Located in Tanzania, with a height of almost 6,000 metres, Mount Kilimanjaro is Africa’s tallest mountain. Over the years, there’s been extensive urbanisation and development at the base of the mountain, which could change what lives on the mountain.
Satellite images show that, between 1976 and 2000, there have been dramatic changes to the strips of land between Mount Kilimanjaro and Mount Meru, which is located less than 100km to the east. Areas that originally had dense natural vegetation were forced to make room for intensive agriculture and residential development to accommodate a growing population.
Nowadays, Mount Kilimanjaro is almost completely surrounded by developed areas that include wheat farms, commercial sugar cane plantations and rice paddies. There are also smallholder farms all around and growing, built-up settlements.
This means the mountain’s ecosystem is at risk of turning into an “ecological island”, entirely surrounded by cultivation and development. When natural habitats are isolated in this way, species are less able to migrate, leading to less genetic variation and diversity.
Less genetic variation makes ecosystems more vulnerable during environmental changes. Diversity increases the ability to adapt. For instance, if there’s a warming event some plants or animals might not survive and others might.
We carried out a study to uncover what the long‐term effects of this land‐cover change could be on biodiversity. Specifically, we wanted to know how important natural vegetation bridges are.
We did this by studying a group of insects called Orthoptera – commonly known as grasshoppers, crickets and bushcrickets. These serve as an early warning system because they are adapted to a very special microclimate. This means they’re affected by even small changes in environmental conditions and this is immediately reflected in which species are living in a certain community in a certain habitat. Any changes to grasshoppers can signal far-reaching effects for other animal groups, which are often difficult to study.
Grasshoppers can also easily be collected and identified.
We found that – because of the distribution of different species – Orthoptera used vegetation between Mount Meru and Mount Kilimanjaro as natural bridges. These bridges allowed species in the area to develop to their current diversity levels.
If bridges of vegetation between the mountains weaken or vanish altogether, it is not just the mobility of these insects that is affected. Larger animals such as antelope, small mammals, snakes, or chameleons are at an even higher risk of becoming isolated and thus going extinct in the foreseeable future. They need genetic exchange and large habitats.
Bridges of vegetation
We studied the environments of Orthoptera at 500 selected sites on Mount Kilimanjaro and Mount Meru.
We wanted to know whether these insects used natural vegetation as bridges. Endemic species – species that are found only in this region of East Africa – were of particular interest. We studied flightless Orthoptera that have to crawl on the ground if they want to reach a new habitat on another mountain. Widespread, mobile species which can fly would have been not suitable for our study.
We found an especially high proportion of common endemic species in the forest areas at lower altitudes shared by these two mountains. We believe that this is a clear indication that the Orthoptera once used dense vegetation between the mountains as bridges to spread out in both regions.
Conspicuously, there are also a few endemic flightless species, shared by both mountains, that are only found in higher forest areas. These species share common ancestors and originated from the same place. This distribution pattern cannot be related to the recently disappeared forest corridor. Species living in a completely different climate zone with much cooler and wetter conditions could never use a migration corridor in the lowlands, at least under the current climate.
We believe the reason for this disconnected distribution of high elevation endemics lies in ancient climate changes. Several tens of thousands of years ago, it was considerably cooler and damper in the lower areas than it is today. Thus, Orthoptera that preferred these climatic conditions settled at the foot of the mountains, travelling by foot via the wooded land route. It was only later, as the temperatures rose and precipitation diminished that they made their way to higher areas. They then no longer had contact with grasshoppers in neighbouring regions.
Our research findings corroborate the thesis that animal and plant species spread out primarily via bridges of vegetation. In contrast, other ways of spreading over long distances, for instance seed transport via wind or the “air travel” of individual insects, play a subordinate role.
Our study showed that forest bridges between East African mountains acted as important migratory corridors and are not only a prehistoric phenomenon, but existed and disappeared in some places more recently. Similar to the missing bamboo belt of Kilimanjaro, this is another example of the long-lasting and accelerating influence of humans on the African landscape.
It’s important for policymakers to address the growing isolation of Kilimanjaro as, without these bridges, many species of animals will be at risk of extinction.
In particular the so-called “Kitendeni corridor”, a seven kilometre-wide strip of savanna bushland on the northern slope, is another bridge of major importance. This links Kilimanjaro with the Amboseli National Park in Kenya.
As a wildlife corridor, Kitendeni is meant to preserve access for elephants and other wild animals to the forests of the mountain’s northern slopes, as far even as the forests of West Kilimanjaro.
Yet increasing human pressure, from cultivation and livestock grazing in particular, is reducing this corridor. It’s forcing wildlife, especially elephants and buffaloes, to remain longer on Kilimanjaro, having a further impact on the forests.