Deadly weeds threaten wildebeest migrations in Serengeti

With names like “devil weed” and “famine weed”, perhaps it’s little wonder that these invasive plant species threaten to disrupt one of the great wonders of the world: the annual migration of 2 million animals across the savannahs of eastern Africa.

Initially planted for decoration at tourist lodges in Kenya’s Masai-Mara National Reserve, the invasive species are now spreading into and displacing natural vegetation out on the savannah. The large animals that cross these grasslands each year depend on them for food.

That’s the grim message from a new survey of the spread of invasive exotic plants in the Serengeti-Mara ecosystem, focusing on six species that pose the most serious threat to the migrating animals.

“Rampant invasions in the Serengeti-Mara ecosystem will certainly reduce forage production, leading to drastic declines in the populations of wildebeest, zebras and other large grazing mammals,” says Arne Witt of CABI Africa in Nairobi, Kenya. “These invasive plants are toxic or unpalatable, meaning there’s less forage available for wildlife to feed on.”

One invader, called famine weed (Parthenium hysterophorus), has already been shown to displace 90 per cent of food in fields for livestock, and the effects would be the same for wildlife, says Witt.

Already, the animals’ food sources have been hit by drought and depletion of the Mara river, so their plight could be exacerbated if the plants continue to spread. The survey shows that the species are already infiltrating areas of grassland, creating impassable thickets of inedible vegetation where once there was only grass.

The devil you know

Devil weed (Chromolaena odorata), a scrambling shrub from Central and Southern America that aggressively invades savannah ecosystems, is one of the worst of the six surveyed species. It has already reduced the chance of survival of Nile crocodiles in South Africa and lowland gorillas in Cameroon.

Meanwhile, famine weed has devastated native grasses of the Kruger National Park in South Africa, rapidly suppressing natural vegetation in rangelands. This invader from tropical America has infiltrated 34 African countries. Witt found that infestations in the Masai-Mara National Reserve had grown substantially denser in 2016 compared with an earlier survey in 2011.

All is not lost, however, if action is taken fast enough to halt the invasions now, while they’re still manageable, say Witt and his colleagues. “We expect an exponential rate of increase both in distribution and abundance of the invasives unless action is taken now,” says Witt.Other invading plants can directly harm animals if eaten. Spines from the erect prickly pear (Opuntia stricta), which was imported from the Americas, can lodge in the gums, tongues and guts of grazing animals, leading to potentially fatal bacterial infections. The cactus also forms dense thickets that obstruct mobility, even blocking roads in Madagascar.

Three-pronged attack

The researchers recommend three main solutions.

The first is to purge all tourist lodges of the invasive plants as soon as possible.

Second, get rid of light infestations already in the wild, as efforts in other parts of Africa have shown this to be more effective at slowing the spread of invasive plants than tackling dense, established populations.

Finally, explore the potential for using beetles and other animals that attack the plants. There are hopes from trials in Tanzania, for example, that the chromolaena stem gall fly (Cecidochares connexa) can control devil weed. “Biological control solutions should be implemented wherever possible,” says Witt.

“Assuming these infestations haven’t been missed before, then this looks very serious,” comments Richard Grenyer at the University of Oxford. “Some of these species are serial repeat offenders and have a track record of ecological damage worldwide.”

“Against a constant background of drought from climate change, poaching, disenfranchisement and the ever-present threat of road construction, this is yet another threat to one of the crown jewels in African biodiversity.”

“The Masai-Mara is of global conservation importance and the encroachment by invasive alien plant species should be a wake-up call for both national and international conservation bodies to take action,” says Philip Hulme of Lincoln University in New Zealand. “Tourism operators appear to be a major pathway by which alien species ingress into the Masai-Mara and perhaps these organisations should contribute to the long-term management of invasive plant species in the region.”

“The new study shows that introduced plant species are a growing threat in the Serengeti-Mara ecosystem, one that must be addressed now,” says David Blanton, co-director of the charity Serengeti Watch. “Compounded with other threats like climate change and human population growth, the challenge is daunting indeed.”