The Chomolaena Story

Dr Alison J. Leslie, (Chair Herpetological Association of Africa), Ecologist/Crocodile Specialist, Dept Nature Conservation, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa.
Tel: +27-(0)21-808-2487, Fax: +27-(0)21-808-3304, ALESLIE@akad.sun.ac.za

What do a reptile, an alien plant and sex have in common? None of my audience at the Fifth International Workshop on the Biocontrol and Management of Chromolaena odorata ventured an answer. Chromolaena may be a strange word, but those who have read Brian Marsh's articles will know what I am talking about. The workshop, held in Durban during October 2000, was organised by the Plant Protection Institute of the Agricultural Research Council.

My audience consisted of 55 research specialists from 15 countries - botanists, plant pathologists, nematologists, entomologists, game park managers and professors, to myself, dubbed the 'conference misfit', being the only zoologist within the group.

So, back to my original question: the reptile being referred to here is the Nile crocodile; the alien plant is Chromolaena; and sex? Well, here I was referring specifically to sex ratios of hatchling crocodiles. But first let me tell you a little about the Nile crocodile and then about Chromolaena itself, before we tie the two together.

Crocodilians belong to the great group of archosaurs, also referred to as the 'ruling reptiles'. This group is so called because for approximately 150-million years they literally ruled the world. The archosaurs also included the extinct thecodonts (small lizards that ran on two legs), the dinosaurs, the pterosaurs or 'flying reptiles' and the ancestors of the birds. Crocodilians of the most advanced kind, known as the Eusuchians, first appeared some 140 to 65 million years ago, and the crocodilians of today all belong to this suborder. There are 22 species in the Crocodylidae Family which is divided into three subfamilies, namely: the Crocodylinae, the Alligatorinae, and the Gavialinae.

Crocodiles and alligators are found between the latitudes of Cancer (23.5º north) and Capricorn (23.5º south), in the rivers and lakes of South America, Africa, Asia and Australia. The Nile crocodile (Crocodylus niloticus), is one of three species inhabiting Africa which it shares with the smaller, shy African dwarf crocodile (Osteolaemus tetraspis) and the now scarce African long-snouted crocodile (Crocodylus cataphractus). The Nile crocodile is widespread throughout the continent, south of the Sahara Desert, but is absent from much of the extreme south and south-west, namely the desert regions of Namibia and Botswana. It occurs in the island of Madagascar, and historical records indicate that its range formerly extended into Israel and Jordan and the species was also established on the Comoros Islands.

Nile crocodiles have wide habitat preferences, reflecting their success and distribution as a species. They are present in a variety of wetland habitats, including rivers, lakes, swamps and brackish water, and are even found in coastal areas of west and southern Africa.

As most of us know, crocodilians are highly efficient predators. They have powerfully constructed armour-plated bodies, stout limbs, streamlined tails, impressive bony heads and extremely strong, tooth-studded jaws; every tooth in their jaws adapted for catching and holding prey. Added to this is the fact that they are 'opportunistic' hunters - they will eat pretty much whatever comes their way.

There is a distinct shift in dietary preferences as they go through a 4000-fold increase in size from 80gm as a hatchling to over 800kg as an adult. Hatchlings survive primarily on small aquatic and shoreline insects. As they increase in size they start feeding on larger and larger prey items. At about one year of age they feed on tadpoles, frogs, snails, crabs, shrimp and even small fish, thereafter feeding on an even greater variety of insects, larger fish and small waterfowl and mammals, eventually progressing to a diet of large fish and crabs, other reptiles including lizards, terrapins and turtles, birds and large mammals, including the odd tasty human being!

The critical temperature varies among species but all crocodilians, whether they live in Himalayan mountain streams, tropical jungles or temperate swamps, incubate their eggs at temperatures close to 30ºC. Exposure to temperatures below 27°C and above 34°C kills embryos of most species. One particular species, Schneider's Dwarf Caiman, nests in dense, closed-canopy rainforest where direct solar radiation hardly contributes to nest temperatures, and this crocodile keeps her eggs warm by selecting a nesting site beside a termite mound (which produce a certain amount of metabolic heat), and builds a nest out of vegetation which decomposes and produces heat, much like a compost heap.

A number of surveys for Nile crocodiles have been conducted in recent years, and information on crocodile status is 'average'. However, for 25 out of 39 African countries there is inadequate information. Hide hunting in the mid-1900s at a time when the Nile crocodile was classified as vermin, caused dramatic declines in populations throughout most of its range, resulting in it being listed on Appendix 1 of CITES in 1975. However, protection by national laws and international regulations has resulted in a recovery in many parts of the species' range. By 1985, due to the crocodile's recovery, expanding human populations and the high value placed on crocodile skin, pressure to recommence exploitation induced CITES to introduce a quota scheme under which a limited number of wild crocodile skins could be exported annually.

The Nile crocodile is one of the most commercially utilized species of crocodilians, producing a 'classic' hide. World trade numbered 80000 skins annually in 1993, with the majority coming from crocodile ranching in Zimbabwe (54%) and South Africa (15%). Commercial utilization is now widespread and many successful management programmes have been established. However, there are still a number of factors threatening Nile crocodile populations throughout Africa, one being conflict with people. Man and crocodiles compete for suitable habitat and for certain food items such as fish, and crocodiles dwelling in Malawian lakes and rivers have become a bone of contention between the government and the local inhabitants who, with their livestock, regularly fall victim to them. But the government insists that it is determined to protect the crocodiles, which is more an exception than the rule.

We all too often hear how vital habitats for both fauna and flora are altered and destroyed by man, a particularly bad example being the accidental introduction of an alien plant known as Chromolaena into South Africa which is invading crocodile nesting sites in the Lake St. Lucia region. Not only is the plant preventing crocodiles from nesting in some traditional nesting areas, but it is also casting shade on others, thereby reducing the incubation temperature and thus altering the sex ratio of the developing hatchlings. More female hatchlings due to the cooler incubation temperatures experienced could lead to eventual extirpation of the species from the area. Chromolaena is also reported to be spreading into Mozambique, Zimbabwe and Botswana - other countries with Nile crocodile populations - so the problem may very well not be limited to South Africa. Invasive alien species are now recognised by the International Union for the Conservation of Nature (IUCN) to be among the top four biggest threats to the maintenance of biological diversity on the planet.

Chromolaena is a herbaceous perennial native to the Neotropics. It occurs in most areas below 1000m elevation from southern Florida to north-western Argentina, and is not a problem in its place of origin. It is a plant of 'secondary succession', in that it invades clearings and persists until shaded out by the overgrowth of forest trees. In the New World, the species is controlled by a number of factors including attack by native insects, other arthropods and diseases, together with competition with related plants. However, due to the absence of these species-specific factors the plant has naturalized in parts of Africa, India, Sri Lanka, Indochina, Malaysia and Indonesia, to mention but a few countries, where it has become an incredibly successful exotic species and a noxious weed.

Chromolaena is incredibly fast growing, is quite difficult to detect, is capable of vegetative reproduction (ie where a new plant can grow from a piece of root stock), it produces hundreds of thousands of wind-dispersed seeds per plant, has a number of genetic strains, has lots of close relatives, promotes fires and is fairly catholic in the habitats it can grow in. It is an invader of primarily the tropical and subtropical forested areas which harbour most of the world's biodiversity usually found in the less developed countries which have some of the world's most rapidly increasing human populations and concomitant increases in human pressure.

It is thought to have been mistakenly introduced to KwaZulu-Natal in seed-contaminated packing materials offloaded at Durban harbour during World War II, but its exact origin is still unknown, and by 1986, invasion of the species had reached alarming proportions in the KwaZulu-Natal coastal region. In the past two decades it has moved into other provinces: south into the Eastern Cape and north into Mpumalanga and Northern Province, as well as Swaziland, Mozambique, Zimbabwe and Botswana. In South Africa, Chromolaena is primarily seen as a threat to conservation, but it also impacts on forestry, pastoral agriculture and other land uses.

Although South Africans are more recently generally aware of the weed, its spread has proceeded largely unchecked. Few private landowners have had the interest to clear Chromolaena in its early stages of invasion, or the resources to clear it at a later stage.

So what are we in South Africa and others in the rest of the world doing about Chromolaena? The answer is plenty - but the big question is, are we doing it fast enough? According to Dr Costas Zachariades of the Plant Protection Research Institute of the Agricultural Research Council (ARC) of South Africa, if Chromolaena is to be reduced to manageable levels in southern Africa, several actions must be taken. This includes the development of a coordinated clearance plan with increased awareness and funding, and the release of biological control agents - natural predators or parasites. With Chromolaena this is tricky because we do not know where our species originated and it is therefore difficult to track down its natural predators. ARC's biological control programme, in operation since 1988, has prioritised ten insect species for investigation and several pathogens collected on Chromolaena plants in the Americas are also being examined for their potential as biocontrol agents. Much is happening in the way of biocontrol, but it is a slow, monotonous and expensive process.

In between all this vital research, much is also happening in the form of manual removal and poisoning of the aliens by S outh Africa's 'Working for Water' (WFW) programme which commenced in 1995. Managed by the Department of Water & Forestry, WFW is a national programme for controlling the 750 tree species and 8000 other plant species that have been introduced into South Africa, combining labour, mechanical, chemical, controlled fire and biological treatments. Chromolaena has kept, and is still keeping, many of the 35000 employees of WFW extremely busy.

Although South Africa has not as yet found a satisfactory biocontrol agent, some other countries have been more successful. According to conference delegate, Dr Haruna Braimah, Chromolaena arrived in Ghana in the late 1960s and by the 1980s had covered almost 60% of the country's land area. A staggering statistic. Its weed status was luckily very quickly realised and efforts involving biocontrol were made to control it, particularly the use of a defoliating arctiid moth obtained from the University of Guam. The insect was released on Chromolaena plants in an experimental field around the research centre in 1989, and ten years later the insect had established itself as an effective natural enemy of Chromolaena in many parts of Ghana, where the land invasion area is now reduced to about 35%.

Ghana now plans to introduce root and stem feeders to complement or synergise the defoliating function of the moth species, which has also established itself in Indonesia, the Philippines and in Papua New Guinea. Unfortunately the same moth species has not established itself in South Africa, possibly due to seasonality/climatic differences.

India is also battling to control the spread of the plant. According to Dr Remadevi of the Institute of Wood Science and Technology, another way of managing it could be to use it - in particular as a biopesticide. A number of alcohols, chalcones, aromatic acids and an essential oil have been isolated from the weed; the oil exhibits insecticidal activity.

Not everyone sees Chromolaena as a problem weed. In the Cameroon local farmers aver that the weed increases soil fertility, and in Ghana it is said to have medicinal properties. For these reasons, and I am sure there are many more, it may be difficult to develop a coordinated clearance plan within Africa.

But in the meantime we can all do our bit. Find out what Chromolaena looks like. Contact your local WFW project managers if you need help with identification. Remove the weed from your garden if you have it and look out for it in neighbours' and friends' gardens. Many towns have a local 'hacking' group of volunteers who go out on field trips to help eradicate all sorts of alien species. Join the group, meet new people, see new areas and make a difference.

Dr Ian MacDonald, of the World Wide Fund for Nature in South Africa, believes that if we can beat Chromolaena it will indicate that we can successfully come to grips with many of the underlying problems which affect conservation in the tropics. And if we can do this, we can then be optimistic that our vast treasure trove of biodiversity can be secured. I can only agree with him.

So whether it is crocodiles, plantation crops, forests or pristine conservation areas, Chromolaena is a serious threat worldwide and we all need to make a concerted effort to help combat the spread of this species.

This article first appeared in Magnum Magazine, January 2001. Dr Alison Leslie is an Aquatic Ecologist in the Department of Nature Conservation at the University of Stellenbosch, and a specialist in the ecology of crocodiles.