Rapid urbanisation has been identified as a so-called “megatrend”, with an estimated 1.5 million people being added to the global urban population every week. While most of this growth is occurring in Africa and Asia, Australia’s rate of urban growth is among the highest in the developed world. The statutory government agency, Infrastructure Australia [1] has projected an additional 11.8 million Australian city dwellers between 2017 and 2046, equivalent to building a new Canberra-sized city every year for the next 30 years.

From a systems perspective, key environmental impacts of urban growth include an upsurge in consumption of resource inputs (energy, water, materials) and in production of detrimental outputs (air and water pollutants, waste) [2]. Further, land clearing to accommodate both outward expansion and internal densification degrades native biodiversity and supplants urban and peri-urban agriculture [3]. However, it should be noted that removal of vegetation is by no means restricted to urban development, and some authors [4] point out that land clearing remains the single greatest threat to terrestrial biodiversity in Australia. In the State of New South Wales (NSW) for example, clearing of native vegetation, predominantly for agriculture and mining, jumped 800% between 2013 and 2016. As at December 2018, 692 plant species were listed as “threatened”, representing about 14% of the total number of native plants in NSW [5].

The above provides the context for a pragmatic approach to the conservation of one particular ecological community, the Eastern Suburbs Banksia Scrub (ESBS), which still survives along some sections of Sydney’s heavily human-modified coastline. The ESBS was the first ecological community to be listed as endangered under the NSW Threatened Species Conservation Act 1995; this Act has been superseded by the NSW Biodiversity Conservation Act 2016, under which the ESBS is now listed as critically endangered. The ESBS has also been listed as endangered under the Australian Federal Government’s Environment Protection and Biodiversity Conservation Act 1999. Both statutes required a recovery plan which led to the preparation of guidelines and management plans for a limited number of sites. The remnant ESBS faces threats from further urban development, weed invasion, fire and erosion. Despite a 2004 recovery plan and a 2009 vegetation management plan [6], only 2.7% of original coverage remains, spread across four coastal local government areas (Figure 1). The NSW Threatened

Species Scientific Committee has indicated that the community faces an extremely high risk of extinction in Australia in the immediate future [7] and, as noted above, the threat to the ESBS is not unique.

Protecting endangered species in their existing habitat in conditions of rapid development is inevitably going to be difficult. One pragmatic but innovative option is to expand our horizons to consider the untapped resource of our urban rooftops. Thus, the framework for this research is conservation biology and the main objective of this proposal is to examine the little remaining vacant space in our metropolises and specifically to evaluate the capacity of green roofs (GRs) to grow and sustain key ESBS species. An additional aim is to determine the ability of GRs more generally to help conserve endangered plant communities and species, acknowledging that there is a limit to the size of plants suitable for rooftop planting. In this way, it may be possible to shrink the human ecological footprint to a small degree.

Multiple environmental, social and economic benefits connected with GRs are widely reported in the literature, as summarized in Table 1. Several landmark studies have been conducted in the past including those by [8] who modelled the monetary benefits of installing green roofs in Toronto; the Centre for Neighbourhood Technology and American Rivers (CNTAR) which also focused on quantifying benefits [9]; the researchers [10] who compiled a professional guide to the design, installation and maintenance of green roofs; and the United States’ General Services Administration (USGSA) [11]. The USGSA study was a meta-analysis of 200 research studies on the costs, benefits, challenges and opportunities of green roofs, and was accompanied by an original cost-benefit analysis and discussion of best practice. It is important to check the assumptions behind the studies that have attempted to monetize benefits. For example, [8] assumed that 100% of available roof space and all roofs over 350 m² would be vegetated. Other research includes a study on stormwater quality from the UK [12]; a “state of the art” analysis of environmental benefits [13] and a white paper on the

potential unintended consequences of reflective cool roofs, increasingly proposed as an alternative to more costly green roofs to mitigate urban overheating [14].

Collectively, the GR benefits cited in the international literature are wide-ranging. Reduced stormwater runoff together with improved water quality are common findings, although [12] as a rare exception, found high concentrations of heavy metals in runoff from an old GR in Manchester, England. Microclimatic effects include a contribution to reducing the urban heat island [13] [15] [16] and [17]. Using advanced simulation techniques, [15] found that 30% greenery coverage of the Darwin CBD (including tree canopy as well as GRs) would reduce local maximum temperatures by 2.66˚C and 2.41˚C with 5 m/sec NW and SE winds, respectively. Installing green roofs alone on all buildings would result in a maximum temperature drop in the CBD of about 0.49˚C and 0.53˚C for the same wind speed and the same two wind directions respectively. These are not dramatic reductions and they lessen at lower wind speeds. On the other hand, researchers [18] using a heat index comprising temperature and relative humidity, found that a combination of green roof and green wall retrofits offered a distinctly helpful role in attenuating interior heat stress in residential buildings.

Green roofs reduce cooling and heating loads for the floors immediately beneath the roof [19] and [20] and some particulate atmospheric pollutants are adsorbed, while greenhouse gases as well as other gaseous pollutants are absorbed [21], CO₂ being removed through photosynthesis. Biodiversity improvements are indisputable compared with conventional roofs. However, biodiversity protection and conservation of endangered flora have rarely been addressed in the GR literature compared with the research carried out on stormwater quality and detention, on building energy savings, or indeed on faunal biodiversity [22]. With the continued pace of research on GRs, however, there are recent signs of researchers investigating the capacity of the artificial environment of green roofs for biodiversity protection and conservation of endangered flora, as noted in Table 1.

Other somewhat less conspicuous benefits of GRs include noise attenuation [23], the potential for urban agriculture [24] and enhanced roof membrane durability. In the last case, the USGSA [11] suggests that a conservative estimate puts the average life of a GR membrane at 40 years compared with 17 years for a conventional roof. Green roofs’ role in providing additional passive recreational space in dense urban settings is also valuable [25] and [19]. Site visits to several GRs in Sydney demonstrate that some are used as pleasant settings for passive relaxation. Figure 2 and Figure 3 are typical examples. It may be that one motivation of owners/developers is to provide social benefits on what would otherwise be an unused space. The perceived value-add for leasing space in the building may also have prompted some owners to absorb the extra construction and maintenance costs of GR installation.

There are likely to be significant differences in outcomes from extensive as distinct from intensive roofs¹ for the benefits listed [13] [26]. Moreover, GRs need to be implemented on a large scale for bio-physical benefits to be appreciable and some of those benefits could be obtained by other means [27]. For example, while the CO₂ sequestration capacity of a 500 m² extensive Sedum spp. green roof is measurable, it is negligible compared with planting a single medium size tree [9]. Furthermore, reducing roof temperatures through planting could be achieved equally effectively by using a high albedo surface or “cool roof” technique like reflective paint [28]. Recent research from Lawrence Berkeley National Laboratory using a 50-year life cycle costing framework, suggests they could cool our urban areas three times more effectively than green roofs per unit area [29]. On the other hand, [14] point to a series of unintended consequences associated with reflective roofs and pavements such as glare, the health impacts of higher levels of UV radiation and possibly reduced local precipitation.

Regarding the societal goal of reducing atmospheric CO₂, GRs can reduce energy consumption in a low rise building as well as sequester carbon in plants and substrate, but there is a distinct carbon cost involved in installing GR components and maintaining the installation during their lifecycle. Japanese research [61] calculated that the carbon payback time ranged between 5.8 and 15.9 years across two scenarios. Most life cycle elements were encompassed by the researchers and the shortest payback period was represented by an irrigated plant, Festuca arundinacea (Tall Fescue).

Australia’s embrace of green roofs

Green roofs in contemporary building design have a long history with precedents from Modernist architecture, such as the GR on Le Corbusier’s Villa Savoye (circa 1929-1931), conceived as outdoor rooms extending the living space of the house [62]. During the 1970s and ‘80s, France and Germany pursued the installation of GRs vigorously, reflecting a growing concern with sustainable building technologies. In the last 20 years, this interest has spread to Canada, the USA and the UK, with some installations gaining world-wide acclaim, such as the 4300 m² installation retrofitted to the roof of the Chicago City Hall in 2001 [43] and the GR integrated into the design of the California Academy of Sciences Building in San Francisco in 2008 [63]. Significant drivers in parts of Europe and North America have been legislative requirements and a variety of incentives provided by governments to promote the inclusion of green roofs and walls in new construction and encourage retrofits to existing development [64].

Internationally, subsidies and non-financial support vary widely and often carry pre-conditions, for example in Chicago, where biodiversity specifications must be met before floor area bonuses are made available. Toronto’s by-law mandating GRs is accompanied by grants of $75/m² up to a maximum of $100,000 (http://www1.toronto.ca/wps/portal/contentonly?vgnextoid=3a0b506ec20f7410VgnVCM10000071d60f89RCRD) while in New York City, property tax credits of US$45/m² up to US$100,000 are available providing the GR covers a minimum of 50% of available rooftop space. The city is hoping to greatly expand the coverage of GRs through draft legislation presented to the NYC government in July 2018. The intention is to promote vegetation, photovoltaics and small wind turbines either in combination or individually to secure environmentally responsible activity on the City’s roofs (https://www.amny.com/news/green-roof-nyc-1.19912433).

In Australia, GRs have been less popular perhaps due to the perceived expansiveness of the physical environment, with its generous suburban landscape [62]. In the 1960s, a limited number of GRs were installed in Sydney, mainly on new apartment buildings, designed to extend domestic living space. A rare commercial example from that era is the 1967 Reader’s Digest Building, featuring an intensive GR with exotic and Australian native plants, included as an outdoor ‘retreat’ for employees of the company (Figure 2, above).

Research on the bio-physical benefits of GRs has been conducted at several Australian universities, with the University of Melbourne emerging as a major centre, focusing on GR substrates, use of Australian native plants and temperature modification of building interiors [65]. This work is supported by Australia’s first dedicated green roof demonstration, training and research facility (http://thegirg.org/burnley-green-roof/), established in 2012. The University of Western Sydney has completed research on stormwater quality and the evaluation of a mix of exotic and native plants and substrate mixes has been completed [66]. Researchers from the University of Technology, Sydney have examined the suitability of campus roofs for greening [67] and [68] as well as their urban agricultural potential [69]. Lastly, there is UNSW’s exploratory move towards re-establishing a pair of derelict GRs on campus.

At local government level in Australian capital cities, policies are pitched at enabling GRs/GWs rather than prescribing them and any focus on biodiversity conservation is completely absent. The City of Sydney has a green roof policy and implementation plan [70] [71] with installation guidance and offers fast tracking of development applications for developers who wish to install rooftop vegetation. As reported in the Sydney Morning Herald on January 19, 2018, Sydney has 53 green roofs and an undisclosed number of green walls (https://www.smh.com.au/national/nsw/missed-opportunity-for-green-roofs-as-sydneys-apartment-boom-continues-20180118-h0k8pu.html).

The City of Melbourne and three other inner-city councils [72] have produced the Growing Green Guide, technical guidelines to encourage the transformation of Melbourne’s roofs and facades into vegetated, leafy habitats [73]. The program identifies prime sites for the future development of GRs in inner metropolitan Melbourne, but the emphasis again is on encouragement, not concrete incentives, according to the City of Melbourne’s 2017 update on greening the city, with green infrastructure still apparently viewed as an added extra rather than a necessary part of building design [74] and urban enhancement.

In the remaining four capital cities, Adelaide City Council points out that roof design in new developments should be structurally capable of facilitating sustainability functions such as GR installation (South Australian Department of Planning, Transport and Infrastructure 2012) [75]. Similarly, Brisbane’s Plan for Action on Climate Change and Energy contains recommendations for market-led GR installation. It is not accompanied by any form of incentive [76] although a 2018 proposed change to Brisbane’s City Plan will encourage developers to include rooftop gardens by exempting GRs from building height limits (https://theurbandeveloper.com/articles/developers-encouraged-to-incorporate-green-roofs-as-part-of-planning-changes-).

One of the smaller municipalities in the Perth metropolitan area has a water sensitive urban design policy in which green roofs are listed as a contributory element [77] and Darwin has a fleeting reference to green roofs in general discussion in its master plan [78]. However, a recent report commissioned by the city [15] presents convincing evidence that counterbalancing high ambient temperatures and the impact of urban heat islands is entirely feasible by adopting mitigation strategies like cool roofs and pavements, green roofs and urban greenery, shading and the use systems like water sprinklers and fountains [15]. Lastly, the local government of Australia’s national capital of Canberra also refers to the value of green infrastructure and green roofs [79].

Natural ecosystems provide a critical range of services such as food, fuel, and timber; purifying air and water; sequestering and storing carbon; detoxifying and decomposing wastes; regulating climate; regenerating soil fertility; and pollinating crops. These ecosystem services have been estimated to be worth trillions of dollars annually [80] [82]. There is consensus in the scientific community that environmental degradation and destruction of many of the Earth’s biota, is taking place on a catastrophically short timescale [82] [83] [84] [85]. Urbanization is a particularly significant threat but at the same time, urban green space, offers many opportunities for biodiversity conservation if it is managed with this objective in mind [57]. Ecological restoration [86] aims to slow the rate of species extinction and ecosystem service decline with two methods in particular. They are the conservation of currently viable habitat, and the restoration of degraded habitat, both forming part of the four-tier urban ecological hierarchy established by [87] for New South Wales. Actions might include erosion control, reforestation, removal of non-native species and weeds, revegetation of disturbed areas, and the reintroduction of native species. Also, part of the hierarchy is a third technique, the creation of new habitat, for example by using spaces previously uncolonized by the target species or indeed, any plant species at all, for example the roofs of buildings, the subject of this research proposal.

One researcher [88], noting a lack of basic information on how green roofs contribute to biodiversity, investigated the diversity of beetle communities, finding that it is the nature of the vegetation employed, in this case meadow grass species rather than forbs, that promotes faunal biodiversity, not so much the placement, age or height of the roof. However, [89] inventoried 51 GRs in Helsinki, Finland and found that substrate depth was a critical factor in structuring plant communities and vegetation abundance. It also was apparent that roof age was highly influential in structuring vegetation [89]. The plant communities changed from young sedum-moss dominated roofs and meadow-species communities chiefly characterized by the presence of sedums, into moss-dominated or almost pure meadow-species communities on older roofs. Meanwhile in northern France, [90] installed green roofs consisting of 176 vascular plant species, 86% of which were indigenous, across 115 roofs. They tested several variables, also finding that plant diversity was strongly related to substrate depth as well as green roof age, its surface area and height above grade, and even maintenance intensity at building scale [90].

While not formally endangered, bees are critical to human food security and their population is declining rapidly in some parts of the world. Field research in Illinois [32], indicated in the USA that urban green roofs may enhance populations of both native and exotic bees. While not a focus of their work, [89] noted a number of plant species that were in decline as well as the presence of host plants of threatened faunal species occurring in GR habitats. For example, Hylotelephium telephium is an important food plant for Parnassius apollo, a vulnerable butterfly species listed on the IUCN Red List of Threatened Species (http://www.iucnredlist.org/). The presence of listed and declining species on the roofs supports earlier findings [91], [92] that roofs can play a supportive role in the provision of habitat for rare and endangered fauna.

Like [93], [94] have argued for a more disciplined conservation planning to ensure the representation of a region’s biodiversity by separating it from threatening impacts. Separation in the city is difficult because habitat is increasingly fragmented into smaller, numerous remnant patches [95] within a hostile matrix of urbanization. Species richness often declines too, as fragment area decreases [96]. As available land at grade diminishes, the significance of alternative types of urban green space for biodiversity conservation simultaneously grows [97], with the vacant space of flat and even moderately sloping rooftops, potentially taking on special value in inner city areas. Green roofs designed to facilitate habitat conservation on new developments could help resolve the invariable conflicts between ecosystem goals at ground level and economic aims [35], replenish space for greening and provide new opportunities for fulfilling our biophilic inclinations [98] [99]. With the majority of the world’s population now city-dwellers—89.5% in Australia according to https://tradingeconomics.com/australia/urban-population-percent-of-total-wb-data.html—many of us come into contact with nature solely within the urban fabric [100].

Green roofs can act as connectors which provide links between habitat fragments, more relevant to fauna than flora, but nonetheless the spread of organisms (and exchange of genes) between functioning fragments may occur. One of the few studies to evaluate the level of faunal connectivity between green roofs concluded that greater proximity facilitated a greater exchange of individuals between green roofs [101]. However, the habitat and corridor potential of a green roof needs to be evaluated in the context of its physical characteristics, microclimate and its relation to the urban landscape [31]. Other [102] accept that establishing a viable network of green roofs would support biodiversity by serving to shorten links between existing habitats but see this as an ideal. They caution that confounding issues of roof age, size and height above ground level, substrate depth and roof load bearing capacity as well as identifying roof tops in strategic locations to connect the fragmented networks of threatened fauna, may be difficult in practice.

Despite some scepticism about the value of corridors, the evidence from well-designed studies suggests that they are valuable conservation tools according to a detailed meta-analysis by [103]. Figure 4 shows schematically how green spaces such as backyards and green roofs could be planted with indigenous species to act as links between more significant fragments of native biodiversity. The diagram represents a progression from a pre-development condition through traditional development with parks and open space (light areas, centre image) to a situation in which backyards and green roofs (the light green areas in RH image) act more as ecological stepping stones than corridors between the parks and reserves. They are valuable in functioning as resting or foraging points for birds and invertebrates [38] and provide an opportunity—albeit small—for seeds to disperse, settle and germinate.

A citywide green roof strategy to support biodiversity conservation would also be supported by the numerous other ecosystem services offered by GRs, noted earlier in this paper. Such a strategy represents an important principle, using the ecosystem synergies provided by GRs to build on the positive aspects of city living and mitigate in a small way some of its negative aspects, such as air pollution and stormwater flows. Several other principles operate too:

l Sites which might be individually unimportant might become significant if they can be linked together into a web of habitat conservation sites;

l The greater the number of ecosystem connections, the greater the chance of robustness and resilience [105] thus strengthening the goal to improve current linkages as well as create new ones;

l Full restoration of the pre-development plant communities is ecologically unrealistic in the city (particularly on rooftops with limited load-bearing capacity), so partial depiction or indication is a key goal [106];

l Implementing a meaningful GR strategy across the roofs of private residential, business, industrial premises and government and other public buildings is likely to be slow and incremental. While government agencies may wish to be seen in supporting a formal strategy, there will be two particular obstacles to overcome regarding non-government buildings. First, there is a lack of flat roofs in the large residential matrix of our cities outside the central business districts; second, business and industry will need to be willing to undertake both retrofits and new installations. It will depend heavily on the principle of reconciliation ecology, the last principle enunciated below [107]; and

l An important principle behind reconciliation ecology is enlisting community support by householders and businesses in ecological care, with the numerous bush care groups operating in the Sydney Region offering ample evidence of community interest in maintaining our native species. Green roof installation and plant maintenance would also entail reaching a compromise between human and non-human use of urban space to support biological conservation [107]. However, a green roof conservation initiative above grade and on private property would entail significantly higher levels of collaborative management than the typical ground level spaces in public ownership tended by residents.

The research should be long term to allow plant conditions and characteristics to be monitored, and especially to observe a full cycle of seed planting and germination, growth into young seedlings, maturation, further seed generation by the maturing plants and natural germination. The research in this case would have the overall goal of supporting Objective 3 of the ESBS Recovery Plan which is to “To restore, and where practical, connect and enlarge remnants of ESBS through appropriate management” [135] and help to combat the apparent on-going loss of species and diminishing gene pool in some fragments of the ESBS [127]. Such research is also applicable broadly to green roofs and biodiversity conservation as well as offering more general ecosystem benefits and will necessitate cross-disciplinary contact among UNSW researchers as well as collaboration with external industry, government and community partners.

Planetary ecosystems provide a wide range of services both to humanity and to life in general, estimated to be worth trillions of dollars each year. Green roofs contribute multiple benefits to these services, including reducing stormwater run-off, moderating heating and cooling loads, carbon sequestration and storage, aesthetic and passive recreation opportunities and biodiversity conservation. It is the last named area that this paper is concerned about and there is growing realization that GRs may offer many opportunities for biodiversity conservation if they are managed with this objective in mind. Thus, researchers like [54] [109] and [138] are raising their objectives from simply obtaining bio-physical benefits from plants on GRs to selecting those which favour flora and fauna conservation, thus extending the function of rooftops as a vehicle for greening. At the same time, such research is showing how the difficulty of safeguarding threatened or endangered flora and fauna at ground level in our urban areas can be countered by harnessing currently vacant spaces on GRs.

In the absence of any Australian material focused on using GRs to rescue threatened or endangered flora, our research proposes to manage a currently threatened community of native Australian plants and support them using GRs. Our expected outcomes are both research and application related. Research-related findings include clarifying what optimum green roof properties might be in relation to, for example substrate thickness and biophysical properties and irrigation regimes for growing selected ESBS species. Outcomes would also include performance parameters for selected ESBS species on green roofs, including germination rates, viability, growth rates and vigour; the capacity of green roofs to provide ESBS seed orchard services and GR performance vis a vis invertebrate, vertebrate and avian faunal biodiversity.

Application related outcomes consist of the provision of seeds for distribution to the four local Councils that still have remnants in their area and to other relevant stakeholders, for example the nursery and garden industry. In the longer term, the research could result in the removal of the ESBS from the endangered communities list.

The authors declare no conflicts of interest regarding the publication of this paper.

Blair, J. and Osmond, P. (2020) Employing Green Roofs to Support Endangered Plant Species: The Eastern Suburbs Banksia Scrub in Australia. Open Journal of Ecology, 10, 111-140. https://doi.org/10.4236/oje.2020.103009