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 11 / 23 contributed to the expansion of specific categories. Notably, savanna conversion was the primary driver of agricultural expansion, with smaller contributions from other LULC classes. Biodiversity trends The number of all plant species per km2 (mean at ward level) exhibited only a slight decrease of approximately 10%, between 1976 and 2022 (Table 4). The decline in plant species of natural habitats, including forest, regeneration, and savanna, was much more pronounced, with a decrease of 46%. Conversely, there was an increase of approximately 25% in neophytes during the same period. This means that not only is species richness changing, but also species composi- tion, i.e., there is an increase in partially invasive and widespread species at the expense of native species with partially restricted distribution. Discussion Demography and economic development as main drivers of land use changes World population is projected to continue growing throughout the century, reaching between 9.6 billion and 12.3 billion at the end of the century, with Africa expected to experience at least a 3.5-fold increase in population. The current population of Africa, approximately 1.5 billion people in 2024, is projected to reach approximately 2.2 billion in 2054 [50]. Tanzania’s population growth rate stands at about 3.0% annually. If this trend persists, Tanzania’s population is estimated to increase

from about 65 million in 2022 to approximately 95 million in 2050 and over 300 million by the end of the century [49,51]. The human population on Kilimanjaro, comprising Moshi Urban, Moshi Rural, Rombo, Siha, and Hai Districts, has multiplied 28 times over 133 years from about 50,000 in 1889 [41] and 100,000 in 1913 [37] to over 1.4 million in 2022 ([49], Fig 4). As a result, population density increased from 15 people/km2 in 1889–30 in 1913, 50 in 1948 [52], and 211 in 1978, eventually reaching 430 people/km2 in 2022. The local population density (outside the cities) rose to unprecedented levels of up to 1500 people/km2 in the most fertile highland area of the Chagga homegardens. This population growth resulted in a severe shortage of cultivable land and the subdivision of land into fragments too small to support a family and too numerous to allow for further expansion [25,53]). Consequently, many young people migrated from the highlands to urban centers such as Moshi, Arusha, and Dar es Salaam, or settled on the foothills of Kilimanjaro. This emigration from the highlands explains why the demographic growth rate on Kilimanjaro is lower than the national average in Tanza- nia and why the population growth rate in the lowlands (2.9%) was much higher than in the highland homegarden areas Table 3. Land cover changes 1911-2022. LULC/ ecosystem type 1911 area (km2) 1911 area (%) 1976 area (km2) 1976 area (%) 2002 area (km2) 2002 area (%) 2022 area (km2) 2022 area (%) Agriculture 66 2 1062 32 1406 43 1486 45 Agroforestry 268 8 395 12 650 20 849 26

Forest 319 (500*) 10 201 6 119 4 60 2 Grassland NA NA 134 4 32 1 28 1 Regeneration 180 5 281 9 87 3 43 1 Savanna 2449 75 1090 33 795 24 588 18 Forest plantat. 0 0 116 4 170 5 150 5 Built-up 0 0 6 0 24 1 74 2 Water body 2 0 2 0 2 0 2 0 (*) estimated number including not covered riverine and ravine forests; NA: anthropogenic grasslands could not be delineated for 1911 (see Supplemen- tary Information S1 Text). https://doi.org/10.1371/journal.pone.0334184.t003