrogen mineralisation was rather influenced by initial species composition than total abundance of decomposers. Effects of phenanthrene on mesofauna species were also influenced by initial composition of mesofauna, with a greater impact on total abundance when species richness increased. But results varied between species and habitats (soil or litter). Furthermore, phenanthrene increased organic matter decomposition (by increasing fungi), but no interaction between phenanthrene and initial composition of fauna was observed.
These results suggest that community effects should be taken into account in ecological risk assessment. Questions concerning ecological relevance of mesocosms (choice of species, stability of the system, time scale...) were discussed.
Biodiversity and organic matter processing on a sandy beach: An experimental approach
Tom Moens *, Barbara Urban-Malinga **, Ilse De Mesel*, Tom Gheskiere*
* University of Gent, Marine Biology Section, Gent, Belgium
**Institute of Ecology PAS, Łomianki, Poland & Institute of Oceanology PAS, Sopot, Poland
Patterns of meiobenthic diversity and phytodetritus decomposition rate were studied across a sandy beach gradient. The meiofaunal community was dominated by Nematoda. Both the meiofaunal densities, number of nematode species and biodiversity changed significantly across the beach gradient. The highest meiofaunal and nematode densities were found near the low water line; a strong increase in species number and biodiversity from high water level towards medium and low water level was observed.
Decomposition rates of organic matter were studied along the same beach gradient by means of litter bags using detritus of the macroalga Fucus sp. as a model. Litter bags with different mesh sizes (10 µm, 125 µm, 125 µm + large openings) were used to assess the contribution of different size classes of biota to the decomposition process. The rates of phytodetritus decomposition changed significantly across the beach gradient, but the link between biodiversity and the observed rates remains to be established.
In addition, a microcosm experiment was performed to study the influence of nematodes and their species diversity on the decomposition process of phytodetritus. In particular, we wanted to assess species redundancy among bacterial-feeding nematodes. The experiment was performed under constant ‘environmental’ conditions, with either 0, 1, 2 or 4 nematode species and Fucus detritus. Nematode diversity had no effects on decomposition rates of Fucus in our experiment, but a species-specific stimulatory effect, linked to the presence and activity of Pellioditis marina, was observed. It is clear from our results that bacterial-feeding nematodes may influence decomposition processes, but the exact nature of the effect remains to be established. At the same time, while our preliminary results show a significant level of species redundancy, this needs confirmation in open systems under variable environmental conditions.
Testing the relationship between biodiversity and ecosystem processes in grassland ecosystems
Wolfgang Weisser, Institute of Ecology, Friedrich-Schiller-University, Jena, Germany
In Jena, two new research projects explore the influence of biodiversity on ecosystem function. Both research projects use grasslands as model systems. In the DIVA-BIOLOG research project, natural managed grasslands in the Thuringian Schiefergebirge are investigated. In 2001, a comparative study of 78 grasslands was carried out to analyse the relationships between plant diversity, insect diversity, land use and ecosystem processes. All sites were visited during a two-week interval in June 2001. First results of the survey will be presented. The research project Forschergruppe Biodiversität is a long-term experiment to study the interactions between plant diversity and ecosystem processes, focussing on element cycling and trophic interactions. 60 plant species, native and common to the Central European Arrhenatherum grasslands serve as species pool. Mixtures of one to 16 plant species and of one to four plant functional groups have been seeded as newly established communities on 20 m x 20 m plots. The experimental design addresses current questions in the biodiversity debate. Further information on both projects can be found under http://www.uni-jena.de/biologie/~ecology.
