The impact of Green Roofs on the Urban Ecosystem

In 2008 the percentage of the global population living in urban areas became 50% and it is predicted this will be 70% by 2050. More and more people are moving from rural areas to cities, which is called urbanisation(1). Also, the human population is still growing, resulting in even more people living in cities. Urbanisation affects the environment in numerous ways. In fact, urbanisation has induced a major loss of natural habitats, which has led to the loss of biodiversity(2). Nevertheless, urbanisation has created a new and unique ecosystem. A new field in biology arose: urban ecology. Urban ecology studies the biology in urban areas and shows how important it is to promote biodiversity in cities. Biodiversity offers the human population many ecosystem services, also within urban areas. Therefore, strategies have been developed to preserve, restore or create green areas to counter the negative environmental impacts of rapid urbanisation. These efforts are underway because green parts of the city such as parks, gardens and ponds foster a lot of life. To promote biodiversity more green areas are created on ground level, but also on the very roofs above our heads. By creating green roofs, people are hoping to offer different species a new habitat and thereby increasing the biodiversity. The purpose of this paper is to investigate whether green roofs benefit the urban ecosystem and promote urban biodiversity. Therefore, the effect of urbanisation on biodiversity and the urban ecosystem will be explored. Secondly, the green roof system will be explained. Then, the ecosystem of the green roof itself will be investigated. Lastly, the role played by green roofs on the urban ecosystem as a whole will be discussed.

The effect of urbanisation on biodiversity

Urbanisation has a major impact on biodiversity, the degree of diversity of life. Biodiversity consists of two components: species richness, which is the total number of species, and species abundance, which measures how common or rare a species is relative to other species in a community. Urbanisation has both negative and positive effects on biodiversity, which varies according to the location in a city. The effects of urbanisation vary greatly for different groups of organisms, such as plants, invertebrates and vertebrates. To promote urban biodiversity it is important to have enough habitats for different species, which can be accomplished by creating more natural areas.

Negative and positive effects of urbanisation

The effects of urbanisation on biodiversity are both profound and complex. Firstly, urbanisation is a primary cause of the extinction of native species(2). Cities are often located in biodiversity hotspots, which are locations with a very high natural biodiversity. Much habitat has been cleared for pavement and buildings, which has caused habitat fragmentation. Another effect is disturbance: the city has a big disturbing effect on green spaces within the city or near to the city. Both habitat fragmentation and disturbance have a strong negative effect on many native species. Nevertheless, there are some species, such as the city pigeon, that can adapt to the high disturbance and therefore flourish in cities.

Some aspects of urbanisation can actually promote urban biodiversity. The arrival of invasive species can (temporarily) increase biodiversity. Humans are doing a good job at dispersing species, thereby introducing invasive ones all around the globe. Often, invasive species replace the native species and can destruct the native ecosystem, leading to a decrease in biodiversity. Sometimes, invasive species appear faster than native species disappear, which results in a net increase in biodiversity. Nevertheless, the introduction of invasive species is undesirable because of their profound negative effects on the native biodiversity.

In addition, urban areas have a high spatial habitat heterogeneity, which means that urban areas harbour many ecological niches. As a result, many different species, occasionally including endangered species, can settle in urban areas(3). Species richness in urban areas is often higher than in the rural areas bordering the city. Organisms that do not require much space to survive and reproduce, such as plants, are abundant in urban areas. Another reason for an increase in biodiversity is the fact that there is a higher primary production in the city, compared to surrounding areas. This is because there is an inflow of water, nutrients and other limiting compounds into urban areas to feed all the humans. Up to a certain point, a higher primary productivity means a higher biodiversity(4).

Overall effect of urbanisation on plants and animals

The overall effect of urbanisation on plant and animal species was investigated in an analysis in 2008)(5). The degree of urbanisation was measured by categorising land plots into different levels of urbanisation. In the study, city areas were said to have either a high, moderate, or a low level of urbanisation, using respectively, a city centre, suburb and rural area plot as a reference. It was found that there is often a net increase in plant diversity in the transition from a low level of urbanisation to a moderate level of urbanisation. In contrast, there is a general decrease in vertebrate and invertebrate diversity. This study concludes that plants species richness usually peaks at intermediate levels of urbanisation, whereas most invertebrates and vertebrates rather reside in less urbanised areas(5)

As the level of urbanisation increases, the species richness of vertebrates and invertebrates decreases. Increasing urbanisation reduces the habitat quality of the remaining vegetation and these species are more affected by fragmentation than plant species. Plant species flourish in moderately urbanised areas, partly because they need a smaller habitat to survive compared to animals. In addition, the increase of plant species at moderate levels of urbanisation is possibly related to the intermediate disturbance hypothesis, which states that moderate levels of (human) disturbance can promote the coexistence of species(6). Therefore, multiple plant species are able to coexist in these urbanised areas, whereas they normally would not. Thus, the effect of urbanisation is different for different groups of species.

Opportunities for urban wildlife

Urban biodiversity is very important because it provides invaluable ecosystem services to cities. These services include the purification of water and air, recreation and aesthetic enjoyment. Furthermore,urban biodiversity has an important role in educating the growing urban population about nature and species conservation. Finally, promoting urban biodiversity will contribute to biodiversity conservation in general(7). It is not difficult to see that it would be profitable to promote urban biodiversity. To make cities more accessible for wildlife, it is necessary to enhance the quality and quantity of green spaces and reduce the negative influence of urban areas on biodiversity. This can be accomplished in many ways, such as creating more parks and promoting the creation of city gardens(8). In addition, creating green rooftops in urban areas offers many advantages for the human population and creates more habitat, partially alleviating the lack thereof in cities. Roofs represent 30% of the total surface of cities and this space is mostly unused, so creating green roofs does not further encroach upon the precious little free surface left in cities like parks do(7). Green roofs might offer some big advantages for urban wildlife, which will be explored further in the next paragraphs.

What are green roofs?

Green roofs are much like conventional (flat) roofs, but with a layer of vegetation. Green roofs are created by mimicking ground-level vegetation which is not found on a conventional roof. The concept of copying or using natural processes is called biomimicry, which you can read up on in the article 'The rise of the biomimetic building', written by Anne Snijders and Lotte Pronk. There are different types of green roofs that will be considered together with the overall benefits of green roofs.

Intensive and extensive green roofs

There are two types of green roofs: Intensive and extensive. Intensive green roofs are roof gardens and have been around for a very long time. They are characterised by deep substrates and a variety of plantings. An example of an intensive green roof is shown in figure 1. Modern green roofs are called extensive green roofs and they are based on the concept of the roof gardens. The primary difference is that extensive green roofs have shallower substrates and require less maintenance than roof gardens. Extensive green roofs are used more than intensive green roofs because they are more practical and less costly to install. A simple extensive green roof consists of a vegetation layer, a layer with growth medium, a waterproof membrane and an insulation layer. An extensive green roof is pictured in figure 2. From this point on, we will focus on the more common extensive green roofs(3).

Figure 1: An intensive green roof.Source.

Figure 2: A green roof in Berlin. Source.

Extensive green roof substrate and vegetation

The substrate of green roofs consists chiefly of minerals, and around 10% of its weight is organic matter. Two commonly used minerals are clay granules and crushed brick, but all kinds of materials can be used. Currently, there is a growing interest in using locally obtained granular waste materials, which is more sustainable. The substrate of an extensive green roof is shallow and is less than 20 centimetres deep. This offers an economic advantage: a shallow substrate is relatively light. A green roof with deeper substrate is heavier, requires a stronger construction, and is therefore more expensive(3).

A shallow substrate results in periodic drought and rapid fluctuations in soil moisture. It follows that natural conditions on a rooftop are very different from those at ground level. It can thus be challenging for plants to grow and survive in this environment: they often suffer from drought stress. In addition, high light intensities and high wind speeds can damage the vegetation and the substrate. Only plants that are adapted to such extreme conditions are suitable for life on a green roof. Plants that are commonly used on green roofs belong to the genus Sedum, whose members are capable of growing on a shallow substrate. On 2 to 5 centimetres thick substrates, Sedum is mostly used because they are resilient to desiccation and fluctuations in temperature. Substrates from 7 to 15 centimetres deep provide a more stable environment. These roofs are capable of supporting different mixes of plant species, such as grasses and alpines, and thus support a greater biodiversity(3). A schematic overview of a conventional green roof is shown in figure 3 in which the vegetation layer, growth medium and the waterproof membrane are shown.

Figure 3: A schematic overview of a conventional green roof. The vegetation layer, growth medium and the waterproof membrane are shown. Source.

Ecosystem services

Green roofs mimic natural ecosystems to provide ecosystem services. Firstly, green roofs can store large quantities of water when storms occur. Urban areas often suffer from heavy water runoff because they are dominated by hard and non-porous surfaces. Green roofs store water in the vegetation layer and in the growth medium. During rainfall, this storage of water delays the runoff, so that it can then be gradually released when the sewage system is not running at maximal capacity. It also facilitates re-entry of moisture into the atmosphere through evapotranspiration (3).

Secondly, green roofs aid in climate control. During winter, they provide insulation. During summer, they reduce the amount of heat transferred through the roof by promoting evapotranspiration and by absorbing most of the incoming sunlight so that it does not heat the building directly. This means that less energy is needed for air-conditioning. The cooling effect is advantageous for the whole city and reduces the urban heat island effect: the fact that urban areas are significantly warmer than surrounding areas. The reason for this is that most vegetation has been replaced by dark surfaces which have a lower albedo. Green roofs can help to increase the albedo and thus decrease the urban heat island effect(3).

In addition, the vegetation on green roofs filters air and water which helps to create a cleaner environment and can improve human health. Moreover, green roofs provide aesthetic and psychosocial effects for people in cities. Research has shown that people who are exposed to more nature live a healthier and happier life, because it aids relaxation(9).

Furthermore, green roofs have a longer lifespan than conventional roofs. The waterproof membranes on conventional dark roofs are damaged rapidly by ultraviolet radiation and fluctuations in roof temperature. The vegetation layer of green roofs protects the waterproof membrane from ultraviolet radiation and helps maintain a stable temperature(3).

Lastly, and perhaps most importantly, green roofs can provide new habitats for urban wildlife, thereby increasing urban biodiversity(3). So apart from all the advantages green roofs have to offer us, such as cleaner air and water and a more efficient water runoff, green roofs might also facilitate an increase in urban wildlife. Green roofs form a new ecosystem in which different species can find a new home. But which species are we talking about, and how does a green roof function?

The green roof ecosystem

A green roof can be considered a small ecosystem, created by humans. Green roofs facilitate plant growth and are therefore useful for creating varied habitats. In the green roof ecosystem microbes, plants, invertebrates and birds form a community and this ecosystem will be considered in this chapter.

Succession and soil formation

The green roof ecosystem becomes successively more complex. Soil formation occurs, fostering a more stable environment, richer in organic carbon and nitrogen. The ecological succession the species community on green roofs undergoes is comparable to that in extreme environments(10). But what plants are chosen to begin this cycle?

Plant species

Native plants often find the extreme conditions on rooftops difficult to cope with. The previously- mentioned members of the genus Sedum are therefore often the plants of choice. Their manifold adaptations, such as drought resistance through succulence, makes them ideal candidates. In addition, these plants form a valuable habitat for springtails and their predators

While one might assume that plants native to the area are ideal occupants of a green roof, there is more to it than meets the eye. Native plants often find the extreme conditions on rooftops difficult to cope with, and many cannot abide the shallow substrate layers. Monocultures of the previously- mentioned members of the genus Sedum are therefore often the plants of choice. Their manifold adaptations make them ideal candidates. For example, Sedum plants are succulents, which means that they can store water and have a low surface: volume ratio (i.e. less surface from which water can evaporate). Additionally, Sedum roofs have a more consistent vegetative cover and retain moisture in the substrate more efficiently than roofs covered with other species. Lastly, Sedum plants are a valuable habitat for springtails (a typical pioneer arthropod) and their predators(10,11).

Despite the harsh conditions, wild plant species are also populating green roofs. A research survey on 115 extensive and intensive green roofs in France(7) identified 176 wild plant species in their plots. This study, together with other research projects, has demonstrated that the substrate depth is the most important factor structuring the wild plant diversity(12).

Figure 4: An example of a Sedum roof. Source

Biodiverse green roofs

In addition to the simple Sedum roofs, more diverse roofs are created to conserve habitat. This type of green roof has a more heterogenic substrate because of the varying depth of the growth substrate. Substrate depth variation on the same roof can establish a gradient of moisture levels across the roof. This causes more niches to open up, which may function as refuges for less drought- tolerant species, facilitating higher biodiversity(13). Also, research has shown there is a bigger arthropod, for example spider, diversity on roofs with increasing plant diversity and habitat heterogeneity(10).

Figure 5: A rooftop which is more biodiverse, with wild plant species. Source

Invertebrates

In sampling invertebrate communities, researchers have found hundreds of invertebrate species per green roof, including rare species. The question that comes to mind is how these species arrive on the roofs. Some species arrive during construction of the green roof, in the substrate, as fly larvae and beetles do, or on the plants, for example aphids and leafhoppers. Others colonise the roof later on by flying, such as bees, wasps, grasshoppers and dragonflies. Species that cannot fly can arrive by wind or by climbing up, for example spiders or ants. The most successful invertebrate species on green roofs are those who are pioneer species that can colonise recently disturbed habitats, such as springtails(10,14).

Birds

Green roofs offer an opportunity to create more habitats suitable for birds in the city. Green roofs can provide birds with water and food, such as insects, berries, and seeds. The availability of food is the most important component of bird habitats. Whether a green roof is suitable to provide nestling place for birds depends on the type of green roof, its maintenance, vegetation and design. Some bird species have very specific habitat requirements that cannot be met by green roofs(15)

In addition to birds profiting from regular green roofs, there are also cases where green roofs are specifically designed to create a habitat for certain endangered bird species. One example is the black redstart (Phoenicurus ochruros), which is rare in England. Green roofs were designed to mimic the conditions that black redstarts prefer. Using recycled crushed concrete and brick aggregate, a new breeding habitat for black redstarts was created. An example of this can be seen in figure 6(16).

Figure 6: A roof especially designed for black redstarts. Source.

Ecology-based green roof classification

Normally, green roofs are classified as intensive or extensive, which does not describe the ecosystem on the green roof in any way. Therefore, F. Madre et al. (2014)(7) designed a new classification system based on the vegetation the roof supports, which provides an indication of the ecosystem the roof creates. This new classification is displayed in table 1 and can be compared to the old classification. In figure 7 these vegetation classes with their substrate are schematically displayed. Note that the higher the vegetation, the more substrate is demanded.

Table 1: The four new vegetation classes for green roofs, classified by their vegetation types.

Vegetation class	Characteristics	Comparable to green roof type
Muscinal stratum	composed of non-vascular plants, lichens, fungi and small herbaceous plants, such as creeping succulents (e.g., Sedum).	Extensive
Herbaceous stratum	dominated by non-woody herbaceous plants such as grasses and flowering plants that can exceed 1 m in height at maturity.	Semi-extensive
Arbustive stratum	shrubs, bushes, and young trees from 1 to 7 m high.	Simple intensive
Arboreous stratum	large trees (over 7 m).	Intensive

Figure 7: The vegetation classes for green roofs schematically displayed. Source.

The classification by vegetation classes will be helpful to determine the contribution of green roofs to the urban biodiversity. As mentioned before, when the green roof has a thicker substrate, more species can live on it and more complex ecosystems can arise on rooftops. The overall contribution of green roofs to the urban ecosystem will be reviewed in the next paragraph.

Green roofs and the urban ecosystem

We have learnt that green roofs provide extra green space in our jungle of concrete and bricks. On our rooftops, species have an opportunity to settle. Because of these benefits, it is now mandatory to build a green roof when building a flat-roofed building in Basel, in Switzerland. If the building is bigger than 500m², the green roof should be manufactured with natural soils from the surrounding region and the substrate should have varying depths to support urban biodiversity(17). What then, is the drive for this obligation? What are the concrete benefits of green roofs?

Better than conventional roofs, but not as effective as ground-level habitats

Green roofs have a greater species abundance and diversity than conventional roofs, because they provide a better habitat(18). But, it is unclear whether green roofs can support as many species as ground-level habitats. There is, as of yet, a shortage of adequate comparative studies. Green roofs are vertically isolated and difficult to reach by species with low mobility. Research has shown that, in ground-level habitats, the abundance of insects is higher than on green roofs. So, the green roof is more desirable for the urban ecosystem than a conventional roof, but cannot replace ground-level habitats. For these reasons, it would be unwise to remove ground level habitats and compensate this with green roofs. Unfortunately, this is already happening in habitat compensation projects in several countries(18).

Green roofs offer potential for urban biodiversity

Just because green roofs do not support as much biodiversity as ground-level habitats, does not mean that they can't play an important role in urban biodiversity. In fact, there are several hints that they do. It has been shown that older green roofs contain more species than younger green roofs. As mentioned before, this is caused by the formation of soil on green roofs and the fact that species have had more time to find the roof and settle on it. In Hannover, new and older green roofs were compared. The outcome was clear: older green roofs were more biodiverse(10). At the moment, many new green roofs are being created. Those new roofs have the opportunity to become more biodiverse in the future, especially if designers keep biodiversity in mind and make sure their green roofs have a deep, varied, substrate layer so that more niches are created.

Furthermore, green roofs can hold rare species, especially when the roof is specifically designed for this purpose, as you have read in the example of the black redstarts in England. Other green roofs have been designed to support rare lizards(10). Unfortunately, the conservation value for rare species on green roofs is poorly documented, but they might already be effective (18).

Lastly, there is research that suggest green roofs can help species to migrate through urban landscapes. Green roofs could supplement the larger system of wildlife corridors in urban and suburban areas, along with parks, gardens and other wildlife habitats(19).

Conclusion

All in all, it is difficult to conclude the concrete contribution that green roofs make to the overall urban biodiversity, because insufficient studies have been done on the subject. More research is needed to draw hard conclusions, but it is very likely that green roofs promote urban biodiversity, especially the green roofs with a thicker substrate layer. Classifying green roofs by their vegetation cover is more informative with regards to the biodiversity they can support, as explained before. Unfortunately, most green roofs are relatively simple with a substrate layer as shallow as possible to save costs. Therefore, it would benefit urban biodiversity to promote green roofs with deeper substrate layers. In the future, more innovative green roofs could be designed that will be even more effective in hosting plant and animal species.