Metacommunity theory meets restoration: isolation may mediate how ecological communities respond to stream restoration

Long-term climate and hydrologic regimes shape stream invertebrate community responses to a hurricane disturbance

Disturbances can produce a spectrum of short- and long-term ecological consequences that depend on complex interactions of the characteristics of the event, antecedent environmental conditions, and the intrinsic properties of resistance and resilience of the affected biological system. We used Hurricane Harvey's impact on coastal rivers of Texas to examine the roles of storm-related changes in hydrology and long-term precipitation regime on the response of stream invertebrate communities to hurricane disturbance. We detected declines in richness, diversity and total abundance following the storm, but responses were strongly tied to direct and indirect effects of long-term aridity and short-term changes in stream hydrology. The amount of rainfall a site received drove both flood duration and flood magnitude across sites, but lower annual rainfall amounts (i.e. aridity) increased flood magnitude and decreased flood duration. Across all sites, flood duration was positively related to the time it took for invertebrate communities to return to a long-term baseline and flood magnitude drove larger invertebrate community responses (i.e. changes in diversity and total abundance). However, invertebrate response per unit flood magnitude was lower in sub-humid sites, potentially because of differences in refuge availability or ecological-evolutionary interactions. Interestingly, sub-humid streams had temporary large peaks in invertebrate total abundance and diversity following recovery period that may be indicative of the larger organic matter pulses expected in these systems because of their comparatively well-developed riparian vegetation. Our findings show that hydrology and long-term precipitation regime predictably affected invertebrate community responses and, thus, our work underscores the important influence of local climate to ecosystem sensitivity to disturbances.

Impacts of habitat connectivity on grassland arthropod metacommunity structure: A field-based experimental test of theory

Metacommunity theory has advanced scientific understanding of how species interactions and spatial processes influence patterns of biodiversity and community structure across landscapes. While the central tenets of metacommunity theory have been promoted as pivotal considerations for conservation management, few field experiments have tested the validity of metacommunity predictions. Here, we tested one key prediction of metacommunity theory-that decreasing habitat connectivity should erode metacommunity structure by hindering species movement between patches. For 2 years, we manipulated an experimental old-field grassland ecosystem via mowing to represent four levels of habitat connectivity: (1) open control, (2) full connectivity, (3) partial connectivity, and (4) no connectivity. Within each treatment plot (10 × 10 m, n = 4 replicates), we measured the abundance and diversity (i.e., alpha and beta) of both flying and ground arthropods using sticky and pitfall traps, respectively. We found that the abundance and diversity of highly mobile flying arthropods were unaffected by habitat connectivity, whereas less mobile ground arthropods were highly impacted. The mean total abundance of ground arthropods was 2.5× and 2× higher in the control and partially connected plots compared to isolated patches, respectively. We also reveal that habitat connectivity affected the trophic interactions of ground arthropods, with predators (e.g., wolf spiders, ground spiders) being highly positively correlated with micro-detritivores (springtails, mites) but not macro-detritivores (millipedes, isopods) as habitat connectivity increased. Together these findings indicate that changes in habitat connectivity can alter the metacommunity structure for less mobile organisms such as ground arthropods. Because of their essential roles in terrestrial ecosystem functioning and services, we recommend that conservationists, restoration practitioners, and land managers include principles of habitat connectivity for ground arthropods when designing biodiversity management programs.

Has riparian woody vegetation a positive effect on dispersal and distribution of mayfly, stonefly and caddisfly species?

During their adult life stage most EPTs (Ephemeroptera, Plecoptera and Trichoptera) disperse by flying following the riparian corridor. Although it is likely that riparian forest fosters EPT dispersion, this has not been empirically tested in a larger dataset yet and several additional open questions remain. First, it is unclear if the effect of riparian vegetation on EPT community differs and depends on the spatial scale. Second, it is not assessed how the effect of riparian vegetation on EPTs is and how it changes depending on other environmental stressors. Third, the effect potentially depends on riparian vegetation characteristics such as trees species composition and cover. We analysed 98 sites in lowland and lower mountain streams in Northrhine Westfalia, Germany, at two longitudinal and two lateral spatial scales. At each site we calculated the EPT community dispersal ability and quantified other environmental stressors as well as deciduous and coniferous woody cover in the riparian buffer. Generalised Linear Models were used to identify the conditions under which woody riparian vegetation has a significant effect on EPT community dispersal ability. Our results confirmed that the share of weak dispersers increased with deciduous woody riparian cover in low mountain streams, indicating a potential positive effect of natural riparian forest on landscape connectivity. This relationship was only observed at the regional longitudinal scale irrespective of the lateral spatial scale. Tree species composition was relevant as coniferous forests did not contribute to this effect. Finally, there was some indication that the positive effect of deciduous riparian forest occurs at a moderate woody cover and levels off at higher values. This highlights the role of riparian forests not only as habitat but also dispersal corridor in river management and the need to preserve and restore natural woody riparian vegetation to improve EPT communities and macroinvertebrates ecological status.

Ecological drivers of macroinvertebrate metacommunity assembly in a subtropical river basin in the Yangtze River Delta, China

Identifying the underlying ecological drivers of macroinvertebrate community assembly is fundamental to metacommunity ecology. Comparably, determining the influence of different drivers on beta diversity patterns can provide insight into processes governing community organization. Exploring the ecological drivers of metacommunity and beta diversity are major avenues to improve bioassessment, restoration, and river management, which are still poorly explored in China, especially in subtropical highly developed river networks. To address this gap, we use a dataset (macroinvertebrate communities and environmental variables) collected from the Yangtze River Delta, China to test the above ideas. We used the K-means clustering method to divide 405 river sites into three anthropogenic impacted groups, nearly pristine sites, moderately impacted sites, and heavily impacted sites, and subsequently used partial Mantel tests to investigate how species sorting and dispersal shaped the metacommunity that varied with the levels of anthropogenic impacts and to explore the responses of different components of beta diversity to environmental and spatial distances among sites for each group. Our results revealed that both species sorting and dispersal shape communities, but the importance of species sorting and dispersal varied with the levels of anthropogenic impacts. Nearly pristine sites were mostly shaped only by species sorting, while heavily impacted sites were shaped by dispersal. We also found that turnover was by far the dominant component of beta diversity across all levels of impact. Therefore, we encourage that environmental variables and spatial processes should be considered in bioassessment approaches. In addition, it is essential to focus on maintaining habitat heterogeneity and identifying and protecting regional species pools that could improve local biodiversity through dispersal for ecosystem management of the Yangtze River Delta of China.

From meta-system theory to the sustainable management of rivers in the Anthropocene

Regional-scale ecological processes, such as the spatial flows of material, energy, and organisms, are fundamental for maintaining biodiversity and ecosystem functioning in river networks. Yet these processes remain largely overlooked in most river management practices and underlying policies. Here, we propose adoption of a meta-system approach, where regional processes acting at different levels of ecological organization - populations, communities, and ecosystems - are integrated into conventional river conservation, restoration, and biomonitoring. We also describe a series of measurements and indicators that could be assimilated into the implementation of relevant biodiversity and environmental policies. Finally, we highlight the need for alternative management strategies that can guide practitioners toward applying recent advances in ecology to preserve and restore river ecosystems and the ecosystem services they provide, in the context of increasing alteration of river network connectivity worldwide.

Defining Recovery Potential in River Restoration: A Biological Data-Driven Approach

Scientists and practitioners working on river restoration have made progress on understanding the recovery potential of rivers from geomorphological and engineering perspectives. We now need to build on this work to gain a better understanding of the biological processes involved in river restoration. Environmental policy agendas are focusing on nature recovery, reigniting debates about the use of “natural” reference conditions as benchmarks for ecosystem restoration. We argue that the search for natural or semi-natural analogues to guide restoration planning is inappropriate due to the absence of contemporary reference conditions. With a catchment-scale case study on the invertebrate communities of the Warwickshire Avon, a fifth-order river system in England, we demonstrate an alternative to the reference condition approach. Under our model, recovery potential is quantified based on the gap between observed biodiversity at a site and the biodiversity predicted to occur in that location under alternative management scenarios. We predict that commonly applied restoration measures such as reduced nutrient inputs and the removal of channel resectioning could be detrimental to invertebrate diversity, if applied indiscriminately and without other complementary measures. Instead, our results suggest considerable potential for increases in biodiversity when restoration measures are combined in a way that maximises biodiversity within each water body.

The application of metacommunity theory to the management of riverine ecosystems

River managers strive to use the best available science to sustain biodiversity and ecosystem function. To achieve this goal requires consideration of processes at different scales. Metacommunity theory describes how multiple species from different communities potentially interact with local-scale environmental drivers to influence population dynamics and community structure. However, this body of knowledge has only rarely been used to inform management practices for river ecosystems. In this paper, we present a conceptual model outlining how the metacommunity processes of local niche sorting and dispersal can influence the outcomes of management interventions and provide a series of specific recommendations for applying these ideas as well as research needs. In all cases, we identify situations where traditional approaches to riverine management could be enhanced by incorporating an understanding of metacommunity dynamics. A common theme is developing guidelines for assessing the metacommunity context of a site or region, evaluating how that context may affect the desired outcome, and incorporating that understanding into the planning process and methods used. To maximize the effectiveness of management activities, scientists and resource managers should update the toolbox of approaches to riverine management to reflect theoretical advances in metacommunity ecology.

Rethinking Biological Invasions as a Metacommunity Problem

Perhaps more than any other ecological discipline, invasion biology has married the practices of basic science and the application of that science. The conceptual frameworks of population regulation, metapopulations, supply-side ecology, and community assembly have all to some degree informed the regulation, management, and prevention of biological invasions. Invasion biology needs to continue to adopt emerging frameworks and paradigms to progress as both a basic and applied science. This need is urgent as the biological invasion problem continues to worsen. The development of metacommunity theory in the last two decades represents a paradigm-shifting approach to community ecology that emphasizes the multi-scale nature of community assembly and biodiversity regulation. Work on metacommunities has demonstrated that even relatively simple processes at local scales are often heavily influenced by regional-scale processes driven primarily by the dispersal of organisms. Often the influence of dispersal interacts with, or even swamps, the influence of local-scale drivers like environmental conditions and species interactions. An emphasis on dispersal and a focus on multi-scale processes enable metacommunity theory to contribute strongly to the advancement of invasion biology. Propagule pressure of invaders has been identified as one of the most important drivers facilitating invasion, so the metacommunity concept, designed to address how dispersal-driven dynamics affect community structure, can directly address many of the central questions of invasion biology. Here we revisit many of the important concepts and paradigms of biological invasions—propagule pressure, biotic resistance, enemy release, functional traits, neonative species, human-assisted transport,—and view those concepts through the lens of metacommunity theory. In doing so, we accomplish several goals. First, we show that work on metacommunities has generated multiple predictions, models, and the tools that can be directly applied to invasion scenarios. Among these predictions is that invasibility of a community should decrease with both local controls on community assembly, and the dispersal rates of native species. Second, we demonstrate that framing biological invasions in metacommunity terms actually unifies several seemingly disparate concepts central to invasion biology. Finally, we recommend several courses of action for the control and management of invasive species that emerge from applying the concepts of metacommunity theory.

DISPERSE, a trait database to assess the dispersal potential of European aquatic macroinvertebrates

Dispersal is an essential process in population and community dynamics, but is difficult to measure in the field. In freshwater ecosystems, information on biological traits related to organisms' morphology, life history and behaviour provides useful dispersal proxies, but information remains scattered or unpublished for many taxa. We compiled information on multiple dispersal-related biological traits of European aquatic macroinvertebrates in a unique resource, the DISPERSE database. DISPERSE includes nine dispersal-related traits subdivided into 39 trait categories for 480 taxa, including Annelida, Mollusca, Platyhelminthes, and Arthropoda such as Crustacea and Insecta, generally at the genus level. Information within DISPERSE can be used to address fundamental research questions in metapopulation ecology, metacommunity ecology, macroecology and evolutionary ecology. Information on dispersal proxies can be applied to improve predictions of ecological responses to global change, and to inform improvements to biomonitoring, conservation and management strategies. The diverse sources used in DISPERSE complement existing trait databases by providing new information on dispersal traits, most of which would not otherwise be accessible to the scientific community.

Consideration of spatial and temporal scales in stream restorations and biotic monitoring to assess restoration outcomes: A literature review, Part 2

Stream and river restoration practices have become common in many parts of the world. To answer the question whether such restoration measures improve freshwater biotic assemblages or functions over time, and if not, can general reasons be identified for such outcomes, we conducted a literature survey and review of studies in which different types of stream restorations were conducted and outcomes assessed. In the first paper, we reviewed studies of culvert restorations, acid mine drainage or industrial pollution restoration; and urban stream restoration projects. Here, we review studies of restoration via dam removal, changes in dam operation or fish passage structures; instream habitat modification; riparian restoration or woody material addition; channel restoration and multiple restoration measures and develop some general conclusions from these reviews. Biomonitoring in different studies detected improvements for some restoration measures; other studies found minimal or no statistically significant increases in biotic assemblage richness, abundances or functions. In some cases, untreated stressors may have influenced the outcomes of the restoration, but in many cases, there were mismatches in the temporal or spatial scale of the restoration measure undertaken and associated monitoring. For example, either biomonitoring to measure restoration effects was conducted over a too short a time period after restoration for effects to be observed, or the sources and stressors needing remediation occurred at a larger catchment scale than the restoration. Also, many restoration measures lack observations from unimpaired reference sites for use in predicting how much of a beneficial effect might be expected.

Facets and scales in river restoration: Nestedness and interdependence of hydrological, geomorphic, ecological, and biogeochemical processes

Although river restoration has increased rapidly, observations of successful ecological recovery are rare, mostly due to a discrepancy in the spatial scale of the impact and the restoration. Rivers and their ecological communities are a product of four river facets-hydrology, geomorphology, ecology and biogeochemistry-that act and interact on several spatial scales, from the sub-reach to the reach and catchment scales. The four river facets usually affect one another in predictable pathways (e.g., hydrology commonly controls geomorphology), but we show that the order in which they affect each other and can be restored varies depending on ecoregion and hydroclimatic regime. Similarly, processes at different spatial scales can be nested or independent of those at larger scales. Although some restoration practices are dependent of those at higher scales, other reach-scale restoration efforts are independent and can be carried out prior to or concurrently with larger-scale restoration. We introduce a checklist using the four river facets to prioritize restoration at three spatial scales in order to have the largest positive effect on the entire catchment. We apply this checklist to two contrasting regions-in northern Sweden and in southern Brazil-with different anthropogenic effects and interactions between facets and scales. In the case of nested processes that are dependent on larger spatial scales, reach-scale restoration in the absence of restoration of catchment-scale processes can frankly be a waste of money, providing little ecological return. However, depending on the scale-interdependence of processes of the river facets, restoration at smaller scales may be sufficient. This means that the most appropriate government agency should be assigned (i.e., national vs. county) to most effectively oversee river restoration at the appropriate scale; however, this first requires a catchment-scale analysis of feedbacks between facets and spatial scale interdependence.

Ecosystem responses to channel restoration decline with stream size in urban river networks

Urban streams are often severely impaired due to channelization, high loads of nutrients and contaminants, and altered land cover in the watershed. Physical restoration of stream channels is widely used to offset the effects of urbanization on streams, with the goal of improving ecosystem structure and function. However, these efforts are rarely guided by strategic analysis of the factors that mediate the responsiveness of stream ecosystems to restoration. Given that ecological gradients from headwater streams to mainstem rivers are ubiquitous, we posited that location within a river network could mediate the benefits of channel restoration. We studied existing stream restorations in Milwaukee, Wisconsin, to determine (1) whether restorations improve ecosystem function (e.g., nutrient uptake, whole-stream metabolism) and (2) how ecosystem responses vary by position in the urban river network. We quantified a suite of physicochemical and biological metrics in six pairs of contiguous restored and concrete channel reaches, spanning gradients in baseflow discharge (19-196 L/s) and river network position (i.e., headwater to mainstem). Hydrology differed dramatically between the restored and concrete reaches; water velocity was reduced 2- to 13-fold while water residence time was 50-5,000% greater in adjacent restored reaches. Restored reaches had shorter nutrient uptake lengths for ammonium, nitrate, and phosphate, as well as higher whole-stream metabolism. Furthermore, the majority of reaches were autotrophic (i.e., gross primary production > ecosystem respiration), which is not common in stream ecosystems. The difference in ecosystem functioning between restored and unrestored reaches was generally largest in headwaters and declined to equivalence in mainstem restorations. Our results suggest that headwater sites offer higher return on investment compared to larger downstream channels, where ecosystem responsiveness is low. If this pattern proves to be general, the scaling of ecosystem responses with river size could be integrated into planning guidelines for urban stream restorations to enhance the societal and ecological benefits of these expensive interventions.

Community size can affect the signals of ecological drift and niche selection on biodiversity

Ecological drift can override the effects of deterministic niche selection on small populations and drive the assembly of some ecological communities. We tested this hypothesis with a unique data set sampled identically in 200 streams in two regions (tropical Brazil and boreal Finland) that differ in macroinvertebrate community size by fivefold. Null models allowed us to estimate the magnitude to which β-diversity deviates from the expectation under a random assembly process while taking differences in richness and relative abundance into account, i.e., β-deviation. We found that both abundance- and incidence-based β-diversity was negatively related to community size only in Brazil. Also, β-diversity of small tropical communities was closer to stochastic expectations compared with β-diversity of large communities. We suggest that ecological drift may drive variation in some small communities by changing the expected outcome of niche selection, increasing the chances of species with low abundance and narrow distribution to occur in some communities. Habitat destruction, overexploitation, pollution, and reductions in connectivity have been reducing the size of biological communities. These environmental pressures might make smaller communities more vulnerable to novel conditions and render community dynamics more unpredictable. Incorporation of community size into ecological models should provide conceptual and applied insights into a better understanding of the processes driving biodiversity.

A Metacommunity Approach to Improve Biological Assessments in Highly Dynamic Freshwater Ecosystems

Rapid shifts in biotic communities due to environmental variability challenge the detection of anthropogenic impacts by current biomonitoring programs. Metacommunity ecology has the potential to inform such programs, because it combines dispersal processes with niche-based approaches and recognizes variability in community composition. Using intermittent rivers-prevalent and highly dynamic ecosystems that sometimes dry-we develop a conceptual model to illustrate how dispersal limitation and flow intermittence influence the performance of biological indices. We produce a methodological framework integrating physical- and organismal-based dispersal measurements into predictive modeling, to inform development of dynamic ecological quality assessments. Such metacommunity-based approaches could be extended to other ecosystems and are required to underpin our capacity to monitor and protect ecosystems threatened under future environmental changes.

River restoration is prone to failure unless pre-optimized within a mechanistic ecological framework | Insights from a model-based case study

River restoration with the use of in-stream structures has been widely implemented to maintain/improve physical habitats. However, the response of aquatic biota has often been too weak to justify the high costs of restoration projects. The ecological effectiveness of river restoration has thus been much debated over claims that large-scale environmental drivers often overshadow the potential positive ecological effects of locally placed in-stream structures. In this study, we used a two-dimensional hydrodynamic-habitat model to evaluate the ecological effectiveness of habitat restoration with the use of in-stream structures in various water discharges, ranging from near-dry to environmental flows. The habitat suitability of benthic macroinvertebrates and of three cyprinid fish species was simulated for six restoration schemes and at four discharge scenarios, and was compared with a reference model, without in-stream structures. We found that the ecological response to habitat restoration varied by species and life stages, it strongly depended on the reach-scale flow conditions, it was often negative at near-environmental flows, and when positive, mostly at near-dry flows, it was too low to justify the high costs of river restoration. Flow variation was the major environmental driver that our local habitat restoration schemes attempted -but mostly failed-to fine-tune. We conclude that traditional river restoration, based on trial and error, will likely fail and should be ecologically pre-optimized before field implementation. Widespread use of in-stream structures for ecological restoration is not recommended. However, at near-dry flows, the response of all biotic elements except for macroinvertebrates, was positive. In combination with the small habitat-suitability differences observed among structure types and densities, we suggest that sparse/moderate in-stream structure placement can be used for cost-effective river restoration, but it will only be ecologically effective -thus justifying the high implementation costs-when linked to very specific purposes: (i) to conserve endangered species and (ii) to increase/improve habitat availability/suitability during dry periods, thus proactively preventing/reducing the current and future ecological impacts of climate change.

Linkages between flow regime, biota, and ecosystem processes: Implications for river restoration

River ecosystems are highly biodiverse, influence global biogeochemical cycles, and provide valued services. However, humans are increasingly degrading fluvial ecosystems by altering their streamflows. Effective river restoration requires advancing our mechanistic understanding of how flow regimes affect biota and ecosystem processes. Here, we review emerging advances in hydroecology relevant to this goal. Spatiotemporal variation in flow exerts direct and indirect control on the composition, structure, and dynamics of communities at local to regional scales. Streamflows also influence ecosystem processes, such as nutrient uptake and transformation, organic matter processing, and ecosystem metabolism. We are deepening our understanding of how biological processes, not just static patterns, affect and are affected by stream ecosystem processes. However, research on this nexus of flow-biota-ecosystem processes is at an early stage. We illustrate this frontier with evidence from highly altered regulated rivers and urban streams. We also identify research challenges that should be prioritized to advance process-based river restoration.

Biological Diversity in Headwater Streams

Headwaters, the sources of all stream networks, provide habitats that are unique from other freshwater environments and are used by a specialised subset of aquatic species. The features of headwaters that provide special habitats include predator-free or competitor-free spaces; specific resources (particularly detrital based); and moderate variations in flows, temperature and discharge. Headwaters provide key habitats for all or some life stages for a large number of species across just about all freshwater phyla and divisions. Some features of headwaters, including isolation and small population sizes, have allowed for the evolutionary radiation of many groups of organisms within and beyond those habitats. As small and easily engineered physical spaces, headwaters are easily degraded by streambank development, ditching and even burial. Headwater streams are among the most sensitive of freshwater ecosystems due to their intimate linkage with their catchments and how easily they are impacted. As a unique ecosystem with many specialist species, headwater streams deserve better stewardship.

Combined effects of local habitat, anthropogenic stress, and dispersal on stream ecosystems: a mesocosm experiment

The effects of anthropogenic stressors on community structure and ecosystem functioning can be strongly influenced by local habitat structure and dispersal from source communities. Catchment land uses increase the input of fine sediments into stream channels, clogging the interstitial spaces of benthic habitats. Aquatic macrophytes enhance habitat heterogeneity and mediate important ecosystem functions, being thus a key component of habitat structure in many streams. Therefore, the recovery of macrophytes following in-stream habitat modification may be prerequisite for successful stream restoration. Restoration success is also affected by dispersal of organisms from the source community, with potentially the strongest responses in relatively isolated headwater sites that receive a limited amount of dispersing individuals. We used a factorial design in a set of stream mesocosms to study the independent and combined effects of an anthropogenic stressor (sand sedimentation), local habitat (macrophytes, i.e., moss transplants), and enhanced dispersal (two levels: high vs. low) on organic matter retention, algal accrual rate, leaf decomposition, and macroinvertebrate community structure. Overall, all responses were simple additive effects with no interactions between treatments. Sand reduced algal accumulation, total invertebrate density, and density of a few individual taxa. Mosses reduced algal accrual rate and algae-grazing invertebrates, but enhanced organic matter retention and the number of detritus and filter feeders. Mosses also reduced macroinvertebrate diversity by increasing the dominance by a few taxa. Mosses reduced leaf mass loss, possibly because the organic matter retained by mosses provided an additional food source for leaf-shredding invertebrates and thus reduced shredder aggregation into leaf packs. The effect of mosses on macroinvertebrate communities and ecosystem functioning was distinct irrespective of the level of dispersal, suggesting strong environmental control of community structure. The strong environmental control of macroinvertebrate community composition even under enhanced dispersal suggests that re-establishing key habitat features, such as natural stream vegetation, could aid ecosystem recovery in boreal streams.

Evaluating riparian solutions to multiple stressor problems in river ecosystems - A conceptual study

Rivers are among the most sensitive of all ecosystems to the effects of global change, but options to prevent, mitigate or restore ecosystem damage are still inadequately understood. Riparian buffers are widely advocated as a cost-effective option to manage impacts, but empirical evidence is yet to identify ideal riparian features (e.g. width, length and density) which enhance ecological integrity and protect ecosystem services in the face of catchment-scale stressors. Here, we use an extensive literature review to synthesise evidence on riparian buffer and catchment management effects on instream environmental conditions (e.g. nutrients, fine sediments, organic matter), river organisms and ecosystem functions. We offer a conceptual model of the mechanisms through which catchment or riparian management might impact streams either positively or negatively. The model distinguishes scale-independent benefits (shade, thermal damping, organic matter and large wood inputs) that arise from riparian buffer management at any scale from scale-dependent benefits (nutrient or fine sediment retention) that reflect stressor conditions at broader (sub-catchment to catchment) scales. The latter require concerted management efforts over equally large domains of scale (e.g. riparian buffers combined with nutrient restrictions). The evidence of the relationships between riparian configuration (width, length, zonation, density) and scale-independent benefits is consistent, suggesting a high certainty of the effects. In contrast, scale-dependent effects as well as the biological responses to riparian management are more uncertain, suggesting that ongoing diffuse pollution (nutrients, sediments), but also sources of variability (e.g. hydrology, climate) at broader scales may interfere with the effects of local riparian management. Without concerted management across relevant scales, full biological recovery of damaged lotic ecosystems is unlikely. There is, nevertheless, sufficient evidence that the benefits of riparian buffers outweigh potential adverse effects, in particular if located in the upstream part of the stream network. This supports the use of riparian restoration as a no-regrets management option to improve and sustain lotic ecosystem functioning and biodiversity.

Do latitudinal gradients exist in New Zealand stream invertebrate metacommunities?

That biodiversity declines with latitude is well known, but whether a metacommunity process is behind this gradient has received limited attention. We tested the hypothesis that dispersal limitation is progressively replaced by mass effects with increasing latitude, along with a series of related hypotheses. We explored these hypotheses by examining metacommunity structure in stream invertebrate metacommunities spanning the length of New Zealand’s two largest islands (∼1,300 km), further disentangling the role of dispersal by deconstructing assemblages into strong and weak dispersers. Given the highly dynamic nature of New Zealand streams, our alternative hypothesis was that these systems are so unpredictable (at different stages of post-flood succession) that metacommunity structure is highly context dependent from region to region. We rejected our primary hypotheses, pinning this lack of fit on the strong unpredictability of New Zealand’s dynamic stream ecosystems and fauna that has evolved to cope with these conditions. While local community structure turned over along this latitudinal gradient, metacommunity structure was highly context dependent and dispersal traits did not elucidate patterns. Moreover, the emergent metacommunity types exhibited no trends, nor did the important environmental variables. These results provide a cautionary tale for examining singular metacommunities. The considerable level of unexplained contingency suggests that any inferences drawn from one-off snapshot sampling may be misleading and further points to the need for more studies on temporal dynamics of metacommunity processes.