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May 2011
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Ecological, Social, and Policy Factors Influencing Biodiversity on Green Roofs

 This article is a summary of a 51 page report.

By Bracha Y. Schindler

Green roofs are rising in popularity in the United States as a way to control stormwater runoff and mitigate the warming effect of heat-absorbing surfaces in cities, but green roofs can also be used to provide habitat for arthropods (e. g. insects and spiders) and birds in the urban environment, which may provide some connectivity in a fragmented landscape.

A diversity of arthropods, in particular, may improve green roof functioning by resulting in pest control and decomposition of organic materials in soil.  Green roofs are a particularly promising way to protect biodiversity in cities because they can be implemented on a large scale, and potentially provide protected habitat for animals.

Ant collected on water treatment building, Wellesley College.

In this study, I explored factors that could influence arthropod diversity and biodiversity in general on green roofs.  In an ecological study of six green roofs in the Boston area and on the Wellesley College campus from June through October, 2010, I looked for correlations between soil arthropod diversity and characteristics of roof vegetation, size, and isolation from other arthropod habitats, addressing the question: how should green roofs be designed if arthropod diversity is a goal?

Brachi on a green roof.

To determine social factors that could affect biodiversity on green roofs, I surveyed green roof builders, designers, users, and owners about their motivations to build green roofs and how they communicate with others about green roof research.  With this survey, I attempted to address the question: do people care about biodiversity and are they aware of ways to promote it on green roofs?

Finally, I examined green roof policies worldwide to determine whether governments or organizations promote biologically diverse green roof building with their regulations or recommendations.

Green roof at Alumnae Hall, Wellesley College.

In my exploration of ecological factors influencing arthropod diversity on green roofs, I found a positive correlation between arthropod diversity and vegetation cover (Figure 1), no correlation of arthropod diversity with plant diversity, and no correlation of arthropod diversity with roof size or isolation.  The results suggest that plant cover, and not diversity, is influencing arthropod diversity.

Though I found no correlation of arthropod diversity with plant diversity, this factor could be important under other circumstances, or for other green roof functions.  My study only examined species and family richness (number of species), but evenness (how evenly represented each species is) could be important as well, depending on the needs of green roof managers.

Green roof at Alumnae Hall, Wellesley College.

My results also suggest that the size and isolation of a roof from other arthropod habitats may not be important; small and isolated roofs may be as diverse in soil arthropods as larger roofs that are close to other arthropod habitats, though a less isolated roof may be more useful as a bridge between ground-level habitats.

This may be the case because arthropods could be introduced to the roof with the soil, or could be transported by organisms, such as birds, whose abundance on the roofs is not affected by this degree of isolation.

Figure 1. Correlation of mean percent vegetation cover with mean number of arthropod species in
soil samples on six green roofs.  Error bars indicate one standard deviation.

Findings such as mine would only be useful if people responsible for green roofs are interested in protecting biodiversity on green roofs and are aware of these results.  With this in mind, I surveyed 37 people who manage or build green roofs to determine the extent to which they care about biodiversity, whether they are aware of ways to promote it on green roofs, and how they might become aware of this information.

I found that, overall, protecting biodiversity on green roofs was not respondents’ top priority (Figure 2), but it was among the top priorities of about one third of respondents.  People rarely chose soil characteristics based on their impact on biodiversity, but plant choice was sometimes determined by the effects on biodiversity.  This may be indicative of a lack of knowledge about the effects of soil on arthropod diversity, or it may mean that green roof designers are more concerned with plant diversity than diversity of arthropods and other taxa.

Many survey respondents indicated that they publish findings in science journals (57% of those who publicize their findings), but the majority of respondents indicated that they get information from personal communication.  Even if green roof managers hear about the latest research relating to green roof biodiversity, they may not implement it if they are more concerned about other green roof functions, as the survey suggests.

Figure 2. Ranking of importance of six green roof functions by 37 survey respondents, with rank adjusted as described in the complete report.  A rank of 1 indicates greatest importance.  Median value, 25th and 75th quantiles, and minimum and maximum values are shown.

The current priorities of green roof managers, as indicated by the survey results, may be shaped by government policies.  The government policies I examined seemed to prioritize the stormwater management and energy conservation functions of green roofs, showing the same priorities as survey respondents.

Of the 29 city and state green roof policies that I found, only six mentioned biodiversity as a goal.

One of the green roofs at Boston City Hall.

In addition to these governmental regulations, other policies that could influence biodiversity on green roofs include standards by ASTM International for green roofs, and certifications for green building and landscaping by Leadership in Energy and Environmental Design (LEED) and the Sustainable Sites Initiative (SSI), respectively.

In cases where these governmental and non-governmental policies specify how green roofs should be designed, the policies have the potential to enhance green roof biodiversity, or, in some cases, the policies may have a negative effect on biodiversity.

Green roof at Four Seasons Hotel, Boston.

Previous studies of green roof biodiversity suggest that the use of native soil, variable soil depths, and deep soil can have positive effects on arthropods and plants.  Previous studies also suggest that intensive, rather than extensive, soil and/or vegetation also seem to have a positive effect on arthropod diversity, as may diverse native or non-invasive plants.

The results of my ecological research suggest that high vegetation cover may have positive effects on arthropod diversity.

Collected insect samples from water treatment building
at Wellesley College.

Overall, I found 17 green roof policies that address one or more of these characteristics of green roofs, and in all cases at least some of the recommendations would seem to have a positive effect on green roof biodiversity based on the results of my ecological study or previous studies.

Green roof at Massachusetts College of Art and Design, Boston.

Regulations that are aimed at increasing biodiversity can be particularly helpful with promoting arthropod diversity on green roofs.  However, the results of my ecological study suggest that standard regulations of vegetation cover, like the requirement to maintain 80% vegetation cover on green roofs in Toronto, may also be helpful.

Green roof on water treatment building,
Wellesley College Botanic Gardens.

My results suggest that vegetation cover is important for arthropod diversity, but other studies suggest that other vegetation characteristics may also be important, although many effects may be site-specific.

The results of the survey of people associated with green roofs suggest that these results need to be disseminated through direct communication and on green building forums, but communication through free online scientific journals is also important, so that the findings can be readily accessible and can be considered credible.

Green roof at Massachusetts College of Art and Design, Boston.

Green roofs can serve as an important means to protecting biodiversity in cities, but before that can happen, green roof designers need to be able to obtain information on how to do so.

Bracha Y. Schindler

Bracha Y. Schindler conducted this research in 2010 as a thesis project in Environmental Studies at Wellesley College, where she earned her BA.  Brachi is currently a graduate student in the University of California-Riverside’s Evolution, Ecology, and Organismal Biology Program.  This article is a summary of a 51 page report.  To read the full report, email

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