Peter M. Davies

Global threats to human water security and river biodiversity

Protecting the world’s freshwater resources requires diagnosing threats over a broad range of scales, from global to local. Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly 80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global water security for both humans and freshwater biodiversity.

Biological processes in running waters and their implications for the assessment of ecological integrity

Although biomonitoring approaches are being increasingly used in the measurement of stream and river health, critical assumptions about the nature of biological populations and communities that underpin them are often ignored. Many approaches based on pattern detection in plant and animal communities assume high temporal persistence in the absence of anthropogenic disturbances. However, this has been rarely tested with long-term data sets and there is evidence that this assumption is not true in some river systems. Biological processes, such as predation and recruitment, can account for considerable spatial and temporal variation in the structure of some stream communities. These processes may prevent the development of robust predictive models or indices based on pattern detection. Measurements of population or community attributes also are often used to infer ecosystem processes, yet the link between pattern and process has rarely been demonstrated. Many goals of river management relate to the maintenance of natural ecological processes and ecosystem function; direct measurement of these processes is, however, often neglected in assessment programs. Such measures are often sensitive to causal factors that are known to affect river health and it is possible to develop simple but powerful predictive models. Perhaps more importantly, should an impact to be detected, strategies for remediation are more obvious as the causal processes are generally better known. The ultimate success of biomonitoring approaches depends on how well we understand the biophysical processes that influence the structure and dynamics of stream and river systems, and the way they function.

River and wetland food webs in Australia’s wet–dry tropics: general principles and implications for management

The tropical rivers of northern Australia have received international and national recognition for their high ecological and cultural values. Unlike many tropical systems elsewhere in the world and their temperate Australian counterparts, they have largely unmodified flow regimes and are comparatively free from the impacts associated with intensive land use. However, there is growing demand for agricultural development and existing pressures, such as invasive plants and feral animals, threaten their ecological integrity. Using the international literature to provide a conceptual framework and drawing on limited published and unpublished data on rivers in northern Australia, we have derived five general principles about food webs and related ecosystem processes that both characterise tropical rivers of northern Australia and have important implications for their management. These are: (1) the seasonal hydrology is a strong driver of ecosystem processes and food-web structure; (2) hydrological connectivity is largely intact and underpins important terrestrial–aquatic food-web subsidies; (3) river and wetland food webs are strongly dependent on algal production; (4) a few common macroconsumer species have a strong influence on benthic food webs; and (5) omnivory is widespread and food chains are short. The implications of these ecosystem attributes for the management and protection of tropical rivers and wetlands of northern Australian are discussed in relation to known threats. These principles provide a framework for the formation of testable hypotheses in future research programmes.

Riparian Ecosystems in the 21st Century: Hotspots for Climate Change Adaptation?

Riparian ecosystems in the 21st century are likely to play a critical role in determining the vulnerability of natural and human systems to climate change, and in influencing the capacity of these systems to adapt. Some authors have suggested that riparian ecosystems are particularly vulnerable to climate change impacts due to their high levels of exposure and sensitivity to climatic stimuli, and their history of degradation. Others have highlighted the probable resilience of riparian ecosystems to climate change as a result of their evolution under high levels of climatic and environmental variability. We synthesize current knowledge of the vulnerability of riparian ecosystems to climate change by assessing the potential exposure, sensitivity, and adaptive capacity of their key components and processes, as well as ecosystem functions, goods and services, to projected global climatic changes. We review key pathways for ecological and human adaptation for the maintenance, restoration and enhancement of riparian ecosystem functions, goods and services and present emerging principles for planned adaptation. Our synthesis suggests that, in the absence of adaptation, riparian ecosystems are likely to be highly vulnerable to climate change impacts. However, given the critical role of riparian ecosystem functions in landscapes, as well as the strong links between riparian ecosystems and human well-being, considerable means, motives and opportunities for strategically planned adaptation to climate change also exist. The need for planned adaptation of and for riparian ecosystems is likely to be strengthened as the importance of many riparian ecosystem functions, goods and services will grow under a changing climate. Consequently, riparian ecosystems are likely to become adaptation ‘hotspots’ as the century unfolds.

Effects of patchy shade on stream water temperature: how quickly do small streams heat and cool?

Summer field observations in five 2nd order streams (width 1–2 m, depth 5–15 cm, velocity 5–10 cm s–1) in Western Australia and south-east Queensland showed that daily maximum temperatures changed by ±4°C over distances of 600–960 m (travel time 2–3 h) immediately downstream from 40–70% step changes in riparian shade. There was a strong linear relationship between the rate of change of daily maximum temperature and the change of shade such that downstream from a 100% change of shade the heating/cooling rates are ±4°C h–1 and ±10°C km–1 (upper bound ±6°C h–1 and ±15°C km–1) respectively. These high rates only apply over short distances and travel times because downstream water temperatures adjust to the new level of shade and reach a dynamic equilibrium. Shade was too patchy in the study streams to measure how long water takes to reach equilibrium, however, using an existing computer model, we estimate that this occurs after ~1200 m (travel time 4 h). Further modelling work is desirable to predict equilibrium temperatures under given meteorological, flow and shade conditions. Nevertheless, landowners and regulators can use this information to determine whether the presence/absence of certain lengths of bankside shade are likely to cause desirable/undesirable temperature decreases/increases.

Primary Production in Tropical Streams and Rivers

Net primary production is a fundamental ecological process that reflects the amount of carbon synthesized within an ecosystem, which is ultimately available to consumers. Although current ecosystem models of streams and rivers have placed variable emphasis on the importance of instream primary production to aquatic food webs, recent research indicates that aquatic algae are a significant contributor to food webs in tropical rivers and streams. This is in contrast to many well-studied north temperate latitude streams in deciduous forest ecosystems, which are thought to depend mainly upon terrestrial leaf litter and detritus-based food webs. A review of available literature suggests that rates of in-stream primary production in tropical regions are typically at least an order of magnitude greater than comparable temperate systems. Although nutrient status can significantly modify rates, the ultimate driver of aquatic primary production is light availability. Rates of benthic gross primary productivity in tropical streams range from 100 to 200mgCm-2 d-1 under shaded conditions to much higher values associated with open canopies. Light inputs to the channel can be controlled by stream orientation, with east-west channels receiving much more light compared to those orientated north-south. Rates of production for large tropical rivers are similar to those for streams, although factors that regulate production are different and hence they respond differently to human impact. Values for rivers range from 10 to 200mgCm-2 d-1 to more than 1000mgCm-2 d-1. Production is often limited by turbidity, which tends to be at a maximum after high flow events. In polluted tropical rivers, productivity responds to nutrient enrichment and can attain rates of 6000mgCm-2 d-1. The highest rates of production in tropical river systems typically occur in floodplains subject to seasonal inundation, where aquatic vascular plants dominate total productivity. These macrophytes (herbaceous vascular plants that can be primarily terrestrial or aquatic) can proliferate in situ or be transported from upstream. Rooted aquatic plants with emergent or floating leaves respond to the rising water level, sometimes elongating their stems at a rate of 20cmd-1, and many terrestrial plants tolerate prolonged submergence. These ecosystems can attain very high rates of primary production that rival those of intensively managed agro-ecosystems. Floodplain forest can also be a productive component of these ecosystems. Both attached algae (periphyton) and phytoplankton contribute substantially to algal production in floodplain waters. Floodplains are important for fodder and for nursery habitat for fish, which re-invade main channels when floods recede. Tropical rivers may flow into coastal mangrove ecosystems, where rates of productivity are variable and often dependent of methodologies of measurement. Rates of mangrove production range from 1300mgCm-2 d-1 in the T鲭inos Lagoon, Mexico to 1900-2700mgCm-2 d-1 in the Fly River estuary (Papua New Guinea). However, rates of phytoplankton growth within mangrove forests are low, probably controlled by shading and turbidity, and are comparable to those of tropical streams. As pressures for water resource development intensify, tropical fluvial ecosystems are coming under increasing pressure. It is important to understand how these ecosystems function and to ensure problems of developing water resources in temperate regions are not repeated in the tropics.

A relationship between environmental degradation and mental health in rural Western Australia.

Australia is currently experiencing a process of escalating ecosystem degradation. This landscape degradation is associated with many outcomes that may directly or indirectly impact on human health. This study used a Bayesian spatial method to examine the effects of environmental degradation (measured as dryland salinity) on the mental health of the resident rural population. An association was detected between dryland salinity and depression, indicating that environmental processes may be driving the degree of psychological ill-health in these populations.

Flow–ecology relationships: closing the loop on effective environmental flows

Providing flows for biota and environmental processes is a challenging water management issue. For society the ability and willingness to allocate water to sustain the environment is increasingly competitive due to escalating demand and as a consequence of climate change. In response, an array of environmental flow (E-flow) methods have developed. Our view is that few E-flows have been implemented and even fewer evaluated in a research and management context. Much of our science effort in E-flows has been directed primarily at method development, with less attention being given to monitoring, evaluation and subsequent revision of E-flow strategies. Our objectives are to highlight the lack of connection between current trends in E-flow literature and theory with assessment of the efficacy and practical application of these methods. Specifically, effective E-flows need to be explicit about flow-ecology relationships to adequately determine the amount and timing of water required. We briefly outline the historical development of E-flows and discuss how serial development of methods and techniques has restricted implementation, evaluation and revision. We highlight areas where methods are lacking, such as incorporation of data on flow-ecology relationships into operational use of E-flow methods. We suggest four initial steps that will improve the applicability, implementation and ultimate success of E-flows.

Community structure of the macroinvertebrate fauna and water quality of a saline river system in south-western Australia

Many streams and rivers in the south-west of Western Australia have marginal salinities and only about half of the divertible surface water is fresh (< 3 g 1−1). Although salinization is recognised as the greatest threat to water resources throughout Australia, little is known of its effect on the biota. This paper gives the first quantitative description of the macroinvertebrate fauna of a salinized river system in south-western Australia.Two sites on an intermittent stream (Thirty-four Mile Brook) and two sites on a perennial river (Hotham River), above and below the confluence with the above tributary, were sampled on three occasions for benthic macroinvertebrates. Only 68 taxa were recorded in quantitative samples from the sites; a further 11 taxa were recorded in qualitative samples from these and two additional sites on the tributary. The river system shared several faunal elements with athalassic salt lakes across southern Australia. Crustaceans, particularly the amphipod Austrochiltonia subtenuis and several species of ostracods, were numerically dominant at all sites. In contrast to the benthic fauna of most lotic systems, insects formed only a small proportion of the individuals collected from the Hotham River. Chironomids were the dominant insect group, both in terms of the number of species and individuals, particularly at one of the tributary sites where species of Tanytarsus comprised almost half of the total number of individuals. Overall, the benthic fauna was characterised by very high densities and low richness, diversity and evenness. This is likely to be a direct consequence of the poor water quality of the system where high salinities, often greater than 3 g 1−1, were recorded.Classification and ordination revealed major differences in community structure of the benthic fauna between the Hotham River and its tributary. This was attributed to differences in the physical nature of the two streams, particularly substrate characteristics and stream permanence, rather than differences in water quality. Temporal differences in community structure were also apparent, but were more obvious in the Hotham River than in the tributary.

Predicting Novel Riparian Ecosystems in a Changing Climate

Rapid changes in global climate are likely to alter species assemblages and environmental characteristics resulting in novel ecosystems. The ability to predict characteristics of future ecosystems is crucial for environmental planning and the development of effective climate change adaptation strategies. This paper presents an approach for envisioning novel ecosystems in future climates. Focusing on riparian ecosystems, we use qualitative process models to predict likely abiotic and biotic changes in four case study systems: tropical coastal floodplains, temperate streams, high mountain streams and urban riparian zones. We concentrate on functional groups rather than individual species and consider dispersal constraints and the capacity for genetic adaptation. Our scenarios suggest that climatic changes will reduce indigenous diversity, facilitate non-indigenous invasion (especially C4 graminoids), increase fragmentation and result in simplified and less distinctive riparian ecosystems. Compared to models based on biota-environment correlations, process models built on mechanistic understanding (like Bayesian belief networks) are more likely to remain valid under novel climatic conditions. We posit that predictions based on species’ functional traits will facilitate regional comparisons and can highlight effects of climate change on ecosystem structure and function. Ecosystems that have experienced similar modification to that expected under climate change (for example, altered flow regimes of regulated rivers) can be used to help inform and evaluate predictions. By manipulating attributes of these system models (for example, magnitude of climatic changes or adaptation strategies used), implications of various scenarios can be assessed and optimal management strategies identified.