Abteilung Aquatische Ökologie

Material fluxes are ubiquitous in nature within and across ecosystems, connecting habitats with vastly different characteristics, like forests to rivers and lakes.^1,2,3 Although individual fluxes and their cascading effects are well known,^4,5,6 very few studies address the intra-annual phenology of ecosystem processes, despite the pronounced seasonality of fluxes. Here, we empirically quantified and resolved fluxes of recalcitrant and labile types of leaf litter in temperate riparian forests and streams across a year, representing one of the most emblematic examples of seasonal systems. We quantified intra-annual variation in litter inputs from terrestrial plants to forest floors and streams and estimated aquatic and terrestrial decomposition rates across the year at 6-week intervals. Our data show that the autumn pulse of leaf litter is complemented by smaller magnitude but more constant-through-the-year lateral flows to the stream ecosystems. Decomposition of labile litter fluctuated seasonally, on a different phenology, with generally higher rates in summer, but rates of recalcitrant litter decomposition remained largely constant. Microorganisms were the main contributors to the decomposition process in both forests and streams. Overall, our work highlights the asynchronous and seasonally variable changes in decomposition rates between recalcitrant and labile detritus despite their initial synchronized availability and suggests that the dominating presence of recalcitrant litter buffers ecosystem responses to the concentrated temporal distribution of litter resources.^7,8 Investigating such ecological processes both across ecosystem borders and at fine intra-annual resolutions is imperative to understand complex system responses in the context of species’ shifts in phenologies and resource quality.^9,10,11

Human activities drive a wide range of environmental pressures, including habitat change, pollution and climate change, resulting in unprecedented effects on biodiversity^1,2. However, despite decades of research, generalizations on the dimensions and extent of human impacts on biodiversity remain ambiguous. Mixed views persist on the trajectory of biodiversity at the local scale³ and even more so on the biotic homogenization of biodiversity across space^4,5. We compiled 2,133 publications covering 97,783 impacted and reference sites, creating an unparallelled dataset of 3,667 independent comparisons of biodiversity impacts across all main organismal groups, habitats and the five most predominant human pressures^1,6. For all comparisons, we quantified three key measures of biodiversity to assess how these human pressures drive homogenization and shifts in composition of biological communities across space and changes in local diversity, respectively. We show that human pressures distinctly shift community composition and decrease local diversity across terrestrial, freshwater and marine ecosystems. Yet, contrary to long-standing expectations, there is no clear general homogenization of communities. Critically, the direction and magnitude of biodiversity changes vary across pressures, organisms and scales at which they are studied. Our exhaustive global analysis reveals the general impact and key mediating factors of human pressures on biodiversity and can benchmark conservation strategies.

Biodiversity underpins the functional integrity of ecosystems. At present, our understanding of the relationship between biodiversity and ecosystem functioning (BEF) is essentially based on manipulative experiments. Compelling data at large spatial scales are scarce, especially for river networks. BEF patterns across landscapes are complex because they unfold in the context of environmental gradients and compositional turnover of natural communities. Leaf litter decomposition, a pivotal ecosystem process in streams, is no exception to this context dependency. The dendritic structure of river networks plus the unidirectional water flow shape both environmental conditions and the distribution of leaf resources and consumers. However, it is difficult to predict how spatial gradients of resource and consumer composition can overlap across a river network, and thus govern spatial patterns of decomposition. Here, we investigated the capacity of macroinvertebrate biodiversity to control decomposition rates of heterogeneous leaf resources at the river-network scale. We deployed five litterbags containing either one of four single leaf species or a mixture of all species at 51 sites across the Thur River network (Switzerland). We measured litter decomposition rates, variation of decomposition among leaf resources, and the effect of leaf litter diversity on decomposition. We found that decomposition rates decreased from headwaters to downstream reaches mainly due to the parallel decrease in the abundance of key shredder taxa (namely, Amphinemura, Nemoura, Leuctra, Habroleptoides, and Stenophylacini). Macroinvertebrate diversity had a minor, negative effect on decomposition rates. However, high functional macroinvertebrate diversity at the reach scale reduced the variation of decomposition among leaf resources, thus alleviating nutritional constraints exerted by nutrient-poor leaf resources. Furthermore, litter mixtures were preferably decomposed by communities with low evenness and dominated by a few taxa. These findings point to a critical role of macroinvertebrates in controlling litter decomposition at the network scale beyond environmental effects. While shredder abundance and community composition are key to determining decomposition rates across the river network, functional diversity is important in decreasing the variation of decomposition rates among leaf resources. Our results stress the importance of biodiversity controlling ecosystem functioning not only at the local but also at the river network scale.

Ecosystem ecology needs a framework that explicitly considers the roles of organic compounds. The ecology of molecules integrates compound identity, diversity, and interactions to understand ecosystem processes, such as nutrient and carbon cycling. This approach leverages advances in analytical chemistry and molecular biology to unravel the complex chemical interplay within ecosystems.

1. Cryptic species are an important part of freshwater biodiversity, yet it remains unclear how these species integrate into communities from local to regional geographic scales. To protect biodiversity, particularly overlooked cryptic species, an accurate understanding of the underlying processes and adequate level of protection is needed.
2. We analysed patterns of syntopies (local co-occurrences) and sympatries (regional range overlap) to explore how the phylogenetic origin of cryptic species shapes biodiversity patterns. We hypothesised (i) that syntopies were more common among distantly than closely related cryptic species, and (ii) that the existing sympatries were an outcome of phylogenetic relatedness and dispersal.
3. The hypotheses were tested on a polyphyletic species complex of subterranean amphipod species (Niphargus rhenorhodanensis complex) by deploying molecular species delimitation, time-calibrated phylogenies, and co-occurrence analyses with probabilistic and generalised linear models (GLM).
4. The studied complex comprised 37–48 molecular operational taxonomic units (MOTUs) from nine different clades, with syntopies occurring at random or less frequently than expected. GLM indicated age of divergence did not predict species sympatries, although they emerged more frequently among MOTUs from different clades. Sympatries, however, emerged through dispersal, in MOTUs with large geographic ranges. These mostly overlapped at the foothills of the Alps, the Jura and the Central Massif.
5. We conclude that the observed spatial patterns are mainly driven by dispersal and presumably reflect the geographic circumstances of speciation. While species richness on a local scale may be an outcome of competition and dispersal, regional biodiversity patterns presumably arise through a clade-level cascade of historical events, including orogeny and climatic shifts.

Temperate streams are subsidized by inputs of leaf litter peaking in fall. Yet, stream communities decompose dead leaves and integrate their energy into the aquatic food web throughout the whole year. Most studies investigating stream decomposition largely overlook long-term trajectories, which must be understood for an appropriate temporal upscaling of ecosystem processes. Using mesocosms, we quantified changes in carbon, nitrogen, and phosphorus content of three leaf species during decomposition at weekly to multi-month intervals for up to a year; then, we tested how decomposition duration affected the subsequent consumption by a keystone amphipod macroinvertebrate. Over a year, nitrogen and phosphorus percentage increased across all leaf species, but only the recalcitrant species maintained initial levels of absolute nitrogen and phosphorus. Prolonged decomposition barely affected or impaired amphipod consumption of labile leaf species, whereas it enhanced feeding on the recalcitrant species. Overall, we demonstrate that recalcitrant leaves might serve as longer stored potential resources for when labile species have already been consumed and that their increasing palatability observed over multi-month intervals of sustained decomposition may stabilize fluctuations in the rates of leaf litter integration into aquatic food webs. This yearlong perspective highlights the relevancy of slow-decomposing leaves for aquatic detrital communities.

Biodiversity–ecosystem functioning experiments have established generally positive species richness-productivity relationships in plots of single ecosystem types, typically grassland or forest. However, it remains unclear whether these findings apply in real-world landscapes that resemble a heterogeneous mosaic of different ecosystem and plant types that interact through biotic and abiotic processes. Here, we show that landscape-level diversity, measured as number of land-cover types (different ecosystems) per 250×250 m, is positively related to landscape-wide remotely-sensed primary production across all of North America, covering 16 of 18 ecoregions of Earth. At higher landscape diversity, productivity was temporally more stable, and 20-year greening trends were accelerated. These effects occurred independent of local species diversity, suggesting emergent mechanisms at hitherto neglected levels of biological organization. Specifically, mechanisms related to interactions among land-cover types unfold at the scale of entire landscapes, similar to, but not necessarily resulting from, interactions between species within single ecosystems.