Environmental impact propagated by cross-system subsidy: chronic stream pollution controls riparian spider populations

Resource subsidies between habitats are common and create the potential for the propagation of environmental impacts across system boundaries. However, recent understanding of the potential for subsidy-mediated cross-system impact propagations is limited and primarily based on passive flows of nutrients and detritus or short-term effects. Here, we assess the effects of sustained alterations in aquatic insect emergence (active subsidy pathway), due to chronic stream pollution, for riparian spiders. The sustained reduction in aquatic insect densities at the polluted reaches resulted in a marked decline in web spider population density and a shift in spider community composition. Our results provide the first evidence that stream pollution can control populations and community structure of terrestrial predators via sustained alterations in aquatic subsidies, emphasizing the role of subtle trophic linkages in the transmission of environmental impacts across ecosystem boundaries.     

Environmental controls of patch-reef growth and development

The effects of predominant currents on the morphology of coral reefs has been studied in the leeward patch reefs of Anegada, British Virgin Islands. Measurements of current velocity and direction and sediment characteristics together with mapping of approximately 31 patch reefs within a 2 km² area suggest that even under relatively low but constant current velocities (surface current 20 cm sec^-1) the major reef-building corals show definite distribution patterns on the reef. The currents also appear to be responsible, in part, for marked orientation and elongation of patch reefs in a north-west/ south-east direction and for a gradation of patch-reef type across the study area.     

Consistent effects of nitrogen fertilization on soil bacterial communities in contrasting systems

Ecosystems worldwide are receiving increasing amounts of reactive nitrogen (N) through anthropogenic activities. Although the effects of increased N inputs on plant communities have been reasonably well studied, few comparable studies have examined impacts on whole soil bacterial communities, though they play critical roles in ecosystem functioning. We sampled soils from two long-term ecological research (LTER) experimental N gradients, both of which have been amended with NH4NO3; a grassland at Cedar Creek (27 years of N additions) and an agricultural field at Kellogg Biological Station (8 years of N additions). By examining shifts in bacterial communities across these contrasting ecosystem types, we could test competing hypotheses about the direct and indirect factors that might drive bacterial responses to elevated N inputs. Bacterial community structure was highly responsive to N additions. We observed predictable and consistent changes in the structure of the bacterial communities across both ecosystem types. Our results suggest that bacterial communities across these gradients are more structured by N and/or soil carbon availability than by shifts in the plant community or soil pH associated with the elevated nitrogen inputs. In contrast to the pronounced shifts in bacterial community composition and in direct contrast to the patterns often observed in plant communities, increases in N availability did not have consistent effects on the richness and diversity of soil bacterial communities.     

辽东湾大凌河口湿地土壤微生物群落分析

土壤微生物对湿地生态系统的物质转化和能量流动具有重要作用,目前对天然湿地的微生物资源及群落结构缺少研究。应用PCR-DGGE技术研究辽东湾大凌河口湿地土壤中的微生物群落结构,并对其群落组成进行分析,结合冗余分析（RDA）方法考察环境因子对微生物群落结构的影响。共设定7个站位采集土壤样品,直接提取样品中微生物的总DNA,以巢式PCR扩增细菌16S rDNA-V3区片段,应用变性梯度凝胶电泳（DGGE）技术分离片段,研究河口区湿地土壤细菌的群落结构。结果表明：辽东湾大凌河口区土壤具有较为丰富的细菌种群,主要包含Proteobacteria、Actinobacteria和Bacteroidetes几大类门,且同一季节采集的样品在聚类分析图上聚类成簇。RDA分析表明,土壤的温度与盐度2个变量分别解释了6.5%、2.3%的土壤细菌群落结构变化,表明大凌河口区土壤的群落结构变化是多种环境因子共同作用的结果,其中温度的影响略大于盐度。     

Twenty years of stream restoration in Finland: little response by benthic macroinvertebrate communities

The primary focus of many in-stream restoration projects is to enhance habitat diversity for salmonid fishes, yet the lack of properly designed monitoring studies, particularly ones with pre-restoration data, limits any attempts to assess whether restoration has succeeded in improving salmonid habitat. Even less is known about the impacts of fisheries-related restoration on other, non-target biota. We examined how restoration aiming at the enhancement of juvenile brown trout (Salmo trutta L.) affects benthic macroinvertebrates, using two separate data sets: (1) a before-after-control-impact (BACI) design with three years before and three after restoration in differently restored and control reaches of six streams; and (2) a space-time substitution design including channelized, restored, and near-natural streams with an almost 20-year perspective on the recovery of invertebrate communities. In the BACI design, total macroinvertebrate density differed significantly from before to after restoration. Following restoration, densities decreased in all treatments, but less so in the controls than in restored sections. Taxonomic richness also decreased from before to after restoration, but this happened similarly in all treatments. In the long-term comparative study, macroinvertebrate species richness showed no difference between the channel types. Community composition differed significantly between the restored and natural streams, but not between restored and channelized streams. Overall, the in-stream restoration measures used increased stream habitat diversity but did not enhance benthic biodiversity. While many macroinvertebrates may be dispersal limited, our study sites should not have been too distant to reach within almost two decades. A key explanation for the weak responses by macroinvertebrate communities may have been historical. When Fennoscandian streams were channelized for log floating, the loss of habitat heterogeneity was only partial. Therefore, habitat may not have been limiting the macroinvertebrate communities to begin with. Stream restoration to support trout fisheries has strong public acceptance in Finland and will likely continue to increase in the near future. Therefore, more effort should be placed on assessing restoration success from a biodiversity perspective using multiple organism groups in both stream and riparian ecosystems.     

Impact of heavy metals on bacterial communities from mangrove soils of the Mahanadi Delta (India)

This study aimed to assess soil nutrient status and heavy metal content and their impact on the predominant soil bacterial communities of mangroves of the Mahanadi Delta. Mangrove soil of the Mahanadi Delta is slightly acidic and the levels of soil nutrients such as carbon, nitrogen, phosphorous and potash vary with season and site. The seasonal average concentrations (μg/g) of various heavy metals were in the range: 14 810–63 370 (Fe), 2.8–32.6 (Cu), 13.4–55.7 (Ni), 1.8–7.9 (Cd), 16.6–54.7 (Pb), 24.4–132.5 (Zn) and 13.3–48.2 (Co). Among the different heavy metals analysed, Co, Cu and Cd were above their permissible limits, as prescribed by Indian Standards (Co=17 μg/g, Cu=30 μ g/g, Cd=3–6 μ g/g), indicating pollution in the mangrove soil. A viable plate count revealed the presence of different groups of bacteria in the mangrove soil, i.e. heterotrophs, free-living N₂ fixers, nitrifyers, denitrifyers, phosphate solubilisers, cellulose degraders and sulfur oxidisers. Principal component analysis performed using multivariate statistical methods showed a positive relationship between soil nutrients and microbial load. Whereas metal content such as Cu, Co and Ni showed a negative impact on some of the studied soil bacteria.     

The cost of tolerance: sensitivity of stream benthic communities to UV-B and metals.

The ability to tolerate disturbance is a defense strategy that minimizes the effects of damage to fitness and is essential for sustainability of populations, communities, and ecosystems. Despite the apparent benefits of tolerance, there may be an associated cost that results in a deficiency of a system to respond to additional disturbances. Aquatic ecosystems are often exposed to a variety of natural and anthropogenic disturbances, and the effects of these compound perturbations are not well known. In this investigation, we examine whether tolerance to one stressor, metals, results in a cost of increased sensitivity to an additional stressor, ultraviolet-B (UV-B) radiation. Heavy metal pollution is recognized as a major environmental problem in Rocky Mountain streams. These high-elevation, typically clear streams may be at particular risk to elevated UV-B levels associated with reduced levels of ozone. Microcosm experiments were conducted using natural stream benthic communities collected from a reference site and a site with a long-term history of heavy-metal pollution. Direct and interactive effects of heavy metals and UV-B radiation on structural and functional characteristics of benthic communities were evaluated among four treatments: control, UV-B, metals, and metal and UV-B. Communities from the metal-polluted site were more tolerant of metals but less tolerant to UV-B compared to reference communities. Increased mayfly drift and reduced metabolism in response to metal exposure were observed in reference communities but not in the metal-polluted communities. In contrast to these results, UV-B radiation significantly reduced community metabolism, total macroinvertebrate abundance, and abundances of mayflies, caddisflies, and dipterans from the metal-polluted site, but had no effects on benthic communities from the reference site. ANOSIM results demonstrated that community responses differed among treatments at both sites. Metals had the largest impact on community differences at both sites, while UV-B had greater impacts at the metal-polluted site. This research demonstrates the need to account for potential costs associated with tolerance and that these costs can result in behavioral, structural, and functional impacts to benthic communities.     

The role of environmental and spatial factors for the composition of aquatic bacterial communities

This study investigates the importance of local vs. spatial factors on bacterial community composition of 35 rock pools at the Baltic Sea coast. The pools were located in five distinct spatial clusters over a total scale of <500 m and differed widely in terms of water chemistry. To determine the fractions of the variance in bacterial community composition (BCC) between rock pools that are explained by local environmental vs. spatial factors, a variance partitioning procedure using partial canonical correspondence analysis was performed. Three environmental variables (salinity, chlorophyll a concentration, and water color) had a significant effect on BCC, irrespective of the spatial location of the pools. Vice versa, there was a significant effect of spatial factors on BCC irrespective of any of the environmental factors included in this study. Hence, the patchy spatial distribution of the pools was partly reflected in the composition of the bacterial communities in the pools, which might be caused by congruent colonization events of adjacent pools, such as simultaneous seaspray inputs or direct exchange of bacteria via connecting rivulets. This study shows that the composition of planktonic bacteria can show provincialism at small spatial scales, which is likely to be caused by environmental conditions as well as historical events.     

Denaturing gradient gel electrophoresis profiling of bacterial communities composition in Arabian Sea

Denaturing gradient gel electrophoresis (DGGE) was used to elucidate spatial and temporal variations in bacterial community composition (BCC) from four locations along the central west coast of India. DNA extracts from 36 water samples collected from surface, mid-depth (-10 m) and dose to bottom (-20 m) during premonsoon, postmonsoon, monsoon were analyzed by PCRfor amplifying variable region of 16S rRNAgene and subsequently through DGGE. Prominent bands were excised, cloned and sequenced indicated the preponderance of gammaproteobacteria, bacteroidetes and cyanobacteria. Non-metric dimensional scaling of the DGGE gels indicated that the spatial variations in BCC were prominent among the sampling locations. Temporal variations in the BCC appear to be influenced by monsoonal processes. The canonical correspondence analyses suggest that the concentration of chlorophyll a and nitrate are two important environmental factors for both spatial and temporal variations in BCC. Chlorophyll a seems to be impart a top-down control of BCC while nitrate, the bottom-up control. Our results also suggest that BCC can vary over a small geographic distance in highly dynamic, seasonally predisposed tropical coastal waters.     

Environmental change and declining resource availability for small-mammal communities in the Great Basin

Changes in climate and land use can impact natural systems across all levels of ecological organization. Most documented and anticipated effects consider species' properties, including phenologies, geographic distributions, and abundances. Responses of higher-level aggregate community or ecosystem properties have not been considered as they are assumed to be relatively stable due to compensatory dynamics and diversity-stability relationships. However, this assumption may not be as fundamental as previously thought. Here we assess stability in the aggregate properties of total abundance, biomass, and energy consumption for small-mammal communities in the Great Basin, using paired historical and modern survey data spanning nearly a century of environmental change. Results show marked declines in each aggregate property independent of spatial scale, elevation, or habitat type, and a reallocation of available biomass and energy favoring diet and habitat generalists. Because aggregate properties directly reflect resource availability, our findings indicate a regionwide decline in resources of 50% over the past century, which may signal a resource crisis. This work illustrates the power of using aggregate properties as indicators of ecological conditions and environmental change at broad spatial and temporal scales.     

Pelagic biogeography of the South Atlantic Ocean

The biogeography of the South Atlantic was investigated using presence/absence data for euphausiids. Records were taken from recent and historic, as well as published and unpublished data sets. The resulting biogeography is the most complete to date and can be usefully compared with the biogeochemical provinces for the region. A total of six biogeographic provinces were identified from similarity analyses of the 246 five-degree grid squares. These correspond to Antarctic, sub-Antarctic, cold temperate, warm temperate (subtropical) and tropical waters, as well as the Agulhas Current. Congruence with the biogeochemistry of the region is good in the south and emphasises the important determinate role of temperature. However, the biogeography fails to identify coastal and tropical biogeochemical provinces. This can be attributed to the fact that while adjoining areas may share many species in common, their assemblages differ in their quantitative composition. This serves to emphasise differences in provincial functioning. Received: 28 May 1997 / Accepted: 15 July 1997     

Effects of urbanization and urban stream restoration on the physical and biological structure of stream ecosystems

Streams, as low-lying points in the landscape, are strongly influenced by the stormwaters, pollutants, and warming that characterize catchment urbanization. River restoration projects are an increasingly popular method for mitigating urban insults. Despite the growing frequency and high expense of urban stream restoration projects, very few projects have been evaluated to determine whether they can successfully enhance habitat structure or support the stream biota characteristic of reference sites. We compared the physical and biological structure of four urban degraded, four urban restored, and four forested streams in the Piedmont region of North Carolina to quantify the ability of reach-scale stream restoration to restore physical and biological structure to urban streams and to examine the assumption that providing habitat is sufficient for biological recovery. To be successful at mitigating urban impacts, the habitat structure and biological communities found in restored streams should be more similar to forested reference sites than to their urban degraded counterparts. For every measured reach- and patch-scale attribute, we found that restored streams were indistinguishable from their degraded urban stream counterparts. Forested streams were shallower, had greater habitat complexity and median sediment size, and contained less-tolerant communities with higher sensitive taxa richness than streams in either urban category. Because heavy machinery is used to regrade and reconfigure restored channels, restored streams had less canopy cover than either forested or urban streams. Channel habitat complexity and watershed impervious surface cover (ISC) were the best predictors of sensitive taxa richness and biotic index at the reach and catchment scale, respectively. Macroinvertebrate communities in restored channels were compositionally similar to the communities in urban degraded channels, and both were dissimilar to communities in forested streams. The macroinvertebrate communities of both restored and urban degraded streams were correlated with environmental variables characteristic of degraded urban systems. Our study suggests that reach-scale restoration is not successfully mitigating for the factors causing physical and biological degradation.     

Abundance and diversity of the phyllosphere bacterial communities of Mediterranean perennial plants that differ in leaf chemistry

Summary.  We studied the epiphytic bacterial communities of the summer leaves of eight perennial species naturally occurring in a Mediterranean ecosystem. The species differ in essential-oil content (from rich in essential oil to non-producers) and composition, and also in life form (from herbaceous species to tall shrubs). We compared the epiphytic bacterial communities on the basis of (i) their abundance, (ii) their metabolic profile (derived by use of the BIOLOG Ecoplate system) and (iii) richness and diversity of substrates that they use, as a measure of functional diversity. Among all species, the aromatic Melissa officinalis was the most abundantly colonized. The bacterial communities on the leaves of the aromatic Myrtus communis, Calamintha nepeta and Melissa officinalis, and also of Cistus incanus catabolized all 31 substrates offered; those on the evergreen-sclerophyllous species, Arbutus unedo and Quercus coccifera, catabolized only 14 and 17 substrates, respectively. Carbohydrates were consistently used abundantly by all communities, whereas carboxylic acids were most variably used. On average, the group of aromatic plants scored higher regarding bacterial abundance, and richness and diversity of substrates used by the bacterial communities on their leaves; the lowest values for both substrate-use indices were recorded in A. unedo. Bacterial abundance or richness or diversity of substrates used did not vary with leaf oil content. Abundance was positively correlated with both substrate-use indices. Results support claims that the antimicrobial effects of essential oils are not exerted so much under natural conditions as reports based on biassays with pathogens usually show. Although essential oils play a part in the microbial colonization of the phyllosphere, it is not likely that inhibition of phyllosphere bacteria is essential oils’ primary role, at least in the Mediterranean environment.     

Molecular dating and biogeography of the neritic krill Nyctiphanes

The genus Nyctiphanes (Malacostraca, Euphausiacea) comprises four neritic species that display antitropical geographic distribution in the Pacific (N. simplex and N. australis) and Atlantic (N. couchii and N. capensis) Oceans. We studied the origin of this distribution applying methods for phylogenetic reconstruction and molecular dating of nodes using a Bayesian MCMC analysis and the DNA sequence information contained in mtDNA 16S rDNA and cytochrome oxidase (COI). We tested hypotheses of vicariance by contrasting the time estimates of cladogenesis with the onset of the major barriers to ocean circulation. It was estimated that Nyctiphanes originated in the Pacific Ocean during the Miocene, with a lower limit of 18 miilion years ago (Mya). An Atlantic–Pacific cladogenic event (95% HPD 3.2–9.6) took place after the closure of the Tethyan Sea, suggesting that dispersal occurred from the Indo-Pacific, most likely via southern Africa. Similarly, the antitropical distribution pattern observed in the eastern Atlantic Ocean likely resulted from recent Pliocene–Pleistocene (95% HPD 1.0–4.97) northward dispersal from the southern hemisphere. Our results imply that dispersal appears to have had a significant role to play in the evolution of this group. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Environmental factors affecting the standing crop of foraminifera in sublittoral and psammolittoral communities of a Long Island salt marsh

Representative samples from the epiphytic and psammolittoral communities were made during the summer of 1968 in North Sea Harbor, Southampton, Long Island, USA. In the epiphytic communities, the foraminifera were very patchy; 2.6% of the total samples accounted for 56.4% of the total foraminifera collected. The standing crop and species composition of the foraminifera changed throughout the summer. Two seasonal peaks were observed in the epiphytes. In early summer Protelphidium tisburyensis was the dominant form. In late July and August Elphidium incertum was dominant. Taken as a whole over the entire summer, the following species comprised the epiphytic foraminiferan community: Elphidium incertum (46.63%); Protelphidium tisburyensis (25.64%); Ammotium salsum (9.88%); Elphidium clavatum (6.74%); E. translucens (3.47%); Ammonia beccarii (2.83%); Ammobaculites dilatatus (2.08%); Trochammina inflata (1.8%); Elphidium advenum (< 1%); E. galvestonense (< 1%); E. gunteri (< 1%); Quinqueloculina lata (< 1%); Q. seminulum (< 1%) and Trochammina macrescens (< 1%). Although fewer species were found, many more foraminifera live in the benthos than in the epiphytic community. Taken as a whole, over the entire summer, the following species comprised the foraminifera in the psammolittoral community: Trochammina inflata (49.6%); Elphidium incertum (31.8%); Ammotium salsum (10.9%); Quinqueloculina seminulum (4.2%); Elphidium sp. (1.5); Protelphidium tisburyensis (0.9%); Ammonia beccarii (0.7%); Elphidium clavatum (< 0.1%); and E. translucens (< 0.1%). Three species, Ammotium salsum, Elphidium incertum and Trochammina inflata bloomed successively in the psammolittoral community. The distribution of the latter two species was correlated with the vertical and horizontal changes of grain size; Elphidium incertum distribution clustered around a median grain size of 0.1 mm, whereas Trochammina inflata clustered around a median grain size of 0.46 mm. The sediments were sampled both at high and low tide. No evidence was obtained to suggest migration of foraminifera through the sediments as a function of tidal cycle. Horizontal distribution of the foraminifera in the marsh was correlated with the flow patterns of very small rivulets in the study area. With respect to many species of foraminifera, the overlying epiphytic communities are not continuous with the psammolittoral communities below them in the water column. Protelphidium tisburyensis was an early summer dominant epiphyte, but was rare in the psammolittoral communities. Trochammina inflata, on the other hand, was dominant in the coarser, deeper sediments and was rare in the epiphytic community. Elphidium incertum is presumably a generalist species. It formed half the foraminiferan population throughout the water column. Ammotium salsum was also abundant in both communities.Supported by US AEC Contract AT (30-1) 3995. Ref. No. NYO 3995-16.Summarized from a thesis submitted by N. J. Matera in partial fulfillment of the requirements of Master of Arts at The City College of New York.University Institute of Oceanography of CUNY, Contribution No. 1.     

Environmental variability and allocation trade-offs maintain species diversity in a process-based model of succulent plant communities

Ecological theory suggests that environmental variability can promote coexistence, provided that species occupy differential niches. In this study, we focus on two questions: (1) Do allocation trade-offs provide a sufficient basis for niche differentiation in succulent plant communities? (2) What is the relative importance of different forms of environmental variability on species diversity and community composition? We approach these questions with a generic, individual-based simulation model. In our model, plants compete for water in a spatially explicit environment. Species differ in their size at maturity and in the allocation of carbon to roots, leaves and storage tissue. The model was fully specified with independent literature data. Model output was compared to characteristics of a species-rich community in the semi-arid Richtersveld (South Africa). The model reproduced the coexistence of plants with different sizes at maturity, the dominance of succulent shrubs, and the level of vegetation cover. We analyzed the effects of three forms of environmental variability: (a) temporal fluctuations in precipitation (rain and fog), (b) spatial heterogeneity of water supply due to run-on and run-off processes and (c) ‘rock pockets’ that limit root competition in space. The three types of variability had differential effects on diversity: diversity exhibited a strong hump-shaped response to temporal variation. Spatial variability increased diversity, with the strongest increase occurring at intermediate levels of temporal variability. Finally, rock pockets had the weakest effect, but contributed to diversity by providing refuges for small species, particularly at low temporal variability. The model thus shows that spatio-temporal variation of resource supply can maintain diversity over long time scales even in small systems, as is the case in the Richtersveld succulent communities. Trade-offs in allocation provide the basis for necessary niche differentiation. By describing resource competition between individual plants, our model provides a mechanistic basis for the link from species traits to community composition at given environmental conditions. It thereby contributes to an understanding of the forces shaping plant communities. Such an understanding is critical to reduce the threats environmental change poses to biodiversity and ecosystem services.     

Sea ice retreat alters the biogeography of the Bering Sea continental shelf.

Seasonal ice cover creates a pool of cold bottom water on the eastern Bering Sea continental shelf each winter. The southern edge of this cold pool, which defines the ecotone between arctic and subarctic communities, has retreated approximately 230 km northward since the early 1980s. Bottom trawl surveys of fish and invertebrates in the southeastern Bering Sea (1982-2006) show a coincident reorganization in community composition by latitude. Survey catches show community-wide northward distribution shifts, and the area formerly covered by the cold pool has seen increases in total biomass, species richness, and average trophic level as subarctic fauna have colonized newly favorable habitats. Warming climate has immediate management implications, as 57% of variability in commercial snow crab (Chionoecetes opilio) catch is explained by winter sea ice extent. Several measures of community distribution and structure show linear relationships with bottom temperature, suggesting warming climate as the primary cause of changing biogeography. However, residual variability in distribution not explained by climate shows a strong temporal trend, suggesting that internal community dynamics also contribute to changing biogeography. Variability among taxa in their response to temperature was not explained by commercial status or life history traits, suggesting that species-specific responses to future warming will be difficult to predict.     

Enforcement costs and the incentive to evade controls on production

Economists have long favored emission taxes as a method of controlling pollution. One reason for the widespread support of emission taxes has been the assumption that emission taxes would be easier to enforce than other policies. However, recent developments in the discussion of pollution control allow more specific comparisons of enforcement costs and yield the conclusion that there may exist at least one alternate regulatory scheme that would be easier to enforce than emission taxes.     

Influence of occupant microhabitat on the composition of encrusting communities on gastropod shells

The exploitation of microhabitats is widely considered to increase biodiversity in marine ecosystems. Although intertidal hermit crabs and gastropods may inhabit the same shell type and shore level, their microhabitat may differ depending on the state of the tide. On the south coast of Wales the hermit crab Pagurus bernhardus mainly inhabits the shells of Nucella lapillus (84%). Hermit crab shells had a significantly different encrusting community compared with live N. lapillus shells. At low tide the live gastropods were found on exposed rock surfaces whereas hermit crabs were restricted to tidal pools. Communities encrusting live gastropod shells were characterised by lower species richness and abundance compared with shells inhabited by hermit crabs (12 species found in total). A greater abundance and richness of epibionts was recorded from both shell types during the summer compared with winter. Differences in community composition between shell occupant types were attributed to microhabitats used by gastropods and hermit crabs and the associated desiccation pressures, rather than competitive interactions or shell characteristics. This contradicts earlier studies of subtidal shells where biological processes were considered more important than physical factors in controlling species abundance and richness patterns. The use of rockpool microhabitats by hermit crabs increases the biodiversity of rocky shores, as some species commonly found on hermit-crab-inhabited shells were rare in other local habitats.Communicated by J.P. Thorpe, Port Erin     

Alternatives to biodiversity offsets for mitigating the effects of urbanization on stream ecosystems

Globally, offset schemes have emerged in many statutory frameworks relating to development activities, with the aim of balancing biodiversity conservation and development. Although the theory and use of biodiversity offsets in terrestrial environments is broadly documented, little attention has been paid to offsets in stream ecosystems. Here we examine the application of offset schemes to stream ecosystems and explore whether they suffer similar shortcomings to those of offset schemes focused on terrestrial biodiversity. To challenge the applicability of offsets further, we discuss typical trajectories of urban expansion and their cascading physical, chemical and biological impacts on stream ecosystems. We argue that the highly connected nature of stream ecosystems and urban drainage networks can transfer impacts of urbanization across wide areas, complicating the notion of like‐for‐like exchange and the prospect of effectively mitigating biodiversity loss. Instead, we identify in‐catchment options for stormwater control, which can avoid or minimize the impacts of development on downstream ecosystems, while presenting additional public and private benefits. We describe the underlying principles of these alternatives, some of the challenges associated with their uptake, and policy initiatives being trialed to facilitate adoption. In conclusion, we argue that stronger policies to avoid and minimize the impacts of urbanization provide better prospects for protecting downstream ecosystems, and can additionally, stimulate economic opportunities and improve urban liveability.