Green stealth bombers in our midst

2010-06-08 00:00

ASK a dozen biologists what biodiversity is, and you could get a dozen different answers.

Basically, biodiversity is the end result of millions of years of evolution where every species has evolved to interact and depend on all other species within its immediate environment. A healthy biodiversity provides a number of natural services for everyone and includes protecting water resources, protecting soils and their nutrients, pollution breakdown, climate stability and recovery from unpredictable events. We find new foods and pharmaceuticals from the plants and we can breed new plants or animals­ from a varied and stable genetic base. Each and every species, no matter how small, plays a crucial role in this enormous but ultimately fragile process. So you may ask, how can the introduction of an alien species interfere with this? Does this not actually add another species to the system?

Initially this may appear to be true. However, a species that has not evolved with all the natural checks and balances of its new home, suddenly finds that it can do what it wants because now there is nothing to stop it. It flourishes, spreads and soon it starts to take over. A plant such as this will exhaust the soil of nutrients and water, it will steal all the light and oxygen­. It will create fire hazards, block rivers and it will eliminate local species. As this progresses, the damage to the environment increases and biodiversity decreases.

South Africa has one of the most varied and threatened ecosystems in the world. It has an enormous range of climates and vegetation zones or biomes­, and yet because of its history and these amenable climates, alien plant species have arrived in abundance and have flourished. They came with the early settlers as reminders of home. They came hidden in animal fodder during various wars. They were brought in for timber or other agricultural uses. They came by accident and they came in from deliberate introduction often as beautiful and fascinating plants for the garden. At the same time, agriculture, forestry and urbanisation have grown at an enormous pace, and these activities on their own have caused enormous strains on the natural environment and biodiversity.

It is something we have to live with, but it is also something we have to manage as our existence on this planet could depend on it. It stands to reason that if we upset biodiversity elsewhere, if we destroy water reserves and fertile soils, then we cannot produce the food that we are increasingly going to need.

Around Pietermaritzburg, we are all familiar with the serious alien invaders such as lantana and Chromaleana odorata. We can see them invading river beds and waste places. We think they are messy and untidy. But go to the old Transkei and you will see lantana destroying the bush and grasslands on which the local people live and exist. Despite this, lantana is still a popular­ garden plant in Tshwane (Pretoria) and other African cities such as Harare and Nairobi, but are they aware of the risks of these plants? Could this plant invade the local­ grasslands and cause the havoc that it does in KwaZulu-Natal? Surely, the most logical solution to this is to remove it completely from these places and make it illegal and forbidden.

This is one aspect to the approach of making so many popular garden plants illegal is that they have the potential to escape and become seriously invasive. It is better to remove these risks than to find out later that we have created another lantana.

But what about the Ardisia, the statice and other popular and pretty plants that perhaps could escape, that maybe do not have the characteristics of becoming an aggressive invader? Firstly, nothing is for certain. The pom pom weed lived happily around Tshwane for more than 30 years, but only in the past few years has it set off and started to spread alarmingly. It is a pretty flower. The bees and butterflies love it, say gardeners, and this is where we have to be alert.

Our indigenous flora and fauna have evolved together. Many of these species rely on butterflies and birds to spread them around and can suffer severely if other plants suddenly appear that produce an abundance of fragrant flowers and juicy berries. The natural range of the hadeda has been greatly enhanced by fruit-bearing aliens, which is okay if you are a hadeda, but it is a problem if you are another species of bird which previously occupied this niche.

Every meaningful countermeasure we can take to protect the environment and biodiversity will help. It can raise awareness and it can bring stability. Local extinction of a species is a precursor to total extinction. Already several plant species have been made completely extinct in the Cape fynbos as a result of alien plant invasions. In Pietermaritzburg, you only have to look over the garden fence to see local extinctions.

Out of more than 2 000 alien plants that now grow unassisted or have become naturalised in South Africa, between about 198 and 380 have been or are about to be declared a current or potential danger. This is not a huge number. Statice (Limonium sinuatum) for example, is invading the fynbos in the Western Cape. It is not prohibited in KZN, but beware, it is being watched. Some plants that are used for crops or forestry are under sus-picion and will require permits — the granadilla, pine trees and poplars. We should all become aware of what these species are that pose a threat and which restrictions apply. We should avoid risky species and we should remove unintended plants while supporting those nurseries involved with the plant-me-instead campaign.

The Conservation of Agricultural Resources Act (Cara) list is on various websites, including www.dwa.gov.za/wfw/ and the proposed National Environmental Management: Biodiversity Act (Nemba) list should be declared soon. Both lists and all the plants involved are in my book, Problem­ Plants and Alien Weeds of South Africa.

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