Ecosystem functioning in stream assemblages from different regions : contrasting responses to variation in detritivore richness, evenness and density.

• 1. The diversity of species traits characterising a biological assemblage reflects not only its species richness but also its species evenness and total density, which together influence the concentration of traits within functional guilds, and is ultimately constrained by the regional species pool. Implications of such variation for spatio-temporal variability in biodiversity-ecosystem functioning relationships are potentially complex, but poorly understood.2. In separate microcosm experiments maintained at laboratories in Sweden, Romania and Ireland, we investigated effects of the species richness, evenness and density of stream-living detritivores on two related processes: detritivore leaf processing efficiency (LPE) and growth. Assemblage composition varied among the laboratories, with only one taxonomic order (Plecoptera) investigated in Sweden, whereas two orders, encompassing wider trait variation, were studied in Romania (Trichoptera and Plecoptera) and Ireland (Trichoptera and Isopoda).3. Relationships between density and both LPE and growth ranged from negative to positive across all the study species, highlighting the potential for density-dependent variation in process rates to alter ecosystem functioning, but indicating that such effects vary with species identity.4. Increased diversity was associated with 25-50% higher LPE in the two more heterogeneous assemblages, but whereas LPE in the Romanian study was generally enhanced as richness increased, LPE in the Irish study only increased in less even species mixtures dominated by particular species. Mechanisms underlying these diversity effects also contrasted. The Romanian results were predominantly attributable to the selection effect, whereas there was evidence for positive complementarity among species in Ireland.5. Growth was elevated in Romanian species mixtures, reflecting positive complementarity among species, but lower than expected growth in some Swedish mixtures was associated with negative complementarity.6. Our results emphasize the potential importance of detritivore diversity for stream ecosystem functioning across multiple contrasting assemblages, but variability in both the effects of diversity on the studied processes, and the mechanisms underlying those effects, highlight challenges in generalizing impacts of diversity change for the functional integrity of naturally dynamic stream ecosystems in situ.

Nyckelord

Alnus/*metabolism

Animals

Arthropods/*physiology

Biodiversity

Ecosystem

Fresh Water

Plant Leaves/metabolism

Population Density

Publikations- och innehållstyp

ref (ämneskategori)

art (ämneskategori)