In January, millions of fish died in Australia’s Murray–Darling Basin as the region experienced some of its driest and hottest weather on record. The heat also caused severe water shortages for people living there. Such harsh conditions will become more common as the world warms. Iconic and valuable species such as the Murray cod (Maccullochella peelii peelii) — Australia’s largest freshwater fish — could vanish, threatening biodiversity and livelihoods.
Rivers around the world are struggling to cope with changing weather patterns. In Germany and Switzerland, a heatwave last year killed thousands of fish and blocked shipping on the River Rhine. California is emerging from a six-year drought that restricted water supplies and devastated trees, fish and other aquatic life. Across the US southwest, extended dry spells are destroying many more forests and wetlands.
What should river managers do? They cannot look to tools of old: conventional management techniques that aim to restore ecosystems to their original state. Ongoing human development and climate change mean that this is no longer possible. And models based on past correlations do a poor job of predicting how species might respond to unprecedented changes in future (see ‘Ecosystem change’). A different approach is called for.
To maintain water supplies and avoid devastating population crashes, rivers must be managed adaptively, enhancing their resilience and limiting risk. Researchers must also develop better forecasting tools that can project how key species, life stages and ecosystems might respond to environmental changes. This will mean moving beyond simply monitoring the state of ecosystems to modelling the biological mechanisms that underpin their survival.
Model process
Today, river managers track properties such as species diversity and population abundance, and compare them with historical averages. If they spot troubling declines, they might intervene by, for instance, altering the amount of water released from dams. But by the time trends are detected, they can be impossible to arrest.
Understanding how sensitive ecosystems might change is crucial to managing them in the future. For example, in the American west, native cottonwoods (Populus spp.) are valuable, long-lived trees that anchor river banks and offer habitats for many species. They are finely tuned to seasonal flood patterns, releasing their seeds in early summer when river flows peak. The seeds take root in moist ground after the flood recedes. But if the flood is delayed, even by a few days, many seeds fall on dry ground and die. Drought-tolerant species, such as salt cedar (Tamarix ramosissima), that disperse seeds over a longer period will move in and dramatically alter conditions for native flora and fauna.
Models based on biological processes or mechanisms — that is, how rates of survival, reproduction and dispersal vary with environmental conditions — can follow and predict such shifts. For example, by modelling the impacts of changes in flood timing on aquatic invertebrates, it is possible to predict how the numbers of dragonflies and mayflies in a dryland river will vary with different patterns of dam releases.
Process-based models can be tailored to particular life stages of a species, or sequences of events. They can identify tipping points and bottlenecks. For example, they have revealed that the early juvenile stage of coho salmon (Oncorhynchus kisutch) in the northwestern United States is most sensitive to summer droughts. The salmon spawn in streams that flow into coastal rivers, and might spend a couple of years in fresh water before moving to the sea. Juveniles might not survive, or might find it hard to travel downstream, when the river levels are low.
Armed with all this information, managers can intervene before a problem arises. For example, in wet years, conservationists in the Pacific Northwest could find and support habitats that are crucial to juvenile salmon. They could manage water flows in dry years to enable the salmon to migrate. Similarly, in the US southwest, river flows could be increased strategically from reservoirs to protect important species, such as cottonwoods. And in Australia, letting more water pass through dams in spring could stop rivers drying up while the eggs of Murray cod mature.
Rivers must also be managed for people. Allocating scarce water resources is contentious. Policymakers, water-resource engineers, conservationists and ecologists must work together to decide how much water should be diverted to people, agriculture and industry, and how much is needed to protect ecosystems during drought.
Such models can also track how interactions among species in communities vary under changing conditions. For example, the loss of riparian specialists in dryland river ecosystems and invasion by both non-native and upland species in a drier future could create a vicious cycle. River ecosystems could become more vulnerable to climate change and to alien species.
Some river basins are beginning to be managed adaptively — agencies are trying different management practices, learning from them and updating them as needed. For example, in Australia, state and federal agencies periodically reassess and rebalance water allocations, as climate trends, information and assessment tools develop. Similarly, the Bay–Delta Plan in California proposes to revisit relationships between target species, water flows and water quality in San Francisco Bay and the Sacramento–San Joaquin River Delta every five years.
But adaptive management alone might miss conservation targets. Unexpected consequences could emerge over the long term as impacts mount. Process-based models can look further ahead and save time, money and disruption by limiting the number of interventions as well as avoiding adverse impacts. They would help stakeholders and managers to choose which features of ecosystems to maintain, to justify costly interventions such as major engineering works and to weigh trade-offs to build resilience under increasing climatic uncertainty.
Obstacles to implementation
Process-based models are already used in fisheries and conservation. For example, they have shown conservationists that it is more effective to protect juvenile loggerhead sea turtles from being caught in fishing nets than to safeguard their eggs on beaches. And such models help to guide the management of wetland habitats in the United States for the endangered Everglades snail kite (Rostrhamus sociabilis), the fledglings of which are susceptible to droughts.
But they are rarely used in river management, mainly because data on the basic biology of local species are lacking. Such data are costly for scientists and agencies to collect. Measuring fecundity or survival, for example, takes years and thus requires long-term funding and commitment. Such campaigns are usually reserved for endangered or commercially valuable species.
Simplifying models might help to bridge the data gaps in the interim. Species with similar life histories or characteristics might respond similarly to changing river conditions. Studies of one could inform models and management of similar species in other places. For instance, plains cottonwood (Populus deltoides) in North America, river red gum (Eucalyptus camaldulensis) in Australia, and Euphrates poplar (Populus euphratica) in North Africa and Eurasia are all riparian trees that have similar hydrological requirements and drought tolerances. They share characteristics such as shallow roots and furrowed bark that resists flood scour, and can resprout after being buried by sediment. Analytical methods could also be developed to extrapolate across gaps in data sets.
Four steps
River scientists and managers should take the following steps.
by Jonathan D. Tonkin, N. LeRoy Poff and Colleagues, Nature | Read more:
Rivers around the world are struggling to cope with changing weather patterns. In Germany and Switzerland, a heatwave last year killed thousands of fish and blocked shipping on the River Rhine. California is emerging from a six-year drought that restricted water supplies and devastated trees, fish and other aquatic life. Across the US southwest, extended dry spells are destroying many more forests and wetlands.
What should river managers do? They cannot look to tools of old: conventional management techniques that aim to restore ecosystems to their original state. Ongoing human development and climate change mean that this is no longer possible. And models based on past correlations do a poor job of predicting how species might respond to unprecedented changes in future (see ‘Ecosystem change’). A different approach is called for.To maintain water supplies and avoid devastating population crashes, rivers must be managed adaptively, enhancing their resilience and limiting risk. Researchers must also develop better forecasting tools that can project how key species, life stages and ecosystems might respond to environmental changes. This will mean moving beyond simply monitoring the state of ecosystems to modelling the biological mechanisms that underpin their survival.
Model process
Today, river managers track properties such as species diversity and population abundance, and compare them with historical averages. If they spot troubling declines, they might intervene by, for instance, altering the amount of water released from dams. But by the time trends are detected, they can be impossible to arrest.
Understanding how sensitive ecosystems might change is crucial to managing them in the future. For example, in the American west, native cottonwoods (Populus spp.) are valuable, long-lived trees that anchor river banks and offer habitats for many species. They are finely tuned to seasonal flood patterns, releasing their seeds in early summer when river flows peak. The seeds take root in moist ground after the flood recedes. But if the flood is delayed, even by a few days, many seeds fall on dry ground and die. Drought-tolerant species, such as salt cedar (Tamarix ramosissima), that disperse seeds over a longer period will move in and dramatically alter conditions for native flora and fauna.
Models based on biological processes or mechanisms — that is, how rates of survival, reproduction and dispersal vary with environmental conditions — can follow and predict such shifts. For example, by modelling the impacts of changes in flood timing on aquatic invertebrates, it is possible to predict how the numbers of dragonflies and mayflies in a dryland river will vary with different patterns of dam releases.
Process-based models can be tailored to particular life stages of a species, or sequences of events. They can identify tipping points and bottlenecks. For example, they have revealed that the early juvenile stage of coho salmon (Oncorhynchus kisutch) in the northwestern United States is most sensitive to summer droughts. The salmon spawn in streams that flow into coastal rivers, and might spend a couple of years in fresh water before moving to the sea. Juveniles might not survive, or might find it hard to travel downstream, when the river levels are low.
Armed with all this information, managers can intervene before a problem arises. For example, in wet years, conservationists in the Pacific Northwest could find and support habitats that are crucial to juvenile salmon. They could manage water flows in dry years to enable the salmon to migrate. Similarly, in the US southwest, river flows could be increased strategically from reservoirs to protect important species, such as cottonwoods. And in Australia, letting more water pass through dams in spring could stop rivers drying up while the eggs of Murray cod mature.
Rivers must also be managed for people. Allocating scarce water resources is contentious. Policymakers, water-resource engineers, conservationists and ecologists must work together to decide how much water should be diverted to people, agriculture and industry, and how much is needed to protect ecosystems during drought.
Such models can also track how interactions among species in communities vary under changing conditions. For example, the loss of riparian specialists in dryland river ecosystems and invasion by both non-native and upland species in a drier future could create a vicious cycle. River ecosystems could become more vulnerable to climate change and to alien species.
Some river basins are beginning to be managed adaptively — agencies are trying different management practices, learning from them and updating them as needed. For example, in Australia, state and federal agencies periodically reassess and rebalance water allocations, as climate trends, information and assessment tools develop. Similarly, the Bay–Delta Plan in California proposes to revisit relationships between target species, water flows and water quality in San Francisco Bay and the Sacramento–San Joaquin River Delta every five years.
But adaptive management alone might miss conservation targets. Unexpected consequences could emerge over the long term as impacts mount. Process-based models can look further ahead and save time, money and disruption by limiting the number of interventions as well as avoiding adverse impacts. They would help stakeholders and managers to choose which features of ecosystems to maintain, to justify costly interventions such as major engineering works and to weigh trade-offs to build resilience under increasing climatic uncertainty.
Obstacles to implementation
Process-based models are already used in fisheries and conservation. For example, they have shown conservationists that it is more effective to protect juvenile loggerhead sea turtles from being caught in fishing nets than to safeguard their eggs on beaches. And such models help to guide the management of wetland habitats in the United States for the endangered Everglades snail kite (Rostrhamus sociabilis), the fledglings of which are susceptible to droughts.
But they are rarely used in river management, mainly because data on the basic biology of local species are lacking. Such data are costly for scientists and agencies to collect. Measuring fecundity or survival, for example, takes years and thus requires long-term funding and commitment. Such campaigns are usually reserved for endangered or commercially valuable species.
Simplifying models might help to bridge the data gaps in the interim. Species with similar life histories or characteristics might respond similarly to changing river conditions. Studies of one could inform models and management of similar species in other places. For instance, plains cottonwood (Populus deltoides) in North America, river red gum (Eucalyptus camaldulensis) in Australia, and Euphrates poplar (Populus euphratica) in North Africa and Eurasia are all riparian trees that have similar hydrological requirements and drought tolerances. They share characteristics such as shallow roots and furrowed bark that resists flood scour, and can resprout after being buried by sediment. Analytical methods could also be developed to extrapolate across gaps in data sets.
Four steps
River scientists and managers should take the following steps.
Image: Jose Luis Roca/AFP/Getty
