The Ecology of Urban Habitats

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In a recent study in Atlantic Canada Lundholm and Marlin, , many of the grasslands that contributed urban species were found to be anthropogenic in nature and composed of European species that originally came from permanently open habitats such as cliffs, dunes, and shorelines Grubb, The urban cliff hypothesis predicts that a large proportion of spontaneously colonizing organisms in cities originate in rare and geographically marginal rock outcrop habitats Larson et al.

Likely because the first buildings were simply extensions of rock walls around the mouths of caves in rocky areas. It would have been easy for species originally restricted to rocky environments to opportunistically exploit the expanding rock-wall habitats created by growing human populations that built more of their own optimal habitats rock shelters as they moved out of the caves" Larson et al.

Areas with an abundance of natural hard surfaces have more extreme hydrological conditions than areas with deeper soil. Plants in these areas are forced to deal with the combined stresses of flooding and drought within the same growing season. Decreased infiltration in urban areas causes greater amplitudes of flow rates and soil-moisture availability over time—flooding occurs during and immediately after storms, but shallow substrates and water loss due to overland transport result in drier conditions between storms.

Figure 2a—2c: Natural a , spontaneous urban b , and designed c rock pavement habitats. The natural pavement is a limestone barren on the Bruce Peninsula, in southern Ontario. The designed site is a green roof in Portland, Oregon. Photos by J. It is clear that hard surfaces are responsible for several key environmental impacts of cities, and that these anthropogenic surfaces have analogs in the natural world. Why then should we not look to the vegetation of natural hard-surfaced areas for guidelines in mitigating the impacts of urban areas?

See Table 1 for references to studies describing the natural vegetation of many of the world's shallow-substrate environments. The ability of green roofs to reduce stormwater runoff and insulate buildings depends in part on the depth of the substrate and corresponding vegetation biomass.

But there is a trade-off between the maximization of environmental benefits and the minimization of costs: Increasing substrate depth adds to the cost of implementation, especially if reinforcement is required, and so roofers attempt to minimize load on the roof surface. The need to select plants that can survive in shallow substrates forces us to target specific habitat templates.

Some natural rock outcrops are largely devoid of vegetation; however, they may still support plant life where cracks, ledges, and other microtopographic features permit the accumulation of organic matter. The adoption of rock outcrop plants on green roofs would thus mimic a particular kind of outcrop—one where vegetation cover is maximized but total biomass production is limited by shallow substrate.

A typical shallow-substrate extensive green roof thus is a manifestation of a very particular habitat template Figures 2a—2c. Other aspects of the habitat template of natural rock outcrop ecosystems have also been incorporated into green roof designs. Spatial heterogeneity in substrate characteristics is a hallmark of natural rock outcrops Larson et al.

The ecology of urban habitats - O. L. Gilbert - Google книги

While most green roofs feature a uniform substrate, recent initiatives have incorporated spatial heterogeneity in the form of varied soil depths in order to increase species diversity in the vegetation and provide a greater range of habitats for invertebrates Brenneisen, Green facades can also be examined through the habitat-template lens. The vegetation that spontaneously colonizes stone walls can be drawn from a variety of habitats but is dominated by cliff and rock outcrop species Rishbeth, ; Woodell, The design of walls and other vertical surfaces determines the degree to which plants can grow on them: Building material, degree of shading, aspect, and the presence of microtopography determine the available niche space, much as they do on natural cliffs Rishbeth, ; Larson et al.

Examination of the original habitats of these species shows that they share their living space with a variety of other organisms that together constitute the "vegetation": bryophytes, lichens, and algae. These tend to be dominated by cyanobacteria, which form mats when water is plentiful. In shallow-substrate green roof systems, it is possible that these cryptogamic mats can contribute directly to the desired functions of green roofs by cooling the roof surface and retaining water.

The key driving force in plant selection for extensive green roofs has been to find plants that can survive and proliferate in very shallow soil environments. While current plantings often feature polycultures of individually selected species, there has been no work on the role of plant species diversity per se on the functioning of green roofs.

Research in other plant communities has identified the potential for larger amounts of species diversity to positively affect ecosystem functions such as biomass production, stability, and nutrient retention or absorption Tilman et al. In general, a community with more species might more completely utilize existing resources due to niche complementarity, which allows the coexistence of species that can use different forms of resources or exhibit resource consumption at different times of the year.

In a green roof context, the consumption of water by plants is likely not to be fast enough to make a difference during heavy storms, but for lighter rain events, greater plant uptake of water might decrease runoff.

On the other hand, there may be a danger of drought if water consumption occurs more rapidly in more diverse communities. The emerging green roof industry relies on a set of tried-and-true plants that can tolerate the harsh conditions of rooftops. These tend to be succulents from the Crassulaceae, or stonecrop family. A current international trend in green roof horticulture is to begin incorporating regionally appropriate native plants on green roofs e. Certain green roof functions, such as wildlife habitat provision, might also be enhanced by the use of native species.

Native insects may be more attracted to native green roof vegetation due to the provision of appropriate food sources or pollen resources. The use of native species that can tolerate harsh conditions is welcome in any urban greening project, providing aesthetically pleasing and educationally valuable biodiversity in hard-surfaced environments that are typically low in biodiversity McKinney, The design of vegetated surfaces on buildings has largely proceeded from engineering considerations, with a more recent focus on the horticultural requirements of desired species.

The growing interest in—and potential environmental and economic benefits of—using entire communities of plants on green buildings necessitates a more nuanced understanding of the habitat templates we design and the relationships between community structure, environmental conditions, and ecosystem functions. These concerns must move research on building-surface vegetation into the forefront of current progress in fundamental ecological research.

I thank Doug Larson for comments on the manuscript and discussion of these ideas. I also thank Erica Oberndorfer, Jeff Licht, Karen Liu, the members of the Green Roofs for Healthy Cities research committee, and two anonymous reviewers for critical discussion and support. Adam, P.

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The vegetation of sea cliffs and headlands in New South Wales, Australia. Australian Journal of Ecology, 15 , — Aey, W.

Why do Some Species Thrive in Cities?

Historical approaches to urban ecology. Sukopp, S. Hejny, and I. Kowarik Eds. Akhani, H.

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Photosynthetic pathways and habitats of grasses in Golestan National Park NE Iran , with an emphasis on the C 4 -grass-dominated rock communities. Phytocoenologia, 32 , — Alberti, M. Integrating humans into ecology: opportunities and challenges for studying urban ecosystems. BioScience, 53 , — Alves, R. Penumbral rock communities in campo-rupestre sites in Brazil.

Urban Habitats

Journal of Vegetation Science, 4 , — Patterns of plant species composition on Amazonian sandstone outcrops in Colombia. Journal of Vegetation Science, 15 , — Ashton, D. The ecology of granite outcrops at Wilson's Promontory, Victoria. Australian Journal of Ecology, 2 , — Bartlett, R.

Ecology Urban Habitats by Oliver Gilbert

Organization of the Niagara Escarpment cliff community. Characterization of the physical environment. Canadian Journal of Botany, 68 , — Baskin, J. A floristic plant ecology study of the limestone glades of northern Alabama. Bulletin of the Torrey Botanical Club, , — Belnap, J. Land Degradation and Development, 8 , — Brenneisen, S. From biodiversity strategies to agricultural productivity. Toronto: The Cardinal Group. Bunce, R. An ecological study of Ysgolion Duon, a mountain cliff in Snowdonia.

Journal of Ecology, 56 , 59— Burbanck, M. Evidence of plant succession on granite outcrops of the Georgia Piedmont. The American Midland Naturalist, 45 , — Granite outcrop communities of the Piedmont plateau in Georgia. Ecology, 45 , — Camp, R. Natural Areas Journal, 17 , — Catling, P. A review of the alvars of the Great Lakes region: distribution, floristic composition, biogeography and protection. The Canadian Field-Naturalist, , — Chin, S. The limestone hill flora of Malaya I. Garden's Bulletin, Singapore, 30 , — Collins, J. A new urban ecology.

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American Scientist, 88 , — Collins, S. Vegetation-environment relationships in a rock outcrop community in southern Oklahoma. American Midland Naturalist, , — Cox, J. Spatial heterogeneity of vegetation and environmental factors on talus slopes of the Niagara Escarpment. Canadian Journal of Botany, 71 , — Davis, P. Cliff vegetation in the eastern Mediterranean. Journal of Ecology, 39 , 63— Dunnett, N. Planting green roofs and living walls. Portland, Oregon: Timber Press.

Vegetation and structure and composition significantly influence green roof performance. Our designs are, therefor, a hybrid between conservation biology and landscape architecture. The method is also finely tuned to meet and reflect the ecological nuance and demands of unique and varied local landscapes.

When these designs are put to action, landscapes are spawned that are not only functional combinations of cultural and natural elements, but also bastions of biodiversity renewal. We are encouraging people to embrace the ecological beauty and wisdom apparent in natural plant communities. CUH is proud to be at the leading edge of a modern movement to usher in a new era in landscape architecture.

Native Landscape Design and Installation. A study published in Nature's Scientific Reports journal in found that people who spent at least two hours per week in nature, were 23 percent more likely to be satisfied with their life and were 59 percent more likely to be in good health than those who had zero exposure. The study used data from almost 20, people in the UK. The results remain the same, whether it was in one trip or multiple, and benefits increased for up to minutes of exposure.

The benefits applied to men and women of all ages, as well as across different ethnicities, socioeconomic status, and even those with long-term illnesses and disabilities.