Freshwater Biology (2007) 52, 589–596
doi:10.1111/j.1365-2427.2007.01737.x
Restoring freshwater ecosystems in riverine landscapes:
the roles of connectivity and recovery processes
R O L A N D J A N S S O N , C H R I S T E R N I L S S O N A N D B J Ö R N M A L M Q V I S T
Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden
SUMMARY
1. This paper introduces key messages from a number of papers emanating from the Second
International Symposium on Riverine Landscapes held in August 2004 in Sweden, focusing on
river restoration. Together these papers provide an overview of the science of river
restoration, and point out future research needs.
2. Restoration tests the feasibility of recreating complex ecosystems from more simple and
degraded states, thereby presenting a major challenge to ecological science. Therefore,
close cooperation between practitioners and scientists would be beneficial, but most river
restoration projects are currently performed with little or no scientific involvement.
3. Key messages emanating from this series of papers are: The scope, i.e. the maximum and
minimum spatial extent and temporal duration of habitat use, of species targeted for
restoration should be acknowledged, so that all relevant stages in their life cycles are
considered. Species that have been lost from a stream cannot be assumed to recolonise
spontaneously, calling for strategies to ensure the return of target species to be integrated
into projects. Possible effects of invasive exotic species also need to be incorporated into
project plans, either to minimise the impact of exotics, or to modify the expected outcome
of restoration in cases where extirpation of exotics is impractical.
4. Restoration of important ecological processes often implies improving connectivity of
the stream. For example, longitudinal and lateral connectivity can be enhanced by
restoring fluvial dynamics on flood-suppressed rivers and by increasing water availability
in rivers subject to water diversion or withdrawal, thereby increasing habitat and species
diversity. Restoring links between surface and ground water flow enhances vertical
connectivity and communities associated with the hyporheic zone.
5. Future restoration schemes should consider where in the catchment to locate projects to
make restoration most effective, consider the cumulative effects of many small projects,
and evaluate the potential to restore ecosystem processes under highly constrained
conditions such as in urban areas. Moreover, restoration projects should be properly
monitored to assess whether restoration has been successful, thus enabling adaptive
management and learning for the future from both successful and unsuccessful
restorations.
Keywords: connectivity, landscapes, recovery, restoration, rivers
Introduction
Correspondence: Roland Jansson, Landscape Ecology Group,
Department of Ecology and Environmental Science,
Umeå University, SE-901 87 Umeå, Sweden.
E-mail: [email protected]
Riverine ecosystems belong to the ones most degraded by humans (Naiman & Turner, 2000; Sala et al.,
2000; Gleick, 2003). The hydrology of rivers has been
altered by the construction of structures, such as dams
and weirs, and water diversions for hydropower and
2007 The Authors, Journal compilation 2007 Blackwell Publishing Ltd
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R. Jansson et al.
other industrial purposes, irrigation and domestic
uses (Jackson et al., 2001; Arthington & Pusey, 2003;
Nilsson et al., 2005b; Dudgeon et al., 2006). These
activities profoundly change the processes that drive
ecosystem function and structure (Poff et al., 1997;
Jansson et al., 2000). By occupying the lowest-lying
portions of landscapes, riverine ecosystems are also
recipients of pollutants and excessive nutrients from
agriculture, industries and domestic sources (Naiman
et al., 2002). In addition, rivers and adjoining riparian
zones have been transformed by wetland reclamation,
dredging, channelisation and clearing of riparian
zones (Malmqvist & Rundle, 2002). Human societies
depend on several freshwater ecosystem services,
such as provision of clean water, food from aquatic
organisms, pollution disposal and leisure (Wilson &
Carpenter, 1999; Jackson et al., 2001; Postel & Richter,
2003). Concern that these services are threatened and
might not be sustained in the future has led to major
restoration efforts in streams and rivers (Bernhardt
et al., 2005). As a consequence, river restoration is now
a major enterprise, and still rapidly growing in terms
of numbers of projects and amount of money spent
(Bernhardt et al., 2005).
Restoration presents a major challenge for ecological science, in that it tests the possibility to reshape
more natural complex ecosystems with their defining
characteristics from more simple and degraded states
(Bradshaw, 1983). Consequently, one might expect
close collaboration between river managers involved
in restoration and the scientific community. Instead,
most restoration projects are performed with little or
no involvement of scientists (Palmer et al., 2005). As a
step to remedy this situation, more than 80 scientists
from 17 countries representing several disciplines
convened to form the Second International Symposium
on Riverine Landscapes in Bredsel, northern Sweden, in
August 2004. The aim of the meeting was to bring
leading scientists working in the field of river restoration together to give a state-of-the-art overview of the
science underpinning river restoration, and to identify
the most urgent needs for future research. A central
tenet of the meeting was that any riverine ecosystem
is embedded in heterogeneous landscapes consisting
of several landscape components that each may have
an effect on the target ecosystem (Wiens, 2002). A
corollary of this notion is that ecosystems to be
restored are not only affected by activities in the
target area, but also by processes in the surrounding
landscape, making connectivity among landscape
components a central concept (Pringle, 2001). Analysis
of the process of recovery is another central field in
restoration science as it addresses whether or not
ecosystems respond to restoration in the expected
way and whether or not the project objectives are
met. Manipulation of ecosystems to achieve a certain
state requires attention to ecological theory (Palmer,
Ambrose & Poff, 1997) and the mechanisms by which
recovery may occur (Jansson et al., 2005).
Connectivity
Connectivity from a landscape perspective can be
defined as the flow of energy, matter and organisms
between landscape components (Ward et al., 2002). In
stream ecology, a four-dimensional model has proved
useful, where flow along the stream (longitudinal),
between the stream and riparian and upland areas
(lateral), and between the channel and the hyporheic
zone (vertical) comprise three spatial dimensions, and
variation over time the fourth (Ward, 1989). To this
can be added connectivity at various hierarchical
landscape levels: between subcatchments, between
catchments and between regions (with several catchments), as well as between catchments and marine
environments (Pringle, 2001). Connectivity at any of
these levels may be important in restoring stream
ecosystems.
Lake, Bond & Reich (2007) seek to identify
ecological theories that might enhance the scientific
underpinnings of stream restoration. Specific organisms such as salmon or riparian trees are often the
target of restoration efforts, stressing the need to
understand the life history of these species, such as
their hydrological requirements and their scope, i.e.
the maximum and minimum spatial extent and
temporal duration of habitat use (Schneider, 1994).
Satisfying the scope of species may require enhancing
connectivity of the stream to allow migration and
dispersal of organisms. Many species depend on
different parts of streams for growth, reproduction
and survival during adverse conditions. Dams and
flow regulation reduce both longitudinal and lateral
connectivity (Ward & Stanford, 1995). Increasing
connectivity by dam removal or riparian restoration
is therefore an important goal (Kondolf et al., 2006).
Lake et al. (2007) review the importance of subsidies
across ecosystem boundaries in maintaining the
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Restoring freshwater ecosystems in riverine landscapes
trophic structure of riverine communities. The input
of organic matter from riparian zones to streams has
long been recognised (e.g. Hynes, 1975), and blocking
this contribution leads to fundamental changes in
stream communities (Wallace et al., 1997). Thus,
directed changes in energy sources and flow might
be important in achieving effective restoration. Lake
et al. (2007) also discuss the relationship between the
scale of restoration and that of ecological processes. If
they do not match, so that restoration is performed at
smaller spatial scales than are relevant for riverine
processes, restoration may be unsuccessful. Attention
to the scope of organisms is one way to tackle the
scaling problem.
The majority of stream restoration projects are
made under the assumption that if structures and
processes of ecosystems are restored, organisms will
recolonise (Palmer et al., 1997). The question is how
likely species are to recolonise spontaneously within a
certain time frame following habitat improvement.
Studying species dispersal is notoriously difficult,
given that rare occurrences of colonisation may have
far-reaching consequences. As an alternative, Hughes
(2007) uses patterns of genetic relatedness among
populations, as the degree of genetic similarity will
reflect the amount of dispersal and interbreeding
among populations. Drawing upon the extensive
work in her own laboratory and also elsewhere, she
documents patterns of genetic similarity and divergence among populations of aquatic organisms. She
found dispersal across catchment boundaries to be
negligible, making it unlikely that species that have
disappeared from catchments will recolonise spontaneously, except for organisms with good dispersal
capacities, such as insects (Bohonak, 1999). Dispersal
among streams within catchments also appears to be
more limited than expected from life-history characteristics, except for fish in Australian lowland rivers.
In general, dispersal among streams within catchments in Australia was greater in lowland than in
upland streams separated by high-relief land
(Hughes, 2007). Further tests are needed to establish
the generality of these results for other continents.
Vertical connectivity, the link between stream
channels and the hyporheic zone, i.e. the saturated
sediments below and alongside the channel, is rarely
considered in restoration. Boulton (2007) reviews the
ecology of hyporheic zones and their organisms, and
the ecosystem functions they perform. Hyporheic
591
organisms range from animals living exclusively in
subterranean habitats to those that only occasionally
venture into the hyporheic zone, and include a range
of diverse taxa. Crustaceans and insects tend to
dominate in coarse sediment, whereas smaller rotifers
and nematodes dominate sandy hyporheic zones. The
physical habitat, shaped partly by the degree of flood
disturbances, thus controls taxonomic structure and
body size with distinct functional consequences for
the ecosystem. Important functions involving hyporheic organisms include bioturbation with effects on
substratum porosity, stream metabolism, litter breakdown, and supply of matter and energy exchange to
the surface stream and terrestrial environments (Boulton, 2007). Humans have disrupted the exchange
between streams and their hyporheic zones by changing patterns of sediment redistribution and water
flow (Hancock, 2002). Strategies used to restore
vertical connectivity include flushing out sediments
that have contributed to clogging of interstitial space
(colmation), and placing logs in the stream to increase
water exchange between channel and sediment (Boulton, 2007).
The paper by Stromberg et al. (2007) illustrates
that successful ecosystem restoration may require
re-establishing longitudinal river connectivity (e.g. to
allow for unimpeded flows of water, including flood
pulses), lateral connectivity (e.g. to permit overbank
flooding), as well as vertical connectivity (e.g. to
reconnect flow between groundwater and surface
water). Different components of the flow regime
shape riparian ecosystems (Bendix & Hupp, 2000).
This is especially evident in arid regions. The
magnitude and duration of minimum flow determine
the composition of riparian vegetation by influencing
depth to saturated soils (Lite & Stromberg, 2005).
Floods redistribute sediment, inundate riparian zones
and recharge aquifers (Bendix & Hupp, 2000; Stromberg et al., 2007). Floods cause mortality by drowning
and physical disturbance, but also create opportunities for establishment of pioneer species. From a
landscape perspective, floods create a mosaic of
patches with specific geomorphology and hydrological conditions, supporting different types of vegetation, contributing to spatial heterogeneity and high
diversity. In the arid south-western U.S.A., flow
diversions and groundwater pumping have changed
low flow characteristics, whereas flood patterns have
been modulated by dams, urbanisation and other
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R. Jansson et al.
land-use changes in the catchment (Stromberg et al.,
2007). These flow alterations have resulted in concomitant changes in riparian vegetation. While pressure to use water from streams and aquifers remains
high in the arid south-western U.S.A., a number of
restoration projects demonstrate that reintroducing
key aspects of low and high flows may result in
successful restoration of riparian ecosystems. For
example, release of water from dams in flood pulses
may enhance establishment of riparian species such as
Populus and Salix in cohorts that may survive for
decades. In some cases, restoring base flows can be
done at a relatively small cost, using only a small
proportion of total flow, but having large influence on
riparian vegetation (Stromberg et al., 2007).
Streams are tightly connected to their catchments
through the transport of water and matter from
terrestrial habitats to streams. Conversion of native
vegetation to agricultural and urban land changes the
input to streams by increasing the loads of sediments,
pollutants and nutrients, often resulting in degraded
stream ecosystems. Restoration of riparian habitats
has been widely used to improve ecological conditions in streams. Most restoration projects to date are
local in scale, and little is known about how such
restoration efforts should be implemented in terms of
size, location and continuity along streams to be
effective in improving the ecological conditions of
stream ecosystems. Johnson et al. (2007) develop an
empirical model establishing a relationship between
the ecological conditions of stream ecosystems and the
stress imposed on streams as a result of land conversion at different locations in the catchment. Their
model can be used to guide restoration planning by
predicting how relevant indices of ecosystem function
or other properties, such as species diversity, would
respond to the implementation of a proposed restoration project. This provides practitioners with a tool to
estimate the width, extent and location of riparian
restoration needed to recreate desired ecosystem
characteristics in streams degraded by land conversion in the catchments.
Although restoration of lateral, longitudinal and
vertical connectivity is the aspiration in many restoration projects, enhanced connectivity at larger scales
may have unwanted consequences. Barriers to the
movement of aquatic organisms between continents,
between major river systems and among subcatchments and river reaches constitute filters that have led
to the evolution of unique faunas and floras. Human
activities have helped many aquatic species to circumvent geographical barriers by stocking, unauthorised release, construction of canals and water
conveyance systems, and transport in ship ballast
water (Rahel, 2007). This contributes to biotic homogenisation, as once distinct biological communities
come to share increasing numbers of species. For
example, the Colorado Province, forming a part of the
Nearctic Zoogeographic Region, harboured 32 native
fish species, but now also hosts 68 additional nonindigenous species. Fifty-four of these species are
from other provinces within the region, and 14 from
other zoogeographic regions (Rahel, 2007). As far as
these new species only add to the existing native
biodiversity, there may be little need for concern, but
examples abound of cases where native species are
threatened and ecosystem function changed by invading species (Simon & Townsend, 2003). Restoring
natural habitat conditions may reduce homogenisation by favouring native species over non-native ones.
Increasing connectivity, e.g. by removing dams, may
allow native species to recolonise their historic range,
but may also facilitate upstream expansion of nonnative species. If removal of non-native species is
unrealistic, construction of migration barriers might
be needed to protect isolated populations of native
species (Rahel, 2007).
Invasion of non-native species across biogeographic
barriers may alter ecosystem function and even cause
ecosystem replacement (Simon & Townsend, 2003). In
the Mediterranean shrub fynbos biome in the Cape
Region of South Africa, the invasion of trees, a growth
form with high water demand largely absent in native
vegetation, has reduced runoff to streams. It is
estimated that 3–7% of the mean annual runoff is
used by non-native vegetation, which is more than the
total amount of water estimated to be used by native
vegetation (van Wilgen, Nel & Rouget, 2007). This
threatens the supply of water for human use. Therefore, one of the largest invasive plant clearing
programmes globally has been established, the Working for Water Programme. Despite the grand scale of
this programme, only a small proportion of catchments and streams can be dealt with simultaneously,
calling for prioritisation among river catchments to
guide the allocation of control purposes. In response,
van Wilgen et al. (2007) developed a scheme to
prioritise among catchments by combining estimates
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of (1) the number of invasive non-native plant species
present, (2) the potential number of invasive species
that would be present if they occupied the full range
determined by climatic envelope models, (3) the
degree of habitat loss in rivers and (4) the degree of
water stress, i.e. the difference between supply and
demand for water. Over a third of the catchments in
South Africa were in the highest priority category,
implying they scored high on all four estimates.
Substantial differences were also found between the
prioritisation and the current pattern of financial
allocation of resources for plant control, demonstrating the potential of the scheme to make plant control
efforts more effective (van Wilgen et al., 2007).
Recovery
Ecosystem recovery is central to restoration, as
ecological success depends on measurable changes
in components of the target stream or river that moves
towards the desired endpoint (Palmer et al., 2005).
While restoration often focuses on goals and endpoints of restoration, the successional pathways and
mechanisms by which these are achieved are seldom
considered (Lake et al., 2007). Successional processes,
such as facilitation, in which early colonisers modify
the environment to become more favourable for later
arrivals, may control the rate of recovery regardless of
whether target endpoints will be achieved or not.
Lake et al. (2007) distinguish four models of community recovery following restoration: (1) The rubber
band model, in which recovery is rapid and complete,
(2) the hysteresis model, in which recovery is slow,
e.g. because of constraints on species recolonisation,
but eventually complete, (3) the Humpty–Dumpty
model, in which recovery is incomplete and may
follow various trajectories and (4) the shifting target
model, in which recovery is incomplete and a stable
endpoint lacking. Invasion of exotic species, for
example, may block recovery or set it off along a
different trajectory (Rahel, 2007; van Wilgen et al.,
2007).
As demonstrated in the paper by Hughes (2007),
recolonisation of restored sites is only likely to be
possible from within the same stream, except for taxa
with good dispersal abilities, such as insects. Thus, in
many cases complete recovery cannot be expected to
occur without interventions to circumvent dispersal
barriers. Muotka & Syrjänen (2007) studied the
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recovery of boreal streams that were channelised to
enhance timber floating. Restoration of these streams
implies returning stones and boulders to cleared
channels, to re-create natural geomorphic features in
straightened reaches, and to re-open cut-off side
channels (Nilsson et al., 2005a; Muotka & Syrjänen,
2007). Results demonstrate that without consideration
of the scope or life history of target species, recovery
may be slow or incomplete (Muotka & Syrjänen,
2007). Trout populations showed weak responses to
restoration, probably because restoration did not
increase the amount of pools, a key winter habitat.
Similarly, the recovery of benthic macroinvertebrates
is thought to have been constrained by a drop in the
abundance of aquatic mosses, primarily Fontinalis
spp., caused by mechanical damage during the
restoration work. Mosses provide shelter and refugia
during high flow events, and increase retention of
organic matter. Loss of mosses because of restoration
therefore represents a severe harm to stream organisms (Muotka & Syrjänen, 2007). As recolonisation of
aquatic mosses occur mainly by downstream dispersal of fragments, rapid recovery of moss cover relies
on upstream source populations. Thus, protection of
upstream source populations is likely to be crucial, as
restoration typically affects moss abundance negatively. The commonly used strategy to restore entire
streams starting in the upstream end may also need to
be reconsidered.
Streams in urban areas belong to the most degraded
ones, often polluted and with the hydrology and
geomorphology fundamentally altered as a result of
underground piping and rapid runoff from impervious surfaces in the catchment (Bernhardt & Palmer,
2007). Large sums of money are spent towards
restoring urban streams. Bernhardt & Palmer (2007)
argue that proper management of storm water is a
prerequisite for successful restoration of urban
streams, as rapid runoff of surface water may fail to
recharge groundwater, flush pollutants and sewage
into streams, and high peak discharges may destroy
installed habitat structures. Therefore, urban stream
restoration should not be undertaken unless integrated within broader catchment management strategies, and when design options are so constrained that
significantly improving ecological conditions in
streams is unrealistic (Bernhardt & Palmer, 2007).
Proper assessment of the outcome of restoration
is needed in order to determine whether target
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ecosystems have recovered as expected, and whether or
not project objectives have been met. Despite this, most
restoration projects are inadequately monitored or not
monitored at all, making evaluation of restoration
success difficult. Guidelines proposed by Woolsey et al.
(2007) to assess river restoration consist of selecting
among a suite of specific project objectives, selecting a
set of corresponding indicators, and subsequently
evaluating restoration success by comparing indicator
values that are measured before and after restoration
and are standardised to a common scale. Finally, all
standardised indicator values from before and after
restoration are averaged to determine the degree of
overall restoration success. A case study on a restored
reach of the Thur River in Switzerland is presented to
illustrate this proposed scheme (Woolsey et al., 2007).
Lessons for practitioners and challenges for
research
Restoration of ecosystems is likely to pose a major
challenge for ecological science in the coming decades, as pressure on ecosystems to provide ecological
services will increase with the growing human population density and in the face of challenges imposed
by global warming and other large-scale environmental changes. These pressures and challenges call for
ecologically sustainable water management (Bernhardt et al., 2006), including restoration of degraded
streams and rivers. The papers in this special issue
present many lessons for future restoration projects, of
which we mention a few to illustrate the point:
• The importance of acknowledging the scope of
target species, so that all relevant stages in their life
cycle are considered (Lake et al., 2007; Muotka &
Syrjänen 2007).
• Integration into project plans of strategies to
ensure the return of target species, as species that
have been lost from a stream cannot be assumed to
recolonise spontaneously (Hughes, 2007). Better
understanding of the potential for spontaneous recolonisation and the opportunities for reintroductions of
species following restoration is needed.
• The importance of increasing habitat diversity
and quality by restoring ecological processes. This
often implies improving the connectivity of rivers by
reintroducing aspects of natural flow regimes. For
example, longitudinal and lateral connectivity may be
enhanced by reintroducing floods (Stromberg et al.,
2007), and vertical connectivity may be restored by
increasing the exchange between surface water and
groundwater flow (Boulton, 2007). Future studies
should establish what aspects of natural flow dynamics are needed to regain a specific function or species.
• Consideration of the best possible location for
restoration projects to maximise the desired response
(Johnson et al., 2007). The choice should explicitly take
into account the cumulative effects of many small
projects (Palmer & Bernhardt, 2006) and the potential
to restore ecosystem processes under highly constrained conditions such as below dams or in urban
settings (Bernhardt & Palmer, 2007). This will require
better understanding of how far the positive response
of a restoration action extends.
• Integration of the effects of invasive exotic species
into project plans, either to minimise the impact of
exotics, or to modify the expected outcome of restoration in cases where extirpation is impractical (Rahel,
2007; van Wilgen et al., 2007).
• Ensuring proper assessment of restoration success (Woolsey et al., 2007) and thus enable adaptive
management and learning for the future from both
successful and unsuccessful restorations.
Acknowledgments
We are grateful to Special Issues Editor Mark Gessner
for helpful assistance in producing this work, and to
the many reviewers of the manuscripts included in
the special issue. The Swedish Research Council
Formas, the Kempe Foundations, The Swedish Research Council, the World Wide Fund for Nature,
Sveaskog, the Wenner-Gren Foundations, the Swedish
Environmental Protection Agency and the Swedish
University of Agricultural Sciences provided financial
support for the symposium.
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