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Wildlife Habitat Relationships and Biodiversity

Assessing Wildlife Habitat Contributions of Green Roofs in Urban Landscapes: Measuring Avian community response to green roof factors

Wildlife habitat degradation is a leading cause of biodiversity loss, and largely attributed to urbanization. Green roofs have potential to provide wildlife habitat in urban areas by creating vegetation types. Vegetation structure and composition and green space cover were quantified for 12 green roofs and their surrounding landscapes in Michigan and Illinois in 2010 and 2011.

Roofs sampled included the Plant and Soil Sciences Bldg on the MSU campus, Ford Motor Company’s Truck Assembly Plant in Dearborn, MI, Haworth Headquarters located in Holland, MI, Aquascape Headquarters in St. Charles, IL, and several roofs in Chicago (Schwab Rehabilitation Hospital, Gary Comer Youth Center, Downtown Chicago Park, McCormick Parking Structure, Chicago Cultural Center, Chicago Nature Museum, Michigan Avenue Structure, and Chicago City Hall). This project evaluated if green roofs could contribute to bird conservation by providing additional green space in urban landscapes. Associated objectives were addressed by collection and analysis of data on vegetation characteristics and bird communities on and surrounding green roofs.

Analysis of vegetation on and surrounding green roofs demonstrated significant differences between intensive and extensive green roof vegetation characteristics. Intensive roofs had taller perennial and woody species, whereas extensive green roofs generally had low growing, drought-tolerant perennial or shrubs species. Analyses of bird surveys demonstrated a tendency for birds to use intensive green roofs for more time than extensive roofs. In addition, intensive green roofs appear better suited to support a greater richness of bird species and successful nesting because of increased niche opportunities in vegetation. Ground foragers were observed on intensive and extensive roofs, and those in shrub and low canopy foraging and bark gleaning guilds (6 species) were only observed on intensive roofs with shrub and/or tree cover. However, successful nesting attempts have been observed on large extensive green roofs, indicating the ability of these roofs to also support nesting activities. Wildlife species that require shorter vegetation and less woody cover may be better supported on extensive roofs.

Comparisons of vegetation characteristics provided on green roofs with those required for various native grassland bird species habitat requirements demonstrated the ability of green roofs to provide bird habitat. Green roofs may be able to support native bird species due to the ability of green roof vegetation to fulfill grassland bird species habitat requirements. In addition, 25 non-invasive, native bird species were observed on green roofs, and nearly all bird species observed in 194 landscapes and not on green roofs are estimated to have a > 70% probability to occur on green roofs. Among these bird species with high occurrence probabilities are those with populations in decline throughout the Midwest United States. These estimates coupled with the ability of vegetation on green roofs to support the habitat requirements of species in decline demonstrate the ability of green roofs to provide habitat for species of conservation concern.

Green roofs were estimated to have higher median bird species richness than in surrounding landscapes. However, comparatively low use probabilities on green roofs indicate that birds primarily use landscapes and green roofs may function as complimentary bird habitat. The high bird species richness and low use probability for green roofs also suggest that birds may use green roofs as stepping stones to traverse urban areas. Bird species present in landscapes directly surrounding green roofs appear to influence which bird species frequently use green roofs, as those on green roofs are generally also observed in landscapes.

Future research is needed to examine the effect other green roof factors (roof height, human presence, non-native plant species, landscape matrix, lack of mesopredators) have on bird use and wildlife habitat connectivity. Green roofs with greater variability in roof size and vegetation structural diversity and/or roofs with similar structure could be studied to hone in on the bird community that uses a specific type of roof (e.g., intensive, native prairie vegetation with >60% cover, mean height of 1.1 m) and the effect various green roof factors have on observed and predicted bird communities. Telemetry studies could be used to further understand how birds move through the landscape, when they are on green roofs, and where they are nesting. Pre- and post-construction bird surveys of green roofs would help explain how to achieve bird conservation goals (i.e., abundance, species richness, diversity). Wildlife managers, land planners, environmental designers and policy makers who aim to improve ecosystem function and wildlife habitat quality in urban and developing landscapes may refer to this manuscript to better understand how wildlife communities may interact with green roofs, green roof vegetation, and surrounding landscapes. Presented information could be used to help select a wildlife group to target with conservation efforts through green roof installation. Results from our study could also demonstrate the ability of green roof installations to address conservation objectives at the landscape scale.

Our research has demonstrated that green roofs have the ability to drastically increase (>300%) the amount of green space that may provide ecosystem functions (i.e., stormwater management, air pollution mitigation) and that is important for wildlife conservation. Wide-spread implementation of green roofs focused on creating bird habitat throughout urban areas could help minimize population declines of various grassland and neo-tropical migratory bird species, and promote biodiversity conservation in urban areas.  This study is not yet complete.

a bird house with a single entrance, buildings in the background
Birhouse on Chicago City Hall

A goose with several goslings
Canada goose nest on the Ford Truck Assembly Plant in Dearborn, MI

Tall grass on a green roof; apartment buliding in the background.
900 North Michigan Avenue Parking Structure, Chicago (Oct 2011)

Comparison of Insect and Spider Species Presence and Composition Across Multiple Vegetated Roof Systems

With increased development of the built environment, the natural landscape continues to be lost and along with it much of the habitat necessary for invertebrate survival. This study attempted to determine how successful green roofs are at creating invertebrate habitat for insects and spiders and to identify the characteristics associated with green roofs that make them successful for invertebrate establishment. It was predicted that some vegetated roofs would provide a more favorable environment than others as invertebrate establishment would depend on vegetation type, roof area, roof height, roof age, substrate depth, and vegetation coverage.

The principle field methods for invertebrate collection included sweep net and pitfall traps; sampling occurred over a seven-month period (April-October) in 2010. Roofs sampled included the Plant and Soil Sciences Bldg on the MSU campus, Ford Motor Company’s Truck Assembly Plant in Dearborn, MI; Haworth Headquarters located in Holland, MI, Grand Rapids Community College in Grand Rapids, MI, Aquascape Headquarters in St. Charles, IL, and the Schwab Rehabilitation Hospital, 900 North Bldg, and the Lurie Garden within Millennium Park in Chicago. All spiders and insects collected were identified to family level and organized and counted to calculate species abundance and richness, Simpson’s diversity index (D), Shannon Weiner diversity index (H’), and Evenness (E). Analysis resulted in significance between the percent of roof covered with vegetation and insect and spider species abundance and richness. The size (area) of a green roof also significantly influenced the richness of insects and spiders present. Roof height, roof age, substrate depth, and vegetation type were not found to be significant factors influencing insect and spider abundance and diversity. In all cases, green roofs provided habitat for a variety of insects and spiders.

In this study the amount of vegetative cover present on a roof was a significant determinant of insect and spider abundance and richness. This study has shown that higher richness and abundance numbers will exist on green roofs with higher proportions of vegetation cover. The size (in area) of vegetated roofs was also found to be a significant factor contributing to insect and spider richness. Abundance numbers, however, were not influenced by the size of a green roof. Between the eight study sites, three were considered to have a large area (over 20,000 m2) with the other five having a smaller area (under 5,000 m2). The high richness could be attributed to the fact that two of the three larger roofs exhibited high plant variety -- Aquascape and Lurie – therefore, resulting in increased insect and spider variety. Other characteristics of green roofs (vegetation type [diversity and structure], substrate depth, roof height, and roof age) were found to have a much smaller influence on insect and spider presence than was originally hypothesized. Within this study it is of particular interest that the vegetation cover of green roofs had more of an impact on insect and spider abundance and diversity, than vegetation type as was recorded in other studies. This might be attributed to the fact that this study categorized vegetation into the two broad categories of low-growing (less than 0.5 m) Sedum roofs compared to mixed perennial roofs with more vegetation structure. Perhaps if vegetation data from the study sites were classified into individual species counts, the results would have been different.

Although age of a roof can prove to be a significant factor when determining insect and spider abundance and diversity, it was found to have no influence within this study. This is due to lack of age difference between the study sites. The oldest green roof was installed in 2003 and the newest was installed in 2008. With a difference in installation dates of only five years among the study sites, small variations existed in the number of years available for each roof to promote insects and spider establishment. Substrate depth and green roof height also were found to have no significant impact on abundance and diversity of insects and spiders between the study sites. Even the extremes of an urban sky scraper (900 North) and limited substrate profile (Ford) were not enough to prevent invertebrates from colonizing green roofs.

Results of this study also may have been affected by the collection methods used. The two methods of sweep net and pitfall traps were effective means of invertebrate collection, but the methods did display shortcomings at particular times. On certain roofs with a low substrate profile (Ford, PSS), the pitfall traps were difficult to fashion into small enough traps to effectively place in the substrate. Collection of pitfall traps one time a month also led to an over flow of collected invertebrates on certain sites. Sweep netting was found to be a limited collection method on roofs with low growing Sedum vegetation (Haworth) or during unfavorable weather conditions (ie, high wind, overcast sky, and cold temperatures). Another method of collection, sticky traps, was undertaken with little success. Although certain green roof characteristics were found to influence the presence or absence of insects and spiders, the results cannot be asserted as being conclusive. Several of the green roof characteristics that were analyzed in the study hold the potential to influence each other. For example, if a green roof is large in size, it may be expected to have high amounts of invertebrates present, but the roof also may possess the shallowest substrate profile and undesirable vegetation, both of which also influence invertebrate habitat. The confounding of these green roof characteristics is important, and caution needs to be taken when interpreting the results. This study is not yet complete.

A pitfall trap made of plastic and wood.
Pitfall trap on the Haworth Building in Holland, MI

A butterfly perched on a purple flower.
Butterfly on roof of Schwab Rehabilitation Hospital in Grand Rapids

Brown bird with yellow face
Golden-crowned Kinglet on the Chicago Cultural Center

Two geese walking in front of a large Ford logo.
Canada geese on the Ford Truck Assembly Plant in Dearborn, MI

A green roof with several large buildings in the background.
Chicago City Hall (October 2011)

A skylight in the middle of a green roof.
Gary Corner Youth Center, Chicago

A man with a net among tall brown grasses.
Collecting insects with sweep net

A man with a plastic bag checking an insect trap.
Collecting insects from pitfall trap at Grand rapids Community College

A grassy green roof with purple flowering plants.
Aquascapes, Inc. headquarters in St. Charles, IL