The intrinsic value of biodiversity is a fundamental tenet of nature conservation. At a basic level, because a range of co-existing species can exploit more resources than can a single species on its own, diverse mixtures tend to out-perform any single species in terms of total biomass production. However, the greatest claim for the value of biodiversity is that diverse plant communities are considered to be more stable and resistant to change than simple systems. There are two main theoretical arguments to back this assertion (McCann 2000). One explanation is based on the assumption that as long as species do not react in identical ways to environmental variation, the greater the number of different species present, the greater the number of different responses, and that, as a consequence, variation will be smoothed out at the total community level. The second general explanation is based upon the idea that at greater diversity there is a greater chance of having species present that are capable of functionally replacing important species that may be adversely affected by external pressures, and that can therefore maintain ecosystem functioning.
Having said this, there is remarkably little scientific research evidence to fully back these claims: theory is definitely ahead of experience. It is clear that monocultures and very simple systems of low diversity are vulnerable to environmental fluctuations. But it is also apparent that chasing high biological diversity for its own sake is also open to question—certainly the notion that the greater the number of species the better (i. e. the greater the biodiversity) is not necessarily tenable on ecological grounds. The main indicators of ecosystem health and functioning, such as productivity, carbon sequestration, water relations, nutrient cycling and storage, and resistance and resilience to environmental change, are primarily dictated by the performance of vegetation dominants (i. e. those species that contribute the greatest amount to the total biomass of the community) and these are likely to be relatively few in number (Grime 1998), perhaps only 20-25% of the total numbers in a plant community (Schwartz et al. 2000). So do the remaining species have any ecological value, or are they merely exploiting available niches without contributing significantly to the functioning of ecosystems? Whilst many argue that the loss of any species can have profound and unforeseen consequences, more evidence is required to answer this question fully on purely ecological grounds (Purvis and Hector 2000).
Some of these arguments may seem rather obscure and irrelevant to designers and managers of landscape vegetation, especially when maintenance techniques can be used to remove the dynamic element from designed plantings. However, an understanding of the value of biodiversity in landscape vegetation, and the mechanisms that maintain it, become crucial if visually and ecologically-rich vegetation is to be created with reduced maintenance input. Given that promoting biodiversity is one of the often-quoted advantages of an ecological approach to landscape planting, what are the real benefits in the context of designed vegetation? These fall into a number of areas, as follows.
– Aesthetics and visual pleasure. The aesthetics of naturalistic vegetation is a complex
topic and is explored in full in Chapter 11 by Anna Jorgensen. Whilst simple low – diversity plantings work well in more formal settings where there may be a requirement for neatness, order and predictability, there is little doubt that diverse naturalistic vegetation has its own beauty in other less-controlled contexts. This may partly be a result of a rich assemblage of textures, forms and colours, or that in more diverse mixtures there is a greater chance at any one time of components of the vegetation being at the height of their visual display Diversity and richness are also one component of complexity: one of four key factors identified by Kaplan and Kaplan (1989) that are said to result in attractive natural landscape. Certainly, many scientists who may question the absolute ecological value of biodiversity also say that they value it purely on aesthetic grounds.
– Stability: removing vulnerability from simple systems. This argument has the closest
affinity to pure ecological theory. Introducing greater diversity into landscape plantings could be seen as an insurance policy against the failure of one or more component species caused by environmental disturbances such as climatic extremes or disease.
– Setting up succession. One of the most distinctive features (and one that is often the
most difficult to accept) of naturalistic or ecologicallyinformed plantings is their unpredictability. As illustrated later in this chapter, different species or components rise and fall in their abundance over time. This may be a result of environmental disturbance, but is also likely to be a result of differences in the length of lifecycles of different species, and a result of the outcome of competition between component species. Including a diversity of functional groups of plants within an initial mix, both facilitates succession and again insures continuity of the integrity of the vegetation. A functional group in ecological terms refers to organisms (that may not necessarily be related) which behave in the same way in response to environmental change, or perform the same ecological function. For example, in planting new naturalistic woodland, both pioneer and longer-term forest trees may be included in the same mix to enable long-term species replacement to occur.
– Supporting other types of organisms. In general, the greater the diversity of plant
species in a unit of vegetation, the greater the diversity of other types of organism (e. g. birds and insects) that it supports, through the provision of a wider range of food sources or habitat opportunities (Knops et al. 1999). As discussed in Chapter 1, there is not necessarily any relationship between whether vegetation is composed of purely native species or a mix of natives and exotics in terms of the number of organisms it supports. What is of more importance is the vertical or horizontal structure of the vegetation in terms of the number of layers it is composed of, or of interactions between vegetation types across boundaries or ecotones.
– Filling up available niches. It is a cliche that ‘nature abhors a vacuum’. Bare ground
rarely remains in that state for long. Most weed control in landscape plantings involves the removal of undesired plants from gaps between desired plants. These undesirable plants, or weeds, are simply filling space that is not being exploited by the intended species. This may partly be because the planted species have not expanded to fill the space, or it may be that the other species are filling ecological niches that the components of the designed system are leaving empty. For example, bare ground beneath shrubs quickly colonises with aggressive species tolerant of light shade. By filling niches at the outset through the inclusion of additional species, for example by ensuring full ground cover throughout the year, and promoting a multi-layered vegetation structure, the need for weed control in this situation is reduced.
– Maximising the length of display: phenological change. Filling a wide range of
available ecological niches also enables the length of visual display to be increased through the exploitation of species with different phenologies (specific patterns of growth and flowering) within the same unit area of vegetation. An obvious example is that of spring flowering bulbs and herbaceous plants within a deciduous woodland that exploit the light conditions at ground level before the leaves on the dominant trees cast dense shade below. Similar principles operate in many plant communities. The idea of exploiting phenological change is discussed later in this chapter.