National Geographic News
The feverfew weed.
Santa Maria feverfew spreads through an Ethiopian grassland in a file picture.

Photograph courtesy Arne Witt via IUCN

Ochieng' Ogodo in Nairobi, Kenya

for National Geographic News

Published December 10, 2010

An alien weed creeping through southern Kenya may threaten the largest wildlife migration on the planet, scientists say. The toxic plant, called Santa Maria feverfew, can also sicken people and kill animals.

Native to Central America, the white-flowering weed was recently found growing in the Maasai-Mara National Reserve (see map) during a survey by the African branch of the Centre for Agricultural Bioscience International Africa (CABI), based in Nairobi, and the International Union for the Conservation of Nature (IUCN).

By producing chemicals that inhibit the growth of other plants, the alien plant can dominate whole landscapes—especially those already in bad shape due to human land use, according to Arne Witt, invasive-species coordinator for CABI.

During the annual migration of wildebeests, more than a million animals follow a circular path through Tanzania's Serengeti (see map) and up into Maasai Mara National Reserve, chasing grass and water as the seasons change.

The giant migrating herds—which include other species such as zebras and Thomson's gazelles—could die if forced to eat the poisonous plant in the absence of other food, Witt said.

(Read about great animal migrations in National Geographic magazine.)

"Unlike weeds in the past, which have mainly been confined to roadsides, these weeds are what we would call transformers," he said. "They have the ability to transform landscapes and displace native species."

Alien Plant Fast-Growing and Aggressive

Santa Maria feverfew, first recorded in Kenya in the 1960s, was likely accidentally introduced to the country through contaminated grain shipments.

A more aggressive strain of the weed arrived on the continent during the past decade, Witt said, and has since moved through the country via wind, water, or vehicle transport. (Read "Attack of the Alien Invaders" from National Geographic magazine.)

Seeds have likely also hitchhiked on construction traffic carrying agricultural materials into the Maasai Mara. Roads are the main conduit of the feverfew's spread, and a controversial planned road through the Serengeti would speed up the invasions of feverfew and other alien weeds, Witt pointed out.

The prolific weed can thrive in a wide range of soil types and climates. Considered one of the world's most serious plant invaders, the species has already spread to other parts of Africa, including Ethiopia, South Africa, Mozambique, and Uganda, as well as parts of Australia, experts say.

"Under ideal conditions, it can mature in four to six weeks and produce anything from 10,000 to 25,000 seeds [from] a single plant," Witt said.

"The seed bank contains millions of seeds and they can survive in the soil for a number of years, waiting for the ideal conditions in which to germinate."

Toxic Plant Can Kill

Santa Maria feverfew can produce so much seed because it's mostly left alone in Kenya: Neither grazing animals nor native insects will touch the plant, which produces a toxic chemical called parthenium.

"Livestock generally don't eat the plant, but if they do in sufficient quantities, because there is no other forage available, they will die," he said.

"If eaten in small quantities together with other forage, it will make their milk and meat distasteful."

The economic impact on herders from loss of grazing lands and damage to livestock is already significant, experts say.

Geoffrey Howard, IUCN's global invasive species programme coordinator, said the weed's take over of neighboring countries has already had a serious impact on cattle, goats, and wild herbivores.

"It will replace native vegetation and remove the food sources of many herbivores if it spreads unchallenged," Howard said.

What's more, the plant's explosion could hit herders particularly hard: Wild animals in the reserves may wander onto pastures looking for food, thus competing with livestock.

(Related: "Animals No Safer Inside Kenya's Parks Than Outside?")

Plant Causes Allergies in People

In humans, Santa Maria feverfew's pollen can cause respiratory problems such as asthma. (Learn about the human respiratory system.) And those who regularly come into contact with the plant—especially women and children—develop severe allergies such as dermatitis, according to Witt.

"Because it causes severe allergies, some farmers' manual clearing [of their land] will no longer be possible unless you have access to protective gear and herbicides," he added. "The pollen blown onto your crops can also reduce yields."

The overall economic impact could be massive in Kenya, where most people depend on natural resources for their survival. The weed may also influence wildlife tourism if the region's famous animals move elsewhere, Witt said. (See pictures of migrating animals in National Geographic magazine.)

Other feverfew-infested African countries may give an idea of what's to come in Kenya. For instance, the weed has wreaked havoc on national parks and livestock pastures in Uganda, according to Peter Beine, coordinator of the National Invasive Species Programme at Uganda's National Agricultural Research Organisation.

"In Uganda, it has caused 100 percent loss or total crop failure in sorghum and other cereals in some parts of the country," Beine said.

Feverfew Can Be Stopped

There are signs that the weed is already causing problems in Kenya, Beine added.

"A conversation with a Maasai pastoralist on the outskirts of Nairobi National Park revealed that the milk of nanny goats was distasteful" after eating feverfew, he said.

"This response was totally unsolicited, and an indication that it is already having an impact in Kenya."

But the weed is still localized enough in Kenya that it can be stopped, Beine emphasized. (See Kenya pictures.)

"A campaign involving mass sensitization and awareness-creation is necessary, coupled with physical removal of the weed from known infested areas, spraying, [and] increased vigilance at border points [and] airports," he said.

But "eradication is only possible if we act now—and not tomorrow."

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