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 17 / 23 forest areas on the southern slope became homegardens, and between 1976 and 2002 the Meru-Kilimanjaro forest corri- dor was destroyed (Fig 5). In parallel to the decreasing area of natural habitats, Fig 9A shows not only a similar negative trend over the study period, but also a similar decrease in the number of natural species at a given population density. For example, at an average population density of 1000 people/km2, 245 species per km2 were recorded in 1976, decreasing to 180 by 2003 and 115 in 2022. This decline seems linked to increasing land use intensity driven by economic development, similar to trends in natural vegetation (Fig 7). Peters et al. [68] identified land use intensity as a key factor influencing bio- diversity on Kilimanjaro. Intensified land use, particularly pesticide application, significantly harms susceptible plant spe- cies, such as epiphytes in Kilimanjaro’s agroforestry systems. Chagga homegardens showed higher numbers of epiphytic mosses compared to intensively managed commercial coffee plantations [57]. Like other high mountains in tropical East Africa and Ethiopia, Kilimanjaro’s alpine zone has a high proportion of endemic species, with numbers decreasing at lower elevations [72]. Our dataset identified 35 endemic species, making up only 2% of the total species in Kilimanjaro’s lowlands. Fig 9B shows the declining trends of endemics in relation to population density over time.

There was also a strong negative correlation between number of Red List species and population density, with a clear decreasing trend over time (Fig 9C), resulting in many wards without any endangered species in 2022. IUCN Red List species (26 species in the study area) occurred only in forest and savanna habitats (Table 2), which are ecosystems with a strong decrease since 1976 (Figs 5 and 6). The relatively high number of Red List species at the highest population density of 3500 people/km2 is due to the presence of Rau Forest Reserve located in Moshi Municipal. Introduced invasive species are significant threats to biodiversity worldwide [2] and in Tanzania [73], where 67 inva- sive species have severe negative impacts on local ecosystems habitats and native species. Our dataset records 180 introduced species on Kilimanjaro, including cultivated plants; however, only a few, such as Prosopis juliflora, Tecoma stans, Caesalpinia decapetala, Calliandra houstoniana, Parthenium hysterophorus (causing allergies) and Lantana camara, are invasive and problematic, particularly in disturbed savanna and submontane areas, whereas Acacia mearnsii affects the former half-mile forest strips. A preliminary study (Hemp unpub. data) documented the spread of Prosopis juli- flora since 1990, which now occupies 7% of the area shown in Fig 10 and is spread by livestock [74]. The other mentioned invasive species have (up to now) not significantly impacted Kilimanjaro’s ecosystems. Similarly, Jaureguiberry et al. [14]

noted that, in recent decades, invasive alien species have been less important causes of global biodiversity loss, espe- cially in Africa, compared to land-use change. In this study, neophytes are viewed not as a factor influencing biodiversity, but as a (partly negative) part of it as a consequence of land cover change. Fig 9D shows the positive correlation between population density and neophyte numbers per km2. Our dataset includes 180 introduced species, representing about 9% of the total species. At 1000 people/km2, neophytes numbered approx- imately 109 in 1976, rising to 117 by 2002 and 128 in 2022, reflecting a decline in natural vegetation (Fig 4). The very weak and non-significant correlation in 2022 may be due to more open agriculture being converted into homegardens, which hold the highest number of introduced species on Kilimanjaro, with over 100 cultivated plants originating from out- side Africa [53] (Table 2). Therefore, the density of introduced species peaked in rural areas rather than densely populated urban areas in 2022. The increasing trend of neophytes may be stronger than reported, as our data cannot capture their immigration over time (see “critical remarks” in Supplementary Informations S1 Text). Increased tourism also raises the risk of introducing exotic plant species, especially in Kilimanjaro’s upper zones. One such species, Poa annua, a European-origin weed, is found on roadsides and pastures. Like most other neophytes on Kilimanjaro, Poa annua invades only anthropogenic or

disturbed vegetation, occurring between about 1600 and 4000 m asl. along climbing routes or their vicinity. Its spread has paralleled the rise in climbing tourism over the past 30 years [32], though an invasion of natural vegetation types seems unlikely at present.