The Biological Deserts Fallacy: Cities in Their Landscapes Contribute More than We Think to Regional Biodiversity

Abstract

Cities are both embedded within and ecologically linked to their surrounding landscapes. Although urbanization poses a substantial threat to biodiversity, cities also support many species, some of which have larger populations, faster growth rates, and higher productivity in cities than outside of them. Despite this fact, surprisingly little attention has been paid to the potentially beneficial links between cities and their surroundings. We identify five pathways by which cities can benefit regional ecosystems by releasing species from threats in the larger landscape, increasing regional habitat heterogeneity and genetic diversity, acting as migratory stopovers, preadapting species to climate change, and enhancing public engagement and environmental stewardship. Increasing recognition of these pathways could help cities identify effective strategies for supporting regional biodiversity conservation and could provide a science-based platform for incorporating biodiversity alongside other urban greening goals.

Keywords: biodiversity conservation; cities; habitat heterogeneity; regional ecosystems; urban biodiversity.

Figure 1.

Abiotic and biotic differences between cities and their surrounding landscapes create strong gradients (grey arrow) in environmental and physical conditions. These gradients result from changes caused by urbanization and underlying conditions related to where cities are located. Arrows do not imply directionality in the difference between cities and their landscapes, which differ depending on locally specific conditions and on the type of gradient. Animals and plants respond to gradients in species-specific ways (blue arrows). Some species (a) are urban avoiders with populations that are primarily restricted to landscapes outside cities, (b) actively select urban areas, with populations primarily contained within city boundaries, (c) have large populations that span the urban boundary, (d) have large home ranges in which individuals move across the urban–rural gradient, and (e) are migratory and use the city or the surrounding landscape as stopover sites. Artwork by Katie McKnight.

Figure 2.

How frequently species are found in cities depends on a combination of species traits that either confer urban tolerance or sensitivity, and a response to the available resources and habitat in cities compared with the surrounding landscape. The landscapes around cities are highly variable in the degree of disturbance, habitat quality, and habitat heterogeneity, leading to variation in how cities compare with their surroundings in terms of available resources and habitat. Species that are tolerant of urbanization and for whom resources are more available in cities than elsewhere are likely to actively select urban habitats, whereas those that are sensitive to urbanization and find higher quality habitat in nonurban landscapes are likely to be nearly or entirely excluded from cities.

Figure 3.

Among plant and animal species that use cities, there is variation in the degree to which they actively select urban habitat, and in their reproductive success or survival in cities compared with surrounding landscapes. “Last chance” species, such as the San Francisco lessingia (Lessingia germanorum) and Canberra spider orchid (Caladenia actensis), may not actively select or have higher reproductive success in urban landscapes, although cities may also represent their only remaining chance for survival (Soanes et al. 2019). Species that actively select cities despite lower reproductive success, such as the Eurasian kestrel (Falco tinnunculus) and the Indian flying fox (Pteropus giganteus), may be falling into an “ecological trap.” Species that actively select and have higher reproductive output in cities can be considered urban beneficiaries, including coyotes (Canis latrans) and peregrine falcons (Falco peregrinus). Opportunity species, such as the buff-tailed bumblebee (Bombus terrestris) and gray catbird (Dumetella carolinensis), may have high reproductive success without necessarily actively selecting urban habitats. Photographs: (clockwise from top left) David Marquina Reyes, N. Lewis/NPS, Hari Patibanda, Dru Bloomfield, Tobias Hayashi, Will Elder/NPS, Alastair Rae, and Jan Arendtsz.

Figure 4.

Cities provide habitat and increase population success for some species, with implications for regional biodiversity conservation. The positive contribution of cities can be grouped into five main pathways, each of which includes several factors that vary between urban and nonurban landscapes, depending on what land cover surrounds the city. For example, lower predator densities in cities compared with the surrounding landscape can release some species from predation in urban habitats, although this effect may be most pronounced in cities surrounded by wildland areas with large predator populations. Each cell shows the hypothesized importance of each factor in the city when the city is surrounded by either agricultural/plantation or wildland landscapes. Highly modified landscapes around cities can include intensive and extensive agriculture, or plantation forests.