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Conserving biodiversity in landscaping

Most landscaping projects, both small and large, use the same small range of plants. These plant species are typically chosen for several reasons:

• They are available in bulk (because they are easy to propagate).
• They transplant and grow well.
• They are resilient in urban conditions.
• They are familiar, so their characteristics are understood.
• Landscaping is quick and straightforward.

But this practice ultimately diminishes our built environment. We end up with the same plants everywhere: box hedges, lomandra strips, mondo grass borders, dwarf lilly pillies, spiny grevilleas, kikuyu lawns ... and not much else.

What tends to happen in new developments is that maybe a hundred species are cleared from a site – all the way from unnoticed groundcovers, grasses and sedges up to trees – and are eventually replaced with half a dozen species, none of which grows naturally in the area. This has two serious effects: it reduces biodiversity and it robs many animals species of their food source and homes.

Even in redevelopments, although the developers may specify native species, the site ends up with plants that could originate anywhere in Australia and that are genetically identical to those at thousands of other sites.

Maintaining genetic diversity

Erosion of genetic diversity is a largely unseen but critical threat to biodiversity all over the world. In the wild, every plant (unless it reproduces asexually, such as by suckering) is genetically unique. But under cultivation, such as in a wheat field, every plant is genetically identical to every other plant. If a pest or disease comes along that is able to evade a plant’s built-in chemical or physical defences, that plant becomes a meal. If all plants are the same, the entire crop is lost.

This genetic uniformity lies at the root of most attacks of pests and diseases on crops. The Cavendish banana is an excellent example of how 50 years of vegetative propagation has given a variety of pests and diseases ample leisure to develop complete resistance to the banana’s natural defences. The only reason we can still eat Cavendish bananas (virtually the only banana you can buy anywhere on Earth) is that growers must douse the plants in an arsenal of chemicals.

Meanwhile, as breeding groups develop new plant cultivars for distribution to third-world growers, for both altruistic and commercial reasons, natural genetic diversity is being rapidly lost. With old crop landraces that are abandoned go potentially millions of genes critical to the survival of crops against new pests and diseases, drought, soil salinity, frosts and global warming.

Genetic diversity in the bush

Genetic diversity is critical in the bush, too. For a start, every plant that is cleared and replaced with a standard landscaping species is a unique genetic combination lost. The genes it held are probably all present in the local population, but the more of a population is cleared, the more unique genes are lost. Eventually a population can be reduced to half a dozen plants hanging on in a small pocket, which together retain very few of the unique genes once present in the species. That set of genes – the remnant gene pool – may not hold enough diversity to maintain the species’ resilience against environmental threats, such as pests, diseases and drought. The species could then go extinct.

Even when a species is not threatened, unique combinations of genes can be lost. Those combinations could have given resistance to a particular pest or disease, encoded a unique flower colour, supported a variant butterfly form, flowered earlier or later than normal (giving potential resistance to climate change), or any of an uncountable number of possibilities.

Supporting wildlife

In the wild, plants, animals, fungi and other organisms develop interdependent systems. The sheer diversity of plant species in the bush shows the enormous range of ecological niches where other organisms can live. There are more plant species in Sydney alone than in all of Great Britain. Upon these plants depend hundreds of other species – probably thousands if we include soil fauna and fungi.

If we remove a key component of an ecosystem, many more than that one species can suffer. If, instead, we preserve local plant species in situ, we thereby preserve other species that depend on them. The retained genetic diversity in each species then makes each species more resistant to disturbance by various factors.

Conserving biodiversity on site

If you are developing a new site, a few simple practices will enable you to retain the local biodiversity:

• Pot up representatives of the smaller plants or seedlings. Keep them aside to replant later.
• Collect seeds to sow later.
• Aim to collect species from all vegetation layers: groundcovers and grasses, creepers, shrubs, trees.
• Scrape off the topsoil and store it for putting back later. The topsoil is a repository of seeds and tubers of many species, which will reappear if given a chance.

If you are redeveloping a site, try to reintroduce the local species. Many local councils run nurseries that propagate local species from locally collected material. They record not only the suburb or town where the material was collected, but often also the street.

Soil

What species you grow depends in part on the soil. Although some species tolerate a range of soil types, others will grow on one soil only. So it is important to look around and understand the soil that developed at your site.

The great advantage to growing species specific to the soil is that they require no special maintenance – they are precisely adapted to the conditions.

Diversity = interest

Growing local species reintroduces both plants and animals to a site that may have lost them. It also introduces people to the enormous variety of wildlife that we have in Australia. Landscaping sites with local plants ultimately creates interesting neighbourhoods, instead of a bland uniformity.

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