Ecosystem Functioning In Response Research Articles

Dominant role of grazing and snow cover variability on vegetation shifts in the drylands of Kazakhstan

Decomposing the responses of ecosystem structure and function in drylands to changes in human-environmental forcing is a pressing challenge. Though trend detection studies are extensive, these studies often fail to attribute them to potential spatiotemporal drivers. Most attribution studies use a single empirical model or a causal graph that cannot be generalized or extrapolated to larger scales or account for spatial changes and multiple independent processes. Here, we proposed and tested a multi-stage, multi-model framework that detects vegetation trends and attributes them to ten independent social-environmental system (SES) drivers in Kazakhstan (KZ). The time series segmented residual trend analysis showed that 45.71% of KZ experienced vegetation degradation, with land use change as the predominant contributor (22.54%; 0.54 million km2), followed by climate change and climate variability. Pixel-wise fitted Granger Causality and random forest models revealed that sheep & goat density and snow cover had dominant negative and positive impacts on vegetation in degraded areas, respectively. Overall, we attribute vegetation changes to SES driver impacts for 19.81% of KZ (out of 2.39 million km2). The identified vegetation degradation hotspots from this study will help identify locations where restoration projects could have a greater impact and achieve land degradation neutrality in KZ.

Putting the Asymmetric Response Concept to the test: Modeling multiple stressor exposure and release in a stream food web

Communities in stream ecosystems often respond asymmetrically to increase and release of stressors, as indicated by slow and incomplete recovery. The Asymmetric Response Concept (ARC) posits that this is due to a shift in the relative importance of three mechanisms: tolerance, dispersal, and biotic interactions. In complex natural communities, these mechanisms may produce alternative outcomes through poorly understood indirect effects. To understand how the three mechanisms respond to different temporal stressor scenarios, we studied multiple scenarios using a stream food web model. We asked the following questions: Do groups of species decline as expected on the basis of individual tolerance rankings derived from laboratory experiments when they are embedded in a complex dynamic food web? Does the response of ecosystem function match that of communities? To address these questions, we aggregated data on individual tolerances at the level of functional groups and studied how single and multiple stressors affect food web dynamics and nutrient cycling. Multiple stressor scenarios involved different intensities of salt and temperature increase. Functional groups exhibited a different relative tolerance ranking between the laboratory and dynamic food web contexts. Salt as a single stressor had only minor and transient effects at low level but led to the loss of one or more functional groups at high level. In contrast, high temperature, alone or in combination with salt, caused the loss of functional groups at all tested levels. Patterns often differed between the response of communities and ecosystem function. We discuss our findings with respect to the ARC.

Disturbance theory for ecosystem ecologists: A primer.

Understanding what regulates ecosystem functional responses to disturbance is essential in this era of global change. However, many pioneering and still influential disturbance-related theorie proposed by ecosystem ecologists were developed prior to rapid global change, and before tools and metrics were available to test them. In light of new knowledge and conceptual advances across biological disciplines, we present four disturbance ecology concepts that are particularly relevant to ecosystem ecologists new to the field: (a) the directionality of ecosystem functional response to disturbance; (b) functional thresholds; (c) disturbance-succession interactions; and (d) diversity-functional stability relationships. We discuss how knowledge, theory, and terminology developed by several biological disciplines, when integrated, can enhance how ecosystem ecologists analyze and interpret functional responses to disturbance. For example, when interpreting thresholds and disturbance-succession interactions, ecosystem ecologists should consider concurrent biotic regime change, non-linearity, and multiple response pathways, typically the theoretical and analytical domain of population and community ecologists. Similarly, the interpretation of ecosystem functional responses to disturbance requires analytical approaches that recognize disturbance can promote, inhibit, or fundamentally change ecosystem functions. We suggest that truly integrative approaches and knowledge are essential to advancing ecosystem functional responses to disturbance.

Seeds exhibit the most stable elemental composition with nitrogen addition in an Inner Mongolian grassland

Variation in biomass elemental composition of grassland plants may have important implications for ecosystem functioning in response to global change. However, relevant studies have mostly focused on variation of nitrogen (N) and phosphorus (P) concentrations in plant leaves, while few studies have evaluated other elements and plant organs of grassland species. Here, we examined the effects of N addition on multi-element concentrations, and analyzed their patterns across different organs (leaf, stem, root and seed) of five plant species in a steppe community of the Inner Mongolian grassland. Our results showed that seeds exhibited the most stable elemental composition with N addition, and that manganese (Mn) and iron (Fe) concentrations were substantially more variable than macro-elements in response to N addition. In particular, we identified a set of significant negative relationships between elemental concentrations and their corresponding CVs (coefficients of variation) for all plant organs as a whole and for each individual organ. We further found that changes in soil pH and the availability of soil nutrients contributed mostly to variation in the biomass elemental composition of major plants in this community. These findings are important for accurately assessing the effects of N deposition on the biochemical cycling of nutrient elements in grassland ecosystems, and provide critical clues for developing effective approaches to adaptively managing grassland resources as well as mitigating the impact of global change on the dryland ecosystems in the Mongolia Plateau.

Pollinator-mediated effects of landscape-scale land use on grassland plant community composition and ecosystem functioning - seven hypotheses.

Environmental change is disrupting mutualisms between organisms worldwide. Reported declines in insect populations and changes in pollinator community compositions in response to land use and other environmental drivers have put the spotlight on the need to conserve pollinators. While this is often motivated by their role in supporting crop yields, the role of pollinators for reproduction and resulting taxonomic and functional assembly in wild plant communities has received less attention. Recent findings suggest that observed and experimental gradients in pollinator availability can affect plant community composition, but we know little about when such shifts are to be expected, or the impact they have on ecosystem functioning. Correlations between plant traits related to pollination and plant traits related to other important ecosystem functions, such as productivity, nitrogen uptake or palatability to herbivores, lead us to expect non-random shifts in ecosystem functioning in response to changes in pollinator communities. At the same time, ecological and evolutionary processes may counteract these effects of pollinator declines, limiting changes in plant community composition, and in ecosystem functioning. Despite calls to investigate community- and ecosystem-level impacts of reduced pollination, the study of pollinator effects on plants has largely been confined to impacts on plant individuals or single-species populations. With this review we aim to break new ground by bringing together aspects of landscape ecology, ecological and evolutionary plant-insect interactions, and biodiversity-ecosystem functioning research, to generate new ideas and hypotheses about the ecosystem-level consequences of pollinator declines in response to land-use change, using grasslands as a focal system. Based on an integrated set of seven hypotheses, we call for more research investigating the putative pollinator-mediated links between landscape-scale land use and ecosystem functioning. In particular, future research should use combinations of experimental and observational approaches to assess the effects of changes in pollinator communities over multiple years and across species on plant communities and on trait distributions both within and among species.

Response of grassland ecosystem function to plant functional traits under different vegetation restoration models in areas of karst desertification.

Plant functional traits serve as a bridge between plants, the environment, and ecosystem function, playing an important role in predicting the changes in ecosystem function that occur during ecological restoration. However, the response of grassland ecosystem function to plant functional traits in the context of ecological restoration in areas of karst desertification remains unclear. Therefore, in this study, we selected five plant functional traits [namely, plant height (H), specific leaf area (SLA), leaf dry matter content (LDMC), root length (RL), and root dry matter content (RDMC)], measured these along with community-weighted mean (CWM) and functional trait diversity, and combined these measures with 10 indexes related to ecosystem function in order to investigate the differences in plant functional traits and ecosystem function, as well as the relationship between plant functional traits and ecosystem functions, under four ecological restoration models [Dactylis glomerata (DG), Lolium perenne (LP), Lolium perenne + Trifolium repens (LT), and natural grassland (NG)]. We found that: 1) the Margalef index and Shannon-Wiener index were significantly lower for plant species in DG and LP than for those in NG (P<0.05), while the Simpson index was significantly higher in the former than in NG (P<0.05); 2) CWMH, CWMLDMC, and CWMRDMC were significantly higher in DG, LP, and LT than in NG, while CWMSLA was significantly lower in the former than in NG (P<0.05). The functional richness index (FRic) was significantly higher in DG and LP than in NG and LT, but the functional dispersion index (FDis) and Rao's quadratic entropy index (RaoQ) were significantly lower in DG and LP than in NG and LT (P<0.05), and there was no significant difference between DG and LP, or between NG and LT (P>0.05); 3) ecosystem function, including ecosystem productivity, carbon storage, water conservation and soil conservation, was highest in LT and lowest in NG; and 4) CWMLDMC (F=56.7, P=0.024), CWMRL (F=28.7, P=0.024), and CWMH (F=4.5, P=0.048) were the main factors affecting ecosystem function. The results showed that the mixed pasture of perennial ryegrass and white clover was most conductive to restoration of ecosystem function. This discovery has important implications for the establishment of vegetation, optimal utilization of resources, and the sustainable development of degraded karst ecosystems.

Phosphorus Coupled with High Nitrogen Addition Exerts a Great Influence on Soil Bacterial Community in a Semiarid Grassland.

Nitrogen (N) and phosphorus (P) addition, either individually or in combination, has been demonstrated to enhance plant productivity in grassland ecosystems. Soil bacterial community, which is the driver of litter decomposition and nutrient cycling, is assumed to control responses of terrestrial ecosystem structure and function to N and P addition. Using a high-throughput Illumina MiSeq sequencing platform, we conducted a 9-year field experiment of N (0, 5, 10, and 20 g N m-2 yr-1) and P (0 and 10 g P m-2 yr-1) additions in the Inner Mongolian steppes to elucidate long-term effects of N and P addition on soil bacterial richness, diversity and composition. We found that N addition reduced the relative abundance of Acidobacteria, Chloroflexi, and Nitrospirae, while increased that of Bacteroides. The results showed that the bacterial biomarker was enriched in P addition treatments, either individually or combined with N addition. Both N and P addition altered the bacterial community structure, while only N addition greatly decreased bacterial richness and diversity. More importantly, we showed that all of these effects were most significant in N3P treatment (20 g N m-2 yr-1 and 10 g P m-2 yr-1), implying that P coupled with a high-level N addition exerted a great influence on soil bacterial community. Structural equation models revealed that N and P addition had a great direct effect on soil bacterial community and an indirect effect on it mainly by changing the litter biomass. Our findings highlighted that severe niche differentiation was induced by P along with a high-level N, further emphasizing the importance of simultaneously evaluating response of soil bacterial community to N and P addition, especially in the context of increasing anthropogenic nutrient additions.

Rapid changes in functional trait expression and decomposition following high severity fire and experimental warming

Uncharacteristically severe wildfires are occurring at higher frequency, across larger spatial extents, and in new seasons in many parts of the globe. At the same time, climate change is elevating temperatures and altering precipitation patterns. High severity fires have the potential to produce shifts in ecosystem type and function in communities that are adapted to low severity fire via changes in community composition, functional trait values, and nutrient cycling processes. However, interactive effects between climate warming and fire severity on community composition, trait values, and ecosystem functioning are rarely studied and poorly understood. We assessed the impact of experimental warming via open top warming chambers across a burn severity gradient on the understory plant community of a Ponderosa pine (Pinus ponderosa) forest in northern Arizona, USA. Specifically, we examined community composition, three plant functional traits (specific leaf area (SLA), leaf dry matter content (LDMC), and plant height) and one ecosystem function (decomposition rate) in the first post-fire growing season. High burn severity produced significant shifts in community composition. The combination of experimental warming and high burn severity significantly decreased community weighted mean LDMC and plant height, and increased decomposition rate. Our study demonstrates significant and rapid responses of community composition, trait expression, and ecosystem function in response to burn severity, experimental warming, and their interaction. This suggests that ecosystems experiencing atypically severe fire under future climate and wildfire conditions may recover in fundamentally different ways than in the past, favoring a different suite of species and traits, with altered ecosystem function.

Contrasting responses of soil microbial biomass and extracellular enzyme activity along an elevation gradient on the eastern Qinghai-Tibetan Plateau.

Soil microbial community composition and extracellular enzyme activity are two main drivers of biogeochemical cycling. Knowledge about their elevational patterns is of great importance for predicting ecosystem functioning in response to climate change. Nevertheless, there is no consensus on how soil microbial community composition and extracellular enzyme activity vary with elevation, and little is known about their elevational variations on the eastern Qinghai-Tibetan Plateau, a region sensitive to global change. We therefore investigated the soil microbial community composition using phospholipid fatty acids (PLFAs) analysis, and enzyme activities at 2,820 m (coniferous and broadleaved mixed forest), 3,160 m (dark coniferous forest), 3,420 m (alpine dwarf forest), and 4,280 m (alpine shrubland) above sea level. Our results showed that soil microbial community composition and extracellular enzyme activities changed significantly along the elevational gradient. Biomass of total microbes, bacteria, and arbuscular mycorrhizal fungi at the highest elevation were the significantly lowest among the four elevations. In contrast, extracellular enzyme activities involved in carbon (C)-, nitrogen (N)-, and phosphorus (P)- acquiring exhibited the maximum values at the highest elevation. Total nutrients and available nutrients, especially P availability jointly explained the elevational pattern of soil microbial community, while the elevational variation of extracellular enzyme activities was dependent on total nutrients. Microbial metabolism was mainly C- and P-limited with an increasing C limitation but a decreasing P limitation along the elevational gradient, which was related significantly to mean annual temperature and total P. These results indicated a vital role of soil P in driving the elevational patterns of soil microbial community and metabolism. Overall, the study highlighted the contrasting responses of soil microbial biomass and extracellular enzyme activities to elevation, possibly suggesting the differences in adaption strategy between population growth and resource acquisition responding to elevation. The results provide essential information for understanding and predicting the response of belowground community and function to climate change on the eastern Qinghai-Tibetan Plateau.

Tracking fungal species-level responses in soil environments exposed to long-term warming and associated drying.

Climate change is affecting fungal communities and their function in terrestrial ecosystems. Despite making progress in the understanding of how the fungal community responds to global change drivers in natural ecosystems, little is known on how fungi respond at the species level. Understanding how fungal species respond to global change drivers, such as warming, is critical, as it could reveal adaptation pathways to help us to better understand ecosystem functioning in response to global change. Here, we present a model study to track species-level responses of fungi to warming-and associated drying-in a decade-long global change field experiment; we focused on two free-living saprotrophic fungi which were found in high abundance in our site, Mortierella and Penicillium. Using microbiological isolation techniques, combined with whole genome sequencing of fungal isolates, and community level metatranscriptomics, we investigated transcription-level differences of functional categories and specific genes involved in catabolic processes, cell homeostasis, cell morphogenesis, DNA regulation and organization, and protein biosynthesis. We found that transcription-level responses were mostly species-specific but that under warming, both fungi consistently invested in the transcription of critical genes involved in catabolic processes, cell morphogenesis, and protein biosynthesis, likely allowing them to withstand a decade of chronic stress. Overall, our work supports the idea that fungi that invest in maintaining their catabolic rates and processes while growing and protecting their cells may survive under global climate change.

A trait database for southern African freshwater invertebrates

Invertebrates are often used as indicators of ecosystem health. In community ecology, the use of organism traits (e.g. behavioural, biological or ecological) can be of value in relation to impact diagnosis and assemblage prediction. It has also been recognised that certain traits are related to ecosystem function, and their use in biological monitoring provide insights into ecosystem functional responses to stressors. However, the paucity of trait information and non-availability of a local trait database for southern African invertebrates impede progress in using this approach for biological monitoring. To address this critical gap, we compiled an updateable trait database for southern African freshwater invertebrates. The database contains information on 40 trait categories and 204 trait attribute/modalities. Of the 40 trait categories included in the database, 12 are biological, 12 are behavioural, and 16 are ecological preferences. The database is designed to accommodate taxonomic flexibility. In this regard, trait information is entered at multiple taxonomic levels: family, genus, and species, allowing researchers working at different taxonomic resolutions to retrieve relevant information. While there are still gaps in the database, this effort represents the first attempt to synthesise available trait information on southern African freshwater invertebrates.

Dispersal limitation and host selection drive geo-specific and plant-specific differentiation of soil bacterial communities in the Tibetan alpine ecosystem

Soil bacteria, which are active in shrub encroachment, play key roles in regulating ecosystem structure and function. However, the differentiation characteristics and assembly process of bacterial communities in scrubbed grasslands remain unknown. Taking the Qinghai-Tibet Plateau, a hotspot of shrub encroachment, as the study area, we collected 192 soils near nine natural typical shrubs' roots on a trans-longitude transect (about 1800 km) and investigated the bacterial communities using 16S rRNA amplicon sequencing. We found that the bacterial communities exhibited plant-specific and geographic-specific differentiation. On the one hand, bacterial communities differed significantly across plant species, with widely distributed shrubs harboring high diversity communities but few plant-specific taxa, and narrowly distributed shrubs possessing low diversity communities but more plant-specific taxa. Besides, there was a significant negative correlation between bacterial community similarity and plant phylogenetic distance. On the other hand, bacterial communities differed across geographic sites, with a significant decay in bacterial community similarity with geographic distance. The bacterial alpha diversity varied in an inverted V-shape from west to east, peaking at 91°E, which could be largely driven by mean annual temperature, soil pH and soil total carbon content. Community differentiation increased with the heterogeneity degree of assembly processes, and the dominant assembly process in these two specific differentiations differed. Dominated by stochastic and deterministic forces, respectively, geography diverged bacterial communities primarily through increased dispersal limitation, whereas plants diverged bacterial communities primarily through increased variable selection. Our study provides new insight into the characteristics and mechanisms of root-surrounding soil bacteria differentiation in scrubbed grasslands, contributing to the scientific management of degraded grasslands and the prediction of bacterial community structure and ecosystem function in response to global change.

Interpreting Mangrove Habitat and Coastal Land Cover Change in the Greater Bay Area, Southern China, from 1924 to 2020 Using Historical Aerial Photos and Multiple Sources of Satellite Data

Coastal habitat dynamics and ecosystem function in response to human-induced disturbance, especially urbanization, are of increasing concern. However, how changes in landscape composition as well as habitat quantity and quality may affect the long-term sustainability of rapidly urbanizing coasts remains unclear. This study aimed to quantify the extent, change rate, patterns, change process and interrelationships of mangrove habitats, impervious surfaces, and other land cover types in Deep Bay in the Guangdong-Hong Kong-Macau Greater Bay Area (GBA), China, the world’s largest megalopolis, from 1924 to 2020. We processed historical aerial photos (1924–2020) and multiple sources of satellite data (1973–2020) for different types of land cover mapping. Post-classification analysis, including correlation analysis and change detection analysis, was conducted based on the long time-series land cover classification results. Mangrove habitats increased in Deep Bay from 1924 to 2020, except for a large area decrease from 1954 to 1964 due to the construction of tidal aquaculture ponds. Mudflat areas contributed most to the expansion of mangrove habitats of about 275 ha from 1987 to 2020. During this period, reclamation and urbanization for the construction of the megacity of Shenzhen turned large areas of water and mudflat (about 4000 ha) on the northern shore into impervious surface and urban vegetation. Overall, the landscape pattern of mangrove habitats in Deep Bay showed increasing connectivity and decreasing degree of fragmentation from 1987 to 2020. These changes have significant implications for the ecosystem services, e.g., supporting migratory waterbirds, supported by these wetlands.

River metabolic fingerprints and regimes reveal ecosystem responses to enhanced wastewater treatment.

Although many studies have examined how improvements in wastewater treatment impact river nutrient concentrations and loads, there has been much less focus on measuring river metabolism to evaluate the wider aquatic ecosystem benefits of reducing nutrient inputs to rivers. The objectives of this study were to evaluate the effects of enhanced wastewater treatment (nitrification) on river metabolism in the Grand River, Canada's largest river draining into Lake Erie. Metabolic fingerprints and regimes (calculated from high-frequency dissolved oxygen [DO] measurements) were used to visualize whole-river ecosystem functional responses to these wastewater treatment upgrades. There was a 60% reduction in ecosystem respiration during summer, in response to reductions in effluent total ammonia inputs, causing a shift from net heterotrophy to net autotrophy, and contraction of river metabolic fingerprints. This resulted in major improvements in summer DO concentrations, with reductions in the percentage of days during summer that DO minima fell below water-quality guidelines for protection of aquatic early life stages, from 88% to ≤16%. The results also point to potential cascading impacts on coupled phosphorus and nitrogen cycles, which may generate further improvements in river water quality. During the summer, high rates of river metabolism and nutrient retention may result in measured water-column nutrient concentrations potentially underestimating nutrient pressures. This study also demonstrates the value of combining river metabolism with nutrient monitoring for a more holistic understanding of the role of nutrients in river ecosystem health and function.

Plant community re-organization and increased productivity due to multi-year nutrient enrichment of a coastal grassland.

Nutrient enrichment alters plant community structure and function at a global scale. Coastal plant systems are expected to experience increased rates of nitrogen and phosphorus deposition by 2100, caused mostly by anthropogenic activity. Despite high density of studies investigating connections between plant community structure and ecosystem function in response to nutrient addition, inconsistencies in system response based on the ecosystem in question calls for more detailed analyses of nutrient impacts on community organization and resulting productivity response. Here, we focus on nutrient addition impacts on community structure and organization as well as productivity of different lifeforms in a coastal grassland. We established long-term nutrient enrichment plots in 2015 consisting of control (C), nitrogen (N), phosphorus (P), and nitrogen + phosphorus (NP) treatments. In 2017 we collected graminoid and forb productivity, root productivity, and community composition for each plot. We found no N x P interaction, but N enrichment was a significant main effect on productivity, highlighting N limitation in coastal systems. Importantly, nutrient enrichment treatments did not alter root productivity. However, all treatments caused significant differences in community composition. Using rank abundance curves, we determined that community composition differences were driven by increased dominance of nitrophilous graminoids, re-organization of subordinate species, and species absences in N and NP plots. Results of this study highlight how coastal grassland communities are impacted by nutrient enrichment. We show that community re-organization, increased dominance, and absence of critical species are all important mechanisms that reflect community-level impacts of nutrient enrichment in our coastal grassland site.

Seasonal Dynamics in Carbon Cycling of Marine Bacterioplankton Are Lifestyle Dependent.

Although free-living (FL) and particle-attached (PA) bacteria are recognized as ecologically distinct compartments of marine microbial food-webs, few, if any, studies have determined their dynamics in abundance, function (production, respiration and substrate utilization) and taxonomy over a yearly cycle. In the Baltic Sea, abundance and production of PA bacteria (defined as the size-fraction >3.0 μm) peaked over 3 months in summer (6 months for FL bacteria), largely coinciding with blooms of Chitinophagales (Bacteroidetes). Pronounced changes in the growth efficiency (range 0.05–0.27) of FL bacteria (defined as the size-fraction <3.0 μm) indicated the magnitude of seasonal variability of ecological settings bacteria experience. Accordingly, 16S rRNA gene analyses of bacterial community composition uncovered distinct correlations between taxa, environmental variables and metabolisms, including Firmicutes associated with elevated hydrolytic enzyme activity in winter and Verrucomicrobia with utilization of algal-derived substrates during summer. Further, our results suggested a substrate-controlled succession in the PA fraction, from Bacteroidetes using polymers to Actinobacteria and Betaproteobacteria using monomers across the spring to autumn phytoplankton bloom transition. Collectively, our findings emphasize pronounced seasonal changes in both the composition of the bacterial community in the PA and FL size-fractions and their contribution to organic matter utilization and carbon cycling. This is important for interpreting microbial ecosystem function-responses to natural and human-induced environmental changes.

Ecosystem function responses to nutrient enrichment mediated by mud content in soft sediment habitats

ABSTRACT Estuaries are threatened with increasing sedimentation and nutrients, however research is limited on the interaction of these stressors on ecosystem functions. A field experiment adding nitrogen (N) fertiliser at high (600 g N m−2) and medium (150 g N m−2) concentrations and control plots was established in two sandy and two muddy sites (mud content 0–15%) to investigate context dependency of nitrogen enrichment on nutrient cycling and primary production. There was a significant interaction of mud x treatment on ammonium (NH4 +) fluxes. High N-addition plots had 133x greater dark NH4 + efflux than controls in muddy sediments, compared to 17x greater dark NH4 + efflux in sandy sediments. Net primary production increased with mud content, while photosynthetic efficiency of the microphytobenthos (GPPchl-a ) decreased above 4% mud. Mud content also affected the bivalve, Austrovenus stutchburyi (>30 mm), macrofaunal abundance and functional groups, and their influence on nutrient cycling. Thus minor increases in mud can alter the ecosystem response to nutrient enrichment directly through changes in sediment properties and indirectly by affecting the bivalves and macrofauna that influence these processes. This study highlights how multiple stressors interact to predispose estuaries to tipping points.

Do experimental pH increases alter the structure and function of a lowland tropical stream?

AbstractDisturbances can alter the structure and function of ecosystems. In stream ecosystems, changes in discharge and physicochemistry at short, intermediate, and long recurrence intervals can affect food webs and ecosystem processes. In this paper, we compare pH regimes in streams at La Selva Biological Station, Costa Rica, where episodic acidification frequency across the stream network varies widely due to buffering from inputs of bicarbonate‐rich interbasin groundwater. To examine the effects of acidification on ecosystem structure and function, we experimentally increased the buffering capacity of a headwater stream reach and compared it to an unbuffered upstream reach. We compared these reaches to a naturally buffered and unbuffered reaches of a second headwater stream. We quantified ecosystem structural (macroinvertebrate assemblages on leaf litter and coarse woody debris) and functional responses (leaf litter and coarse woody debris decomposition rates, and growth rates of a focal insect taxon [Diptera: Chironomidae]). Non‐metric multidimensional scaling and analysis of similarity revealed that macroinvertebrate assemblages were relatively homogenous across the four study reaches, although the naturally buffered reach was the most dissimilar. Ecosystem function, as measured by chironomid growth rates, was greater in the naturally buffered reach, while decomposition rates did not differ across the four reaches. Our results indicate that biological assemblages are adapted to pH regimes of frequently acidified stream reaches. Our experiment informs the effects on structure and function at short time scales in streams that experience moderate acidification, but larger magnitude acidification events in response to hydroclimatic change, as projected under climate change scenarios, may induce stronger responses in streams.

Price Equations for Understanding the Response of Ecosystem Function to Community Change

AbstractThe relationship between biodiversity and ecosystem function (BEF) remains unclear in many natural ecosystems, partially for lack of theoretical and analytical tools that match common characteristics of observational community data. The ecological Price equation promises to meet this need by organizing many different species-level changes into a few ecologically meaningful categories that sum to total ecosystem function change. Current versions of the ecological Price equation focus on species richness and presence-absence. However, abundance and relative abundance are better estimated in samples and are likely showing a stronger response to global change. Here, we present a novel, abundance-based version of the ecological Price equation in both discrete and continuous forms and explain the similarities and differences between this method and a related, previously developed richness-based method. We also present new empirical techniques for applying the Price equation to ecological data. Our two demonstration analyses reveal how additive effects of increasing abundance on total function are modified by concurrent selection effects due to shifts in species' composition as well as intraspecific change in species' per capita function. The ecological Price equations derived here complement existing approaches and together offer BEF researchers analytical tools and a unifying framework for studying BEF in observational community data.

Time after time: detecting annual patterns in stream bacterial biofilm communities.

SummaryTo quantify the major environmental drivers of stream bacterial population dynamics, we modelled temporal differences in stream bacterial communities to quantify community shifts, including those relating to cyclical seasonal variation and more sporadic bloom events. We applied Illumina MiSeq 16S rRNA bacterial gene sequencing of 892 stream biofilm samples, collected monthly for 36‐months from six streams. The streams were located a maximum of 118 km apart and drained three different catchment types (forest, urban and rural land uses). We identified repeatable seasonal patterns among bacterial taxa, allowing their separation into three ecological groupings, those following linear, bloom/trough and repeated, seasonal trends. Various physicochemical parameters (light, water and air temperature, pH, dissolved oxygen, nutrients) were linked to temporal community changes. Our models indicate that bloom events and seasonal episodes modify biofilm bacterial populations, suggesting that distinct microbial taxa thrive during these events including non‐cyanobacterial community members. These models could aid in determining how temporal environmental changes affect community assembly and guide the selection of appropriate statistical models to capture future community responses to environmental change.