Confused travellers
The migration season in the Maasai MaraSerengeti region in Africa has begun earlier than usual this year, due to the destruction of habitats
Maasai Mara's wildebeest have begun their seasonal migration earlier this year as their habitat gets fragmented due to human activities, disrupting traditional migratory patterns
THE MAASAI Mara-Serengeti region, spanning across the borders of Kenya and Tanzania, has been a theatre to the great wildebeest migration for thousands of years. An estimated 1.5 million wildebeest or gnus, zebras and gazelles as well as many species of large African herbivores migrate across the Mara river anticipating rains and in the pursuit of fresh pastures and water.
This year, however, the migration of animals from the Serengeti National Reserve in Tanzania to the Maasai Mara National Reserve in Kenya began earlier than usual— in June. The migration usually starts in July and ends in October. Tourist operators are scheduling the African safaris much earlier. In eastern and southern Africa, fragmentation of habitat has occurred along with the decline in the abundance and geographic range of animals. For example, in northern Tanzania, there has been a gradual loss of connectivity between seasonal ranges available to migratory wildebeest. This has led to fluctuations in the abundance of animals.
Such changes in migratory patterns are expected to increase with the loss of availability of water and food. Recent studies have found that a number of factors, including climate change, human activity, growing human population, change in land use patterns, infrastructural development, and recently, the profusion of invasive alien plant species are causing widespread disruption in migratory patterns.
Zebras already face obstacles in migra-
tion routes. A study by German researchers in Botswana, published in the Proceedings of the Royal Society B recently, found that zebras rely on memory to follow migration routes to reach fresh vegetation. “This memory risks making inaccurate predictions of vegetation and water abundance as seasons are changing,” Chloe Bracis of the Senckenberg Biodiversity and Climate Research Centre, Frankfurt, told Down To Earth. “Migration routes of zebras are threatened by climate change and land use changes in southern Africa,” says Bracis.
However, Sam Weru, a wildlife and conservation consultant based in Nairobi, believes animals combine both institutional memory and an “innate” capacity to detect and read environmental conditions to guide their migration. He cites the example of the wildebeest, which is known to delay birth when rains are delayed. Zebras, he says, are more likely to migrate following the ecological influence of other species like wildebeest through the process of “Ecological Facilitation”, where the feeding behaviour of one species influences or facilitates the access to forage material for another.
Bernard Agwanda, a mammal specialist with the National Museums of Kenya, says zebras have been known to traditionally reach food and water using the same path all the time, due to what is called “evolutionary imprisonment” in their genes, a behaviour that helps them avoid risks such as being attacked by predators.
Among the recently studied risks to the disruption of migration patterns is the unprecedented growth of invasive alien plant species in the Maasai Mara-Serengeti region. A study by the Centre for Agriculture and Biosciences International (cabi), the UK, the Kenya Wildlife Services, Nairobi, and South Africa’s Stellenbosch University, published on May 22 this year in Koedoe, found that as many as 160 alien plant species have taken roots in the Maasai MaraSerengeti ecosystem.
Of these, 23 were found to be invasive, spreading aggressively and threatening to colonise indigenous biodiversity. “We predict that in the absence of efforts to contain, or reverse the spread of invasive alien plants, the condition of rangelands will deteriorate, with severe negative impacts on migrating large mammals, especially wildebeest, zebra and gazelles,” the study found.
Some of the invasive plant species include the famine weed (Parthenium hysterophorus), mesquite (P juliflora), devil weed (Chromolaena odorata), lantana or tickberry (Lantana camara), cactus (Opuntia sp) and the Mexican sunflower (Tithonia diversifolia). The researchers found these species in the fields next to the Maasai Mara game reserve, and say they could spread throughout the wildlife reserve very soon.
Invasive plants displace native species and deny animals their feed. “Most invasive plants are toxic and unpalatable. They are not eaten by livestock or wildlife and they are also aggressive in displacing native species. In other words, they are reducing the amount of forage available, while some also form dense thickets preventing the movement of wildlife,” Arne Witt of cabi told Down To Earth.
Experts say that herbivores such as wildebeest, zebra and gazelles will be the most affected, and this will have knock-on impacts on other wildlife, including many predators. Invasive plants will “alter the whole ecosystem as we know it.” Witt says climate change can facilitate plant invasions due to increased disturbances resulting from droughts, floods and other extreme weather events, adding that some invasive plant species may benefit from increased carbon dioxide levels in the atmosphere. “Increased infrastructure development such as the construction of roads and hotels as well as the proliferation of tourist jeep trucks may facilitate plant invasions,” Witt observes.
Among the biggest threats to migration patterns are infrastructural development, human settlements, agriculture and pastoralism. When pastoral communities living close to wildlife reserves graze their animals next to parks, their livestock compete for pastures with wildlife, making wild animals miss out on forage in times of migration. On the other hand, invasive species are of no value to herbivores and even insects, making movement and grazing impossible due to useless vegetation.
There are some ways to buck these disturbing trends. To monitor mammal populations, researchers developed Wild-ID, an unusual wildlife photo-identification tracking method. Using this technology, researchers have seen that as the number of migration routes goes down in the Tanzanian ecosystem, so does the population of the wildebeest. This finding was published in Biological Conservation in 2016. Studies have shown that Wild-ID is more accurate, less invasive, less expensive, less time consuming and covers more territory than traditional mark recapture and aerial survey methods. As for invasive alien species, experts recommend removing species which are invasive and the introduction of control programmes to contain their spread.
Apart from agriculture, pastoralism and infrastructural development, more than 160 alien plant species are threatening the migratory pathways