TRAPPED: Elephant grass on which the rhinos survive is choked by the weed's matting

It droops in the dark and closes its leaflets when touched. But sensitive as that may sound, it is far from sympathetic when it comes to colonising land. Mimosa rubicaulis, a close cousin of the touch-me-not, has spread its toxic tentacles across the 430 sq km Kaziranga National Park in Assam. The fern-green straggling weed which is overrunning the natural grassland is now threatening the already endangered one-horned rhinoceros and countless other herbivores in the natural preserve, 220 km east of the state capital Guwahati.

Because it can help build up nitrogen, an essential nutrient for tea bushes, a few enterprising planters imported the innocous-looking plant from Indonesia in the 1960s. Little did they realise that this creeper would threaten the last Eden of the rhino. A native of tropical and subtropical areas, the prickly weed is popular for its spectacular response to light and mechanical stimuli. It has today covered with its thorny matting parts of Kaziranga where elephant grass-the natural vegetation on which herbivores like elephants, deer and wild buffaloes survive-grows. "The animals could face a tremendous food shortage in the long run if timely action is not taken," warns a harried N.K. Vasu, the park director. Every morning, dozens of workers under the supervision of Vasu and Ranger D.D. Boro are engaged in the titanic task of physically removing the weeds. But getting rid of mimosa manually may not help in checking the weed's prolific growth. "The seeds germinate quickly and before long the plant sprouts again," says Vasu.

THE LETHAL WEED ...

... AND HOW IT HAS MARRED KAZIRANGA » The toxic weed has spread prolifically across the 430 sq km park forming a thorny matting over the natural vegetation. » If the growth is not checked, the herbivores could face starvation. » Reports say rhinos have been nibbling on the mimosa twigs, though they avoid mature creepers. » At last count, Kaziranga had 1,552 of the estimated 2,200 one-horned rhinos worldwide besides 14 other rare mammals.

Vasu's concern is not without reason. At last count, in 1999, this sprawling reserve was home to 1,552 of the estimated 2,200 one-horned rhinos (Rhinoceros unicornis) worldwide, besides 14 other rare mammals listed in the Red Data Book of the IUCN, the World Conservation Union. There are plenty of chemical solutions to the problem, but the park authorities are against using herbicides for fear of endangering the animals in the sanctuary. They have now turned to scientists at the Assam Agriculture University (AAU) in the nearby town of Jorhat for help. Experts at AAU are in favour of a combination of manual and mechanical methods to tackle the problem. "Unless tea gardens around Kaziranga stop using mimosa as a soil regenerator, the problem will persist. Seeds of the unwanted weed will continue to reach the reserve through run-off from the hills or wind dispersion," says I.C. Barua, taxonomist at AAU.

The scientists have already sent out circulars to tea garden managements to replace mimosa with other plants that are equally capable of fixing nitrogen in the degraded soil. "Some of them have already stopped planting mimosa. Those who are still using this weed should opt for cultivable substitutes like citronella and Guatemala grass that help build up nitrogen for tea bushes," says Barua.

But for the immediate future, experts at the AAU have suggested an integrated approach using chemicals to kill the weed in areas not frequented by the animals, besides manually and mechanically removing the weed using tractors.

The mimosa menace is an added concern for the Kaziranga sanctuary, which is already beset with other problems. The rhinos have always been exposed to unmerciful poachers. Believed to have aphrodisiac properties, its horn fetches up to Rs 15 lakh a kilo in the international market, particularly in Thailand, Singapore and Hong Kong. From shooting rhinos dead with silencer-fitted guns to electrocuting them by putting live wires drawn from high-voltage power lines on rhino tracks, poachers have used ingenious methods to kill the rare pachyderm. In 1989, as many as 44 rhinos were hunted down by poachers. The figure went up to an all-time high of 48 in 1992. Last year, due to heightened vigil by park guards, only eight rhinos were lost to poachers. This year, no rhinos have been killed so far but there is no time for complacency. "The threat from poachers is always there. We have to be vigilant at all times," says Vasu.

Yet another threat to Kaziranga's wildlife is the swirling, grey waters of the Brahmaputra, one of Asia's largest river systems. When it inundates vast stretches of the park, the rhinos, particularly the calves, are the worst hit. At times they are washed away by the strong currents. Some of the adults who stray out of the park to the highlands paralleling the national highway fall prey to poachers.

Given the threats from poachers and floods, conservationists and park officials would, therefore, like to get rid of the mimosa menace as soon as possible. There is always a scare of the rhinos and other herbivores being forced to eat the toxic weed for survival. Already there are reports of rhinos nibbling on mimosa twigs, although the animals generally avoid consuming mature creepers. Says Naseruddin Ahmed, an animal toxicologist at the College of Veterinary Science in Guwahati: "Animals instinctively reject all things unfit for consumption. But when they are hungry or when there is a shortage of food, they could either commit a mistake or be forced to feed on weeds and shrubs."

Anwaruddin Choudhury of the Rhino Foundation, a Guwahati-based conservation group, advocates the need for more stringent regulations in barring plant and animal imports into the country. There are already many instances of exotic species wreaking havoc on the indigenous varieties. The water hyacinth, which is native to central and south America, now dominates water bodies throughout Assam, Choudhury points out. Rajasthan's famed Pichola lake also suffers from a water hyacinth invasion that is choking the life out of the picturesque lake.

Parthenium hysterophorus, commonly known as Congress grass, is another exotic coloniser that has suppressed yields of both cereal and horticultural crops in farmlands across the country. Lantana camara or panchphuli is another ornamental plant that has infested thousands of hectares of forest land in Himachal Pradesh and the Bandipur Tiger Reserve in Karnataka.

In Kaziranga, as the fight against mimosa gains momentum, Vasu and his team may face their biggest problem: lack of funds. With its total debt burden exceeding Rs 11,000 crore, Assam is today virtually bankrupt. It will be difficult to convince the state Government to earmark funds to combat the "little-known" mimosa menace, even if Kaziranga has a World Heritage status and the one-horned rhino is the icon of Assamese identity.