Biodiversity on Green Roofs

A diverse array of wildlife, from tiny soil-dwelling insects to large birds of prey, find food and shelter on urban green roofs. Green roofs can also increase local native plant populations if native plants are part of the design or allowed to establish on their own.

Green roofs are wildlife habitat, so keep wildlife in mind when designing them. Try to use native plants when possible and avoid using pesticides. Avoid allowing pet cats out on the green roof, especially during migration. Importantly, avoid large windows in close proximity to vegetation or install bird-friendly glass treatments so that you can avoid causing bird collisions.

Birds on Green Roofs

Birds use New York City in all seasons of the year. Some birds breed in NYC, some spend their winters here, and others just pass through during spring or fall migration. In many parts of NYC, birds don’t have access to enough green space to find the food and shelter they need. Green roofs can provide habitat in areas where birds may not otherwise be able to survive.

Over 48 bird species have been recorded using green roofs in NYC. The majority use green roofs during spring and fall migration. During migration, birds travel long distances, but they need to refuel from time to time. Green roofs act as stopover sites, where birds forage on the abundant insect community found there. Many of the birds using green roofs during the migration would not usually be found in the city outside of parks.

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Credit: American Natural History Museum Bats in Central Park

Bats on Green Roofs

Bats are an important component of ecosystems and help protect people by keeping pest insect populations under control. In North America, many bat species are imperiled due to white-nose syndrome or habitat loss. Green roofs provide habitat for bats in the city.

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Arthropods on Green Roofs

Arthropods are invertebrates with exoskeletons, including bees, true bugs, spiders, and ants. Arthropods provide food for other wildlife, pollinate plants, control pest species, decomposing plant material, and cycle the soil.

The area surrounding a green roof and the connectivity of a green roof influences which arthropods are found, as does green roof design. Green roofs in NYC host a higher arthropod richness and abundance than non-green conventional roofs and can play an important role in increasing arthropod diversity across the city.

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Plants on Green Roofs

Plant diversity on green roofs is determined by the depth of the growing medium. In NYC, several native plant species are known to survive on green roofs of varying depths, and more are being tested by researchers. In addition to what is planted intentionally, new plant species will colonize green roofs, increasing plant diversity.

Introducing plants on a previously impervious roof surface creates habitat for microbes, fungi, arthropods, birds, and bats. At ground level sites, native plants provide a better habitat for native wildlife and the same is likely true for green roofs. However, more research is needed on which native plants can survive on green roofs in NYC.

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Microbes on Green Roofs

Several groups of microbes, including mycorrhizal fungi, decomposer fungi, N-fixing bacteria, and pathogens, are important for the functioning of non-constructed ecosystems, and likely have implications for green roof design and maintenance. These functional groups interact with certain plant taxa and soil properties in complex ways. The bacterial and fungal communities are different on green roofs with mixed vegetation (wildflowers, grasses, and succulents) than on single-genus (Sedum) green roofs. There is a higher proportion of mycorrhizal fungi on mixed-vegetation roofs. Sedum roofs have higher pathogen loads.

The fungal communities of green roofs are compositionally distinct from those in city parks, and only 54 percent of the green roof microbial taxa were found in park soils. Across the city's green roofs, fungal communities are geographically clustered, indicating that local factors such as edaphic variations, wind patterns, and dispersal from proximate parks may have shaped fungal assemblages.