Gain and loss: Human and environmental wellbeing – drivers of Kilimanjaro’s decreasing biodiversity

PLOS One | https://doi.org/10.1371/journal.pone.0334184 October 29, 2025 19 / 23 Climate change had no apparent impact on biodiversity trends in the densely populated lower regions of Kilimanjaro over the past 40 years. The primary factor negatively affecting biodiversity is land use change driven by rapid population growth and economic development that leads to increased land-use intensity replacing native biodiversity with agricultural species and ecosystems. To our knowledge, this study is the first to link human population densities with plant species densities at a 1 km2 scale in a tropical region, utilizing remote sensing data alongside a comprehensive dataset of the species on the ground. This approach enabled us to analyze various plant groups of different environmental importance and not only simple total species numbers for more ecologically meaningful diversity trends. We also cleaned our dataset to avoid multiple counts of species, requiring sufficient ground data on species composition across the different vegetation types present. For this approach, extensive ecological field data are crucial, often not considered or not available. This makes it difficult to transfer this approach to other tropical regions. Furthermore, it highlights the importance of biological col- lections, which enable the identification and storage of collected specimens. Without the collections and taxonomists of the herbaria in Nairobi, Kew, Paris, Vienna, Oslo, Copenhagen, Berlin, Stockholm, Uppsala and Arusha, this study would not have been possible.

The declining trends in certain natural habitats would have been more pronounced if we could distinguish vegetation types at a finer scale; however, older remote sensing data limited this accuracy. For instance, wetlands – important hab- itats for birds on Kilimanjaro – have probably lost over 90% of their area in recent decades due to illegal water pumping and conversion to rice paddies, often abandoned after a few years due to salinization. Most lower slopes of Kilimanjaro were already converted to anthropogenic land use by 1976, at the start of our detailed evaluation. A century ago, when Kilimanjaro’s population was below 100,000, approximately 98% of its savanna vegeta- tion remained intact, and natural vegetation covered 90% of the lower slopes. Although biodiversity data from that time is limited, extrapolating our population density and species richness trends suggests that in 1911, nearly 700 native plant species existed per km2, whereas today, this number has declined to approximately 180. If we consider population density and economic prosperity as indicators of human well-being [6,78] and the area of natural habitats and species counts as indicators of ecological well-being, we can see significant gains in human well-being alongside strong declines for environ- mental well-being at Kilimanjaro in recent decades. However, land degradation from poor management, such as wetland destruction and invasion by non-native species due to overgrazing former habitats of iconic animals such as elephants, giraffes and lions, threatens both the environment

and long-term human well-being, despite current economic growth. For instance, asthma rates have reportedly increased on Kilimanjaro, possibly due to the replacement of permanent vegetation with fields, raising dust and allergen levels, especially during the dry seasons and exacerbated by the spread of allergenic invasive species such as Parthenium in disturbed areas. On the positive side, as a combination of ecological and human well-being, the development of sustainable traditional agroforestry through the conversion of degraded land, the protection of the Rau forest compared to poorly managed reserves and the establishment of protected areas like Namalok are potential strategies to mitigate drastic changes in land cover. Chagga homegardens stand out as a model for sustainable agricultural practices in tropical regions, showcasing land use without soil degradation or erosion for centuries. Furthermore, effective management of Kilimanjaro National Park, which provides essential ecosystem services, benefits the local population. The Namalok nature reserve, which is since 2010 protected from overgrazing by cattle, also provides space for large animals, while the local population benefits from the renewable forests as they are allowed to collect the now abundant firewood. Kilimanjaro remains a biodiversity hotspot, reflecting both the extraordinary diversity of its flora and vegetation and the challenges posed by demographic and ecological changes, exemplifying broader issues and opportunities for other tropi- cal regions.