Research articleBalancing biodiversity outcomes and pollution management in urban stormwater treatment wetlands
Introduction
In 2015, 54% of the world's population lived in urban areas, and this is predicted to be closer to 70% by 2050 (WHO, 2017). The development of infrastructure to support this growth places considerable demand on aquatic ecosystems (Chin, 2006), and changes the nature and distribution of wetlands around cities (Kentula et al., 2004). Urbanisation causes a raft of effects on the biological, chemical and physical characteristics of aquatic environments (Walsh et al., 2005). One of the major changes is an increase in stormwater runoff from impervious surfaces (e.g. roads, buildings) connected to waterways. Stormwater runoff can alter the hydrological regime in receiving waters with subsequent impacts to stream geomorphology (Vietz et al., 2016b), and contain a diverse range of pollutants (e.g. nutrients, metals, herbicides and hydrocarbons: Malaviya and Singh, 2012).
One of the most common methods to manage the impact of urban run-off on receiving environments is to construct stormwater treatment wetlands (Malaviya and Singh, 2012), with tens of thousands of these built in residential, commercial and industrial urban areas throughout the world (Tixier et al., 2011). These wetlands have been shown to effectively treat pollutants in stormwater both in specific case studies (e.g. Al-Rubaei et al., 2017, Schulz et al., 2003) and more general assessments (e.g. for the UK; Lucas et al., 2015). Although not the primary function of these treatment systems, it is common for animals to colonise stormwater wetlands (Hassall and Anderson, 2015, Tixier et al., 2011). In some circumstances, stormwater treatment wetlands can be important habitats (e.g. Holtmann et al., 2018, Holtmann et al., 2017), especially since one of the major challenges facing animals in urban landscapes is habitat loss and fragmentation. Recognising the importance of unconventional habitats like these wetlands may be an important component of urban biodiversity conservation (Soanes et al., In press).
Urban stormwater wetlands can be suitable habitats for some animals, with comparable biodiversity and community structure to natural lakes (Stephansen et al., 2016). While the pollutants that accumulate as part of the stormwater treatment function may not have deleterious effects on some species (Søberg et al., 2016), they have the potential to cause a range of lethal and sub-lethal effects for others. For instance, experiments in the US have shown that exposure to pond sediments with elevated heavy metal and salt concentrations can kill frog embryos, and cause sub-lethal effects such as reduced size at metamorphosis in other species (e.g. Gallagher et al., 2014; Snodgrass et al., 2008). A recent global assessment indicated that while measures of abundance and species richness in stormwater wetlands can be comparable to natural wetlands, the fitness (i.e. growth, survival, reproduction) of animals is often decreased (Sievers et al., 2018a). There appears to be a conflict between the engineered purpose of these wetlands as stormwater treatment systems, and their incidental use as habitats by wildlife.
Understanding the potential ecological costs and benefits of urban stormwater wetlands depends on knowledge about how the presence and fitness of animals that inhabit them varies. If poor fitness outcomes are associated with occupying certain wetlands, the consequences will be exacerbated if animals do not avoid these habitats. From an evolutionary perspective, animals should choose habitats where their fitness is high, but some mistakenly choose habitats where their fitness is reduced – a situation known as an ecological trap (Robertson and Hutto, 2006). Many animals use indirect cues associated with likely future habitat quality to select habitats (e.g. birds use vegetation to predict future food resources or predator densities; Cody, 1985). Ecological traps arise when these cues are poor predictors of habitat quality (Robertson and Chalfoun, 2016, Robertson and Hutto, 2006, Robertson et al., 2013). Perhaps the clearest example of an ecological trap is when insects are attracted to polarised light reflected from artificial surfaces (e.g. street lamps, sides of buildings, roads) (Horvath, 1995, Horvath et al., 1998).
Ecological traps can also arise as unintended consequences of management activities and pose a serious but largely unexplored conservation risk (Battin, 2004, Hale et al., 2015a, Hale and Swearer, 2017). Traps could cause local extinctions if animals that colonise them are unable to breed or survive. Traps may also increase the risk of regional extinction by attracting animals away from high quality sites and into those where their fitness is reduced (Hale et al., 2015b).
There is a strong conceptual basis for predicting that some stormwater wetlands are ecological traps given the potential conflict between their intended stormwater treatment purpose and their use by animals (Hale et al., 2015a, Tilton, 1995). There has, however, been limited work examining how urban stormwater treatment wetlands function as habitats for animals, and less on whether they are ecological traps. Being able to identify wetlands where animals have low fitness and especially those that are traps is critical if impacts to susceptible animals are to be managed. It is equally important to know where high quality wetlands occur to help target conservation and management efforts, since these wetlands may enhance connectivity and increase biodiversity in urban areas where few natural waterbodies remain.
It may be possible to plan management activities to prevent ecological traps arising in the first place, but in instances when ecological traps have arisen as unintended consequences of management actions there are several steps to identify and mitigate their impact (Hale et al., 2015a). The first step is to assess the risks that management activities pose to animals (e.g. how will environmental conditions change and where will changes take place relative to the distribution of animals) and preventative measures such as alternative treatment systems that are less likely to attract animals (e.g. biofilters, raingardens, rainwater tanks) or provide off-line habitat more suitable for animals. The second step involves testing if an ecological trap has formed, requiring information about how the fitness of animals is affected by habitat changes, and the habitat preferences of animals (Robertson and Hutto, 2006). If an ecological trap has formed, the third step is to mitigate its effects. This could involve improving habitat quality (e.g. remediating sediments to reduce pollutant loads) or reducing the attractiveness of habitats (e.g. removing cues, such as particular types of vegetation).
For the past four years, we have been conducting research to test how stormwater treatment wetlands perform as habitats for native aquatic fauna around the city of Melbourne, in south-eastern Australia, and to examine if some might be functioning as ecological traps (step 2, above). Our aim here is to use our empirical findings to highlight key considerations for agencies involved in stormwater management in terms of managing these wetlands for biodiversity outcomes in relation to steps 1 and 3, above. Given that stormwater wetlands are being constructed in most cities globally, we hope this case study and the considerations that have arisen subsequently will fuel discussions and research on the critical knowledge gaps surrounding the ecological impacts and opportunities of constructed wetlands in urban landscapes.
Section snippets
Study system
Melbourne is the capital city of the state of Victoria in south-eastern Australia, with a population of 4.85 million people (ABS, 2018), and is an ideal case study system to examine how stormwater wetlands might perform as habitats for animals for several reasons. First, as Melbourne has expanded over the past 30 years, the increase in stormwater wetlands has been substantial, from fewer than 50 in the mid-1980s to over 700 currently (Hale et al., 2015a, Sharley et al., 2017). Therefore,
Stormwater wetlands can be ecological traps
The potential for stormwater wetlands to be ecological traps was first raised in the mid-1990s (Tilton, 1995), but our research provides the first empirical evidence that this is the case for both fish and frogs. Furthermore, the mechanisms causing these ecological traps were different for the two taxonomic groups. Measures of frog fitness (e.g. tadpole survival) was lower at more polluted sites (Sievers et al., 2018c), but other environmental characteristics, in particular vegetation and
Conclusions
Stormwater run-off is a major environmental issue for aquatic ecosystems in urban areas, and stormwater treatment wetlands can be an effective pollution management tool. However, stormwater wetlands are often used incidentally as habitats for animals. This can be beneficial for the persistence of some species in fragmented urban environments. However, we have demonstrated that there is the potential that animals that colonise these wetlands suffer deleterious impacts such as from accumulated
Role of the funding source
We acknowledge funding from the Australian Research Council (LP140100343) and Melbourne Water. R. Coleman is an employee of Melbourne Water and contributed to the development and preparation of the article, and the decision to submit for publication.
Competing interests
We have no conflict of interest to declare.
Acknowledgements
This work was supported by Melbourne Water and the Australian Research Council under project LP140100343. We thank the following people for attending a workshop to discuss this project, and for providing feedback on an earlier draft of this manuscript: Alison Rickard, David Reginato, Bronwen Hutchison, William Steele, Trish Grant, Birgit Jordan, Rachael Bathgate, Daniella Gerente, Arezou Houshmand, Leon Metzeling, Chris Walsh, Dennis Corbett and Michael Scroggie. Thanks to Rachael Bathgate for
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Current address: Australian Rivers Institute, Griffith University.