Food web properties vary with climate and land use in South African streams

Regional impacts of warming on biodiversity and biomass in high latitude stream ecosystems across the Northern Hemisphere

Warming can have profound impacts on ecological communities. However, explorations of how differences in biogeography and productivity might reshape the effect of warming have been limited to theoretical or proxy-based approaches: for instance, studies of latitudinal temperature gradients are often conflated with other drivers (e.g., species richness). Here, we overcome these limitations by using local geothermal temperature gradients across multiple high-latitude stream ecosystems. Each suite of streams (6-11 warmed by 1-15°C above ambient) is set within one of five regions (37 streams total); because the heating comes from the bedrock and is not confounded by changes in chemistry, we can isolate the effect of temperature. We found a negative overall relationship between diatom and invertebrate species richness and temperature, but the strength of the relationship varied regionally, declining more strongly in regions with low terrestrial productivity. Total invertebrate biomass increased with temperature in all regions. The latter pattern combined with the former suggests that the increased biomass of tolerant species might compensate for the loss of sensitive species. Our results show that the impact of warming can be dependent on regional conditions, demonstrating that local variation should be included in future climate projections rather than simply assuming universal relationships.

The combined effects of treated sewage discharge and land use on rivers

Freshwater ecosystems are increasingly threatened by multiple anthropogenic stressors. Release of treated sewage effluent and pollution from agricultural or urban sources can independently reduce water quality with implications for ecological communities. However, our knowledge of the combined effects of these stressors is limited. We performed a field study to quantify the combined effect of treated sewage discharge and land use on nutrient concentrations, sewage fungus presence and communities of macroinvertebrates and benthic algae. Over three seasons in four rivers we found that a model which included an interaction between sewage pollution and time of the year (i.e. months) was the best predictor of nutrient concentrations and the abundance of algae and sewage fungus. Both macroinvertebrate and algae communities shifted downstream of sewage input. Specifically, more tolerant groups, such as cyanobacteria and oligochaetes, were more abundant. The EPT (Ephemeroptera, Plecoptera and Tricoptera) water quality score was best explained by an interaction between month and agriculture in the surrounding landscape. Overall, our results show that sewage discharge has a significant impact on water quality and benthic riverine communities, regardless of the surrounding land uses. Agricultural inputs, however, could be more important than treated sewage discharge in reducing the abundance of sensitive invertebrate taxa. We need both improvements to wastewater treatment processes and reductions in agricultural pollution to reduce threats to vulnerable freshwater communities.

Observed and projected functional reorganization of riverine fish assemblages from global change

Climate and land-use/land-cover change ("global change") are restructuring biodiversity, globally. Broadly, environmental conditions are expected to become warmer, potentially drier (particularly in arid regions), and more anthropogenically developed in the future, with spatiotemporally complex effects on ecological communities. We used functional traits to inform Chesapeake Bay Watershed fish responses to future climate and land-use scenarios (2030, 2060, and 2090). We modeled the future habitat suitability of focal species representative of key trait axes (substrate, flow, temperature, reproduction, and trophic) and used functional and phylogenetic metrics to assess variable assemblage responses across physiographic regions and habitat sizes (headwaters through large rivers). Our focal species analysis projected future habitat suitability gains for carnivorous species with preferences for warm water, pool habitats, and fine or vegetated substrates. At the assemblage level, models projected decreasing habitat suitability for cold-water, rheophilic, and lithophilic individuals but increasing suitability for carnivores in the future across all regions. Projected responses of functional and phylogenetic diversity and redundancy differed among regions. Lowland regions were projected to become less functionally and phylogenetically diverse and more redundant while upland regions (and smaller habitat sizes) were projected to become more diverse and less redundant. Next, we assessed how these model-projected assemblage changes 2005-2030 related to observed time-series trends (1999-2016). Halfway through the initial projecting period (2005-2030), we found observed trends broadly followed modeled patterns of increasing proportions of carnivorous and lithophilic individuals in lowland regions but showed opposing patterns for functional and phylogenetic metrics. Leveraging observed and predicted analyses simultaneously helps elucidate the instances and causes of discrepancies between model predictions and ongoing observed changes. Collectively, results highlight the complexity of global change impacts across broad landscapes that likely relate to differences in assemblages' intrinsic sensitivities and external exposure to stressors.

Nitrogen loadings affect trophic structure in stream food webs on the Tibetan Plateau, China

Anthropogenic activities, such as agricultural and industrial development, have increased nutrient inputs into waterways, which affect trophic interactions and the flow of energy through food webs in the aquatic ecosystems. However, the responses of food web structure and function to specific anthropogenic stressors in the alpine stream systems remain unclear. Here, we studied the stream food webs in the Lhasa River on the Tibetan Plateau, China. We measured the isotopic ratios (δ¹³C and δ¹⁵N) of macroinvertebrate and fish functional feeding groups (FFGs) and their basal resources in the streams. Dietary contributions of basal resources to consumers and food web metrics including trophic length, diversity, and redundancy were used to quantify changes in stream food webs in response to anthropogenic disturbance. Dietary analysis showed that allochthonous resources contributed more than autochthonous resources to macroinvertebrate primary consumers regardless of the disturbance intensity in the adjacent land areas. Anthropogenic activities increased the δ¹⁵N values in epilithic algae and isotopic variation in basal resources and fish but reduced the trophic length and redundancy (i.e., fewer species or taxon at each trophic level) in food webs. Additionally, the total nitrogen concentration in waters was the most important environmental variable affecting trophic diversity and redundancy. Therefore, the reduction of nitrogen inputs into streams is critical for sustainable river management and biodiversity conservation in the streams on the Tibetan Plateau.

Invasive carp alter trophic niches of consumers and basal resources in African reservoirs

Environmental pollution and biological invasions are key drivers of biodiversity change. However, the effects of invasion and pollution on food webs remain largely unexplored. Here, we used stable isotopes to examine the effects of common carp Cyprinus carpio and pollution on trophic dynamics in six small reservoirs. Our results revealed that the trophic niche widths of invertebrates, vertebrates, and invasive carp did not significantly differ among reservoirs with different pollution statuses. However, we found low niche conservatism among reservoirs, suggesting that while niche width may remain consistent, there is a shift in the position of the niches in isotopic space under both pollution and invasion scenarios. Niche conservatism among reservoirs was generally higher in invertebrates, but this was also regardless of reservoir condition (i.e. presence or absence of pollution and invasion). These results suggest that invasion by species coupled with organic pollution may cause subtle yet differing effects on components of a food web (basal end-members, invertebrates and vertebrates). Our findings provide a baseline measure of the potential in the development of detection and response strategies for carp invasions and organic pollution.