Regeneration sites

Network models show that a series of many small sites placed across developed regions acts like stepping stones, providing corridor-like connectivity, but empirical data is required (Lepczyk, 2017).

Urban components affect quality of ecosystems in various ways. Large, impervious structures – mainly roads, walls and buildings – ​intefere with abiotic aspects, redirecting water & air flows and absorbing & reradiating massive amounts of heat, altering the microclimate & diurnal cycle. This impacts ecosystem services, affecting native species’ survival, quality of photosynthesis and carbon sequestration.

High fragmentation into small sites creates long borders with urban components, producing high proportion of negative-impact edge-effect habitat; consequently, small sites can’t reach as high-density vegetation as is possible in broad, contiguous regions, reducing potential for biodiversity and carbon sequestration.

Contiguous habitats

Contiguous habitats provide better free movement (than ‘stepping stones’) for many species, because discontinuity presents many dangers (vehicles, lack of vegetative cover, lack of water). Large, contiguous sites diminish edge effects and enable habitat heterogeneity (Lepczyk, 2017), supporting specialist and generalist species, as well as those which require large, undisturbed areas. Site continuity also enables high-quality development of functions that support carbon sequestration, including soil quality and depth, canopy coverage, vegetation density, water retention, nutrient cycling. Non-forest habitats also sequester carbon effectively, shown in experimental comparisons between trees and heather (Friggens et al., 2020), so habitat heterogeneity is valuable and even vital. Studies on birds suggest that 10-35ha of unbroken green space is needed by most urbanised species, whilst forest species require larger areas (Lepczyk, 2017).

Isolated sites

Isolated sites, small and large, are difficult to reach & colonise and are more likely to suffer extirpations, which are unlikely to be replaced quickly. Lack of reliable corridors through urban regions to the outside world (peri-urban and rural) tends to produce sink & trap habitats.

A riverside corridor

Riverside corridor can span the city, providing continuous connectivity between rural zones, diminishing sink/trap potential. It can pass under road bridges, maintaining habitat continuity end-to-end. Sharing a border with the river means an entire edge of the corridor is open, providing species with liminal intraconnectivity along the riverbank, through open air and water, unimpeded by urban constructs.

The length required to span the city means the corridor will be very narrow, creating a large edge habitat, for example a 7.5km corridor has average width of only 40 metres. However, one edge borders with river, potentially creating complex riverside-habitat heterogeneity, broadening biodiversity and different carbon sequestration methods. Low-lying riverside land brings potential for wetter habitats, increasing carbon sequestration through faster growth of trees and other plants; and wetlands, the best carbon sinks overall (EEA, 2022).

Riverside habitats are less likely to suffer drought or fire in worsening climate, and can be irrigated easily due to river proximity.