Habitat value
The general presumption is that habitat value of vegetation will be maximised by the use of communities that are based around native species. Again, this issue needs to be seen as a series of greys rather than black and white. If an overriding goal of a project is to provide habitat for a specific native organism whose habitat requirements are well understood, or replace a now lost semi-natural community that once occurred on a site, then native vegetation is most appropriate. However, it is also important to recognise that exotic vegetation also offers a habitat and is not a biological vacuum. Any vegetation that is structurally more complex than mown grass represents a significant habitat gain in urban landscapes, irrespective of where the species come from. This is clearly demonstrated by the research of Owen (1991) on invertebrate diversity in gardens of exotic species. In urban areas, in particular, it is generally necessary to balance habitat value with other values, for example, attractiveness and structure in relation to human preferences.
Ecological fitness and community stability
It is pointless to attempt to create plant communities that are poorly fitted to physical and biotic aspects of the site. Environment fit is, however, best considered at the level of individual species rather than the community, as some species in a stereotype plant community will be adequately fitted whilst others will not, and will disappear. The designed community only fails when sufficient numbers of the component species are poorly fitted. Where only a few species are poorly fitted, the better fitted species used will often expand into the space vacated by the ill-fitted species.
Key factors influencing gross ecological fit are site productivity in relation to the growth potential of individual species, local climate, soil moisture regimes, herbivore density and management regime. Site analyses need to identify likely levels of these factors across the site, so as to inform decisions on ground pattern, location of various plant communities and constituent species within a community. The placement of, for example, ‘dry looking’ plant communities should reflect where dry conditions occur onsite rather than were a designer might like such a community to be.
These factors are often only weakly related to whether species are native or exotic, but again operate at the level of individual species. Small, slow-growing, stress-tolerating forbs sown as part of native wildflower mixes often fail to persist on highly productive urban sites. They are competitively displaced by larger and more vigorous native species. On such sites, sown or planted species with the latter growth characteristics are more likely to survive competition for light and soil resources, irrespective of whether they are native or exotic. Management in the guise of more frequent cutting, etc., may allow stress-tolerating species to be maintained on productive sites. It is more rational, however, to select highly productive species in the first place, for example tall north American prairie species as opposed to native chalk grassland species.
