Novel riparian ecosystems
Novel ecosystems can be broadly defined as those in which historically unprecedented combinations of species occur, primarily as a result of human influences, and which interact with their environments (including artificial elements such as bridges, roads, etc.) in new ways, often exhibiting novel ecological functions.1 In heavily modified, human-dominated catchments, novel ecosystems are increasingly prevalent in riparian areas, as well as in upland areas and aquatic habitats, and are widely expected to become more common under climate change as species move in response to shifting conditions.2,3 Riparian zones are particularly prone to invasion by exotic plant species due to high levels of natural and human disturbance and connectivity, providing multiple pathways for the establishment and spread of invading taxa (Figure 1).4,5 Furthermore, exotic plants in riparian zones often act as ecosystem engineers, strongly influencing the development of novel ecosystems.4
A wide range of risks to rivers and their catchments are posed by novel riparian ecosystems including altered catchment hydrology due to greater water use by some exotic plants, increased fire risk and substantial reductions in native species richness (e.g., of understory plants, birds, aquatic invertebrates) in both terrestrial and aquatic habitats.4, 6, 7, 8, 9 Consequently, conventional riparian management strategies typically focus, often exclusively, on eradicating or controlling non-native plant species.4, 10, 11
In many cases, however, such efforts are expensive, labor intensive and ultimately futile due to continued propagule pressure via a range of dispersal vectors (e.g., birds, floodwaters) or persistent soil seed banks.11Additionally, removal of exotic plants from riparian habitats can result in perverse outcomes including post-removal soil disturbance, habitat loss for native biota as well as potentially promoting opportunities for further invasions.4,11 In many cases, re-establishment of native riparian vegetation will also be difficult or improbable following weed removal due to habitat changes resulting from other catchment modifications, e.g. river regulation, land clearing.4Eliminating non-native species from riparian zones is therefore increasingly recognized in many situations as an implausible goal as well as potentially disadvantageous. Nevertheless, considerable investments of time and effort continue to be poured into riparian weed control globally. More cost-effective catchment management is likely to be instead achieved through working with novel ecosystems by identifying their potential to support critical ecological functions and services.
Benefits of novel riparian ecosystems
While native biodiversity and ecological functions and services are often reduced in novel riparian ecosystems compared with their native counterparts,6,7,9,12 there is growing acknowledgement that exotic species in riparian zones can also support a range of ecological functions and benefits to biodiversity and people.4,11,13 At local scales, for instance, some novel riparian ecosystems may protect riverbanks from erosion by increasing soil cohesion and protecting top soil as well as performing other abiotic functions such as trapping sediments and pollutants from run-off before these enter waterways.14,15 Many of these regulating functions have catchment-scale significance as they can influence downstream water quality and ecological condition in estuaries.16 Terrestrial novel ecosystems in upland habitats, e.g. exotic gully or hillslope vegetation, may similarly contribute to catchment-scale regulation of hydrology and sediment transport, e.g. by slowing down run-off from cleared slopes.13 Furthermore, evidence suggests that positive relationships between species diversity and ecosystem function (e.g., nutrient cycling, carbon storage) hold even in novel forest ecosystems, which often harbor more species than their native counterparts.17
There are numerous examples of exotic riparian plants providing important habitat and food resources to both aquatic and terrestrial fauna. For example, exotic Tamarix in riparian zones of the south-western United States provides breeding habitat for a significant number of bird species without evidence of negative effects on subsequent survivorship.18 In degraded tropical floodplains of north-western Australia, invasive chinee apple trees (Ziziphus mauritiana) support the persistence of native rodents by excluding feral horses, which have trampled surrounding floodplain areas, limiting the development of burrow systems.19 Novel ecosystems can additionally promote the local regeneration of native plant species through mechanisms of direct or indirect facilitation, e.g. provision of shade, soil stabilization, protection from pests or predators.20 Benefits of novel riparian ecosystems to native plants may also manifest at catchment scales as a result of cascading effects, e.g. influences on seed dispersal. In sub-tropical catchments of eastern Australia, for example, novel riparian ecosystems dominated by invasive camphor laurel trees (Cinnamomum camphora) promote the dispersal and recruitment of native rainforest plants by providing habitat for frugivorous birds.21
Further catchment scale effects of novel riparian ecosystems may arise as a result of greater longitudinal continuity of vegetation along watercourses facilitated by the presence of novel patches, providing important corridors or stepping stones for the movement of wildlife – although in some cases, invasive species may impede the movement of certain fauna. Nevertheless, in many situations, increased connectivity afforded by novel riparian ecosystems can be expected to be especially important under a changing climate by allowing wildlife to move to cooler conditions (e.g., higher altitudes) in response to rising temperatures.16 Similarly, the landscape-scale role of novel riparian ecosystems as refuges for wildlife from drought and heatwaves will probably become more significant under climate change.16
In many situations, novel ecosystems are likely to be more resilient to altered environmental conditions and associated disturbances than their native counterparts as they will comprise species that have been able to colonize, establish and reproduce under altered environmental regimes, which may no longer be suitable for many native species. In riparian habitats, novel ecosystems often reflect significant changes to flow regimes and fluvial processes which favor the growth and establishment of exotic plants over those which evolved under historical conditions.5 Such novel riparian ecosystems might therefore be expected to have a greater capacity to resist, recover from or adapt to future changes, such as those associated with climate change, than those comprising native species that may have narrower environmental tolerances that have been exceeded or specific requirements that are no longer met. In turn, the ecological functions and services provided by novel riparian ecosystems will also be more resilient where these are less affected by, or recover faster from, disturbances. Fast-growing, rapid colonizers, for example, may support a range of services (e.g., soil stabilization, shading) following extreme events, such as floods or fires, which otherwise could take longer to develop.
Benefits of novel ecosystems are particularly likely to occur in highly modified catchments where exotic plants may account for a substantial proportion of riparian vegetation. For instance, novel riparian ecosystems can contribute significantly to habitat heterogeneity and complexity at landscape scales, promoting greater diversity of native species by increasing the variety of food and habitat resources available in homogenized catchments.20 Additionally, many key functions of riparian vegetation such as sediment trapping and filtration of runoff are likely to be especially critical in human-dominated catchments, which frequently have high levels of upland clearing and/or a high proportion of impervious surfaces (e.g., roads and concrete), both of which increase run-off and sediments entering watercourses.22 Novel riparian ecosystems may further deliver many of the cultural ecosystem services associated with riparian zones including aesthetic values, recreation, and shade, especially where these become more important in highly cleared, denuded catchments subject to climate change.16
Incorporating novel riparian ecosystems into catchment management
There is a growing need for catchment planning and management to deal with novel riparian ecosystems in ways that go beyond conventional practices of weed control and eradication. The inevitable and increasing presence of novel riparian ecosystems in modified catchments, as well as recognition of their significant and potential values at both local and catchment scales, demands that managers consider retaining novel riparian ecosystems to avoid perverse outcomes associated with their removal, minimize ineffective investments of time and funds, and to capitalize on their potential benefits. In some situations, more holistic approaches to management could even incorporate interventions which enhance established novel riparian ecosystems (e.g, through ecosystem engineering) to promote their capacity to deliver certain functions or services (e.g., planting or encouraging fast-growing, high shade plants). Following major disturbances such as extreme floods or fires, it may also be desirable to protect emerging novel ecosystems, or indeed to encourage their establishment, by managing other pressures or disturbance such as grazing or fire.
In those catchments heavily modified by human development, there may also be a role for utilizing or retrofitting existing infrastructure (e.g., bridges, roads) with respect to its influences on the structure, function and dynamics of novel riparian ecosystems. For example, shading provided by overhead bridges may enable the development of different novel riparian ecosystems in urban catchments under climate change, potentially providing thermal refuges for terrestrial and aquatic fauna (Figure 2). Catchment planning should therefore consider the potential benefits to biodiversity of such shaded novel riparian ecosystems so that the protection of vegetation emerging under such conditions can be prioritized. Similarly, road infrastructure that affects drainage patterns could be capitalized on so to enable the emergence of novel wetlands that provide habitat and support critical catchment services, e.g. flood mitigation.
Effective adaptive management of novel riparian ecosystems must also take into account varying objectives and outcomes in space and time. For example, retention of some exotic riparian vegetation may provide a ‘band-aid’ approach in the short-term by counteracting the negative effects of other pressures or disturbances (e.g., feral animals) until these have been addressed.19 Novel ecosystems could also be used in the short-term to promote regeneration of native plants over the long-term, or to provide habitat to native fauna while native plant communities are restored.21 At landscape scales, consideration should be given to prioritizing the retention or promotion of novel ecosystems in relation to critical functions and landscape-scale heterogeneity. For example, established novel ecosystems might be preferentially retained in riparian zones that are highly susceptible to bank erosion or in areas which promote longitudinal continuity of riparian vegetation.
Effects of climate change on regional patterns of plant distribution in riparian habitats also need to be considered. For example, the high capacity for dispersal of many riparian plants, as well as the high degree of connectivity between riparian ecosystems, could be expected to result in many plant species colonizing new habitats in response to shifting climatic conditions. Eradication or retention of new arrivals should therefore be conducted with respect to broader patterns in species’ ranges since it can be anticipated that some species may become extinct in their historical range, their survival ultimately dependent on their capacity to colonize new areas. For this, as well as many other reasons, coordinated monitoring and adaptive learning across multiple scales will be critical.
We wish to thank our many colleagues and students with whom we have had many fruitful discussions and debates from which this paper has drawn.
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