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 3 / 23 We hypothesize the following: (i) Population growth is the main indirect driver of biodiversity change, while land use is the main direct driver. Demo- graphic change differs between rural and urban areas: The population in urban areas is growing faster than in rural areas, and the population is more stable in mountainous areas than in lowland areas. Therefore, land cover changes are more pronounced in the lowlands than in the highlands. (ii) Climate change affects biodiversity mainly through changes in land cover after catastrophic events (e.g., fires after droughts or landslides after heavy rainfall), while the direct effects are minor. (iii) Species diversity is generally decreasing, but differently for different plant groups. (iv) Direct anthropogenic impacts inside the nature reserves have decreased, while they are increasing outside the reserves. Materials and methods Study area Kilimanjaro, located in North-Eastern Tanzania, is a UNESCO World Heritage site, boasting approximately 3000 vas- cular plant species [23], representing about one-third of Tanzania’s approximately 10,000 vascular plant species [24]. The study area encompasses the inhabited lower slopes and foothills below Kilimanjaro National Park (KINAPA) with 1688 km2 within five districts of Kilimanjaro Region: Moshi Municipal, Moshi Rural, Hai, Rombo, and Siha, covering 3282 km2 (Fig 1). Surrounding the mountain between 700 and 1100m is a dry and hot colline savanna zone. This zone is characterized

by a heterogeneous mosaic of disturbed savanna vegetation, croplands, and built-up areas (Fig 2). The remaining natural vegetation faces significant pressure from human activities such as agricultural expansion and intensification, urban- ization, firewood collection, brick manufacturing, and grazing [21,25–27]. The predominant natural vegetation consists of savanna woodlands with species of the genera Acacia, Terminalia, Combretum, Ozoroa and Sclerocarya in the tree and shrub layer, but the woodlands have degraded typically into grasslands dominated by Themeda, Eragrostis, Hetero- pogon, Chloris and Cenchrus species. Remnants of tall forest canopy exist along the river courses with fig trees (mainly Ficus sycomorus), Sorindeia, Trilepisium and Mimusops and in some areas with high groundwater table closed albeit heavily disturbed lowland forests still exist. Here, Oxystigma msoo a near-endemic trees species occurs together with Khaya anthotheca, Trichilia emetica and Milicia excelsa. Rushes and Sedges (Typha domingensis, Cyperus papyrus and Schoenoplectus corymbosus) dominate the few still existing swamps. Alkaline-resistant grasses (Sporobolus spicatus and S. robustus) and shrubs (Suaeda monoica, Salvadora persica, Megalochlamys revoluta and Dobera loranthifolia) char- acterize the quite widespread dry vegetation on alkaline soils, while table palms (Phoenix reclinata and Hyphaene com- pressa) dominate on alkaline soils with higher groundwater. Smallholder farms cultivate extensive areas with maize, sunflower, beans, and millet. South of Moshi, large rice paddy

fields and sugar cane plantations exist (Fig 3), while large wheat farms are present in the western part. During fallow periods, these fields are covered with ruderal vegetation consisting of mainly introduced species such as Malvastrum cor- omandelianum, Acanthospermum hispidum, Bidens pilosa, Commelina benghalensis and Hyptis suaveolens. A detailed physiographic map by Hemp et al. [28] illustrates the different land use and land cover types (LULC) of Kilimanjaro includ- ing our study area. In the highlands within the submontane zone below KINAPA only small remnants of forest remain in the deepest val- leys, where Newtonia buchananii, Entandrophragma excelsum (the tallest tree species of Africa [29]) and Garcinia tanza- niensis (a critically endangered tree species) grow. Everywhere else, the forests have mostly been converted into Chagga