When the human population grows 28 times in just over a century, nature retreats, inexorably. This is what happened on Kilimanjaro, where between 1911 and 2022 75% of natural species per square kilometer disappeared from the lower slopes. This was revealed by research published in PLOS One, and supported by the Deutsche Forschungsgemeinschaft, which for the first time precisely documents the link between human density and loss of biodiversity in a tropical ecosystem.
The scientists cross-referenced maps from 1911, satellite data, population censuses and 1,600 field surveys, tracing the evolution of nearly 3,000 plant species. The picture that emerges is unequivocal: the population density has gone from 30 people per square kilometer in 1913 to 430 in 2022, transforming savannahs, forests and wetlands into cultivated fields, pastures and urban settlements.
In 1911, 90% of natural vegetation still covered the lower slopes of the mountain. Today that percentage has fallen to 19%. The savannahs, which a century ago occupied 75% of the territory, have been reduced to 18%, replaced by agriculture. Forests have also undergone a drastic reduction, going from over 500 square kilometers in 1911 to just 60 in 2022.
But the most surprising data from the research concerns the role of climate change. Climate change has had no apparent influence on the trends observed in biodiversity in inhabited areas. While in the higher areas of the National Park global warming has accelerated the melting of glaciers and increased the frequency of forest fires, in the lower slopes population growth has been the main driver of the transformation of the territory.
The population around Kilimanjaro has exploded: from around 50,000 inhabitants in 1889 to over 1.4 million in 2022. This boom has triggered a chain of consequences: agricultural expansion, urbanisation, intensive firewood collection, brick production, grazing. Built-up areas grew twelvefold between 1976 and 2022.
Not all news is bad. The study also documents virtuous examples of coexistence between man and nature. Chagga homegardens, traditional four-layer agroforestry systems, have doubled in size to 849 square kilometers and are home to over 500 plant species. Rau Forest Reserve and Namalok Private Reserve show how effective management of protected areas can mitigate the negative effects of population growth.
The research also highlights a worrying paradox: while native species decline, introduced ones increase. Alien species have gone from being a marginal presence to representing a significant component of the local flora, with some, such as Prosopis juliflora, invading 7% of certain areas.
According to the researchers, Kilimanjaro represents both a challenge and an opportunity for other tropical regions. Protecting the Rau forest and establishing protected areas like Namalok are potential strategies to mitigate drastic changes in land cover. The message is clear: halting the loss of biodiversity requires effective policies that balance human well-being with environmental well-being, before the ecological balance of these unique ecosystems is permanently compromised.
