This section looks at the factors of size and isolation in urban green space. In general, the number of species present on a site increases with increasing site area, but eventually reaches a point where few new species are added. However, various studies suggest that although there is some correlation between species number and the size of urban green space sites, factors other than site size may be equally important. These factors include habitat diversity, management and use, site history, vegetation structure, topography and location.
Small site size is not a problem for most plant species, which do not require a minimum viable area, and the mosaic of many small urban green spaces supports a very wide range of plant species, many of which can disperse to unconnected sites.
Given the right conditions of soil, moisture and climate, many plant species have the potential to survive in urban areas, and have the ability to colonise new areas. Plants with very specific soils needs, such as heather, can survive in small, isolated patches, but will never develop the full range of invertebrate and other fauna supported by large tracts of heathland.
For most animal species, factors other than site size are more important. In general, habitats which combine a varied structure with continuity of management tend to have the highest number of species. For invertebrates of limited mobility, continuity of habitat from year to year is of prime importance, as they find it difficult to move to new sites, and unlike plants, they cannot lie dormant as seeds until conditions become suitable. Small mammals and some invertebrates need long grass and clumps of vegetation which provide cover up to about 500mm above the ground. The presence of the complete range of woodland layers, together with clearings, glades, damp ground and dead wood habitats, tends to maximise the number of bird and invertebrate species. Breeding birds in woodland need dense undisturbed thickets and other refuges.
The urban fauna includes many generalist species, which are those that can thrive on habitat diversity and do not need large areas of habitat. Urban areas cannot hope to support species whose requirements are for large tracts of similar habitat, such as woodland, heathland, rough grassland or wetland, or those species that resent human disturbance.
This suggests that even small green spaces and sites can have some value for wildlife. The aim should be to create a matrix, connecting if possible, of small sites, gardens, hedges, plant-covered walls and roofs, nest boxes and feeding stations. All green spaces are valuable and no opportunity should be passed by to create or preserve them. Even very small areas of relict habitat are of importance and need preserving, if necessary by maintaining existing management to prevent succession. Newly disturbed ground, of however small an area, has potential to develop an interesting flora and fauna, and does not necessarily need management. Larger areas of disturbed ground also have the potential to develop a range of interesting habitats, but requirements of site appearance and use usually dictate intervention. Within large sites, habitat diversity can be produced, which encompasses both continuity and change.
Corridors
Hedgerows, walls, ditches and other linear features can be of wildlife and landscape value, and if primarily required as boundary features, create a bit of ‘extra’ habitat if constructed and managed sympathetically. Other linear features include roadside verges, railway embankments, canals, riverbanks and strips of woodland, common and meadow land. Their functions as corridors along which species can spread are of varying value. Some plant and animal species are so particular in their requirements that unless precisely the right conditions are present they will not spread. Mobile species tend to be able to move across ‘hostile’ areas, and corridors have little effect. Linear water features have obvious advantages for the spread of aquatic species and amphibians. Most noticeable, if sometimes unwelcome effects are where introduced species spread rapidly along corridors, particularly road, railway, canal and riversides. Examples include Oxford ragwort, Himalyan balsam, Japanese knotweed, mink, zebra mussel, American crayfish and zander, an Eastern European species of fish now present in the Fens.
The current advice (Are habitat corridors conduits for animals and plants in a fragmented landscape?, English nature, 1994) is that corridors should be preserved, enhanced or created, where cost-effective, as corridors permit certain species to thrive where they otherwise would not. Corridors should be as wide and continuous as possible, with the habitat managed to match the requirements of the target species. However, where resources are limited, alternative measures to conserve biodiversity may have priority.
Corridors along disused railways or linking areas of amenity land are of great value for recreational walking and cycling, which can combine with nature conservation management. Creation and management of corridors in urban areas should perhaps concentrate on this use, for which grant aid and other support may be obtainable.
Very small sites
Even very small areas of only a few square metres can be of value, and are worth conserving or creating. In general, existing patches of scrub or groups of trees should be protected, and areas of boggy ground left undrained and kept clear of any encroaching scrub. Rough grass with tussocks can be invaluable for some invertebrates. Small areas with restricted existing wildlife interest can be planted with nectar or berrying perennials and shrubs which provide ‘feeding stations’ for butterflies and birds. Small ponds attract invertebrates and birds, and with suitable cover nearby can be home to frogs and smooth newts. Small nature gardens, attached to schools, hospitals and old people’s homes have particular value because of the use they receive, by those people who may otherwise have little opportunity to experience gardens or wildlife.
These very small areas can be self-contained, useful and viable, but are of much greater wildlife value if they are part of a larger green area, such as a park or school grounds, as there is a greater chance of species being available to benefit from the new habitat created.
Medium-sized sites
These are sites up to about 2 ha in size, and include many school and other institutional grounds, small parks, and types of semi-natural green space described above. Many of these sites have potential for sustaining a variety of habitats and uses which will increase their value for wildlife and community use.
Large sites
These include large areas of woodland, commons and wetlands, which may need some management to improve public access and habitat quality and variety. There are also large areas of wasteland and derelict industrial sites with potential for sympathetic management to retain existing wildlife value, whilst improving appearance, accessibility and value to the community. Many formal urban parks are underused and expensive to maintain by traditional forms of park management, and have potential to include different uses and types of management.
Large sites have their own microclimate, which is significantly different from the surrounding built up areas, having higher humidity, reduced turbulence, and lower temperatures. Larger sites also have a greater capacity to intercept noise and airborne particles, and to absorb water and reduce the rate of run-off. However, as trees have a major role to play in these actions, the amount and type of woodland cover is likely to be more significant that the size of the site.
