A biogeographical appraisal of the threatened South East Africa Montane Archipelago ecoregion

Abstract

Recent biological surveys of ancient inselbergs in southern Malawi and northern Mozambique have led to the discovery and description of many species new to science, and overlapping centres of endemism across multiple taxa. Combining these endemic taxa with data on geology and climate, we propose the 'South East Africa Montane Archipelago' (SEAMA) as a distinct ecoregion of global biological importance. The ecoregion encompasses 30 granitic inselbergs reaching > 1000 m above sea level, hosting the largest (Mt Mabu) and smallest (Mt Lico) mid-elevation rainforests in southern Africa, as well as biologically unique montane grasslands. Endemic taxa include 127 plants, 45 vertebrates (amphibians, reptiles, birds, mammals) and 45 invertebrate species (butterflies, freshwater crabs), and two endemic genera of plants and reptiles. Existing dated phylogenies of endemic animal lineages suggests this endemism arose from divergence events coinciding with repeated isolation of these mountains from the pan-African forests, together with the mountains' great age and relative climatic stability. Since 2000, the SEAMA has lost 18% of its primary humid forest cover (up to 43% in some sites)-one of the highest deforestation rates in Africa. Urgently rectifying this situation, while addressing the resource needs of local communities, is a global priority for biodiversity conservation.

Figure 1

Location and extent of the South East Africa Montane Archipelago (SEAMA) showing core sites in red, and an outline boundary of the convex hull of the ecoregion (created using QGIS version 3.28.12 LTR https://qgis.org/en/site/).

Figure 2

Contemporary macro-climatology of the South East Africa Montane Archipelago. Also labelled is the Eastern Arc Mountains (EAM) ecoregion to the north, and the Eastern Highlands of Zimbabwe to the southwest (EHZ). Annual rainfall (a) is measured in mm. Rainfall seasonality (b) is the coefficient of variation across months. Aridity index (c) is the ratio of annual rainfall to potential evapotranspiration with values above 0.65 considered humid. Maximum water deficit (d) is a measure of water stress defined across the most arid months of the year, with lower values conducive of potentially dense, evergreen canopy structure. Bioclimatic layers were extracted from WorldClim version2 (https://www.worldclim.com/version2) and presented in QGIS version 3.28.12 LTR (https://qgis.org/en/site/).

Figure 3

Comparative ecoregion endemism according to the species-area function. Within each taxonomic group and ecoregion, we plot the number of endemics divided by A^z, where A is the core area (ha) of the ecoregion and z = 0.25 is taken as a representative value of the slope of the species-area curve. This yields a metric of endemism to estimate the number of endemic species per hectare, which diminishes the impact of larger areas.

Figure 4

Relative survey effort across core sites in the SEAMA. Bars plot a comparative measure of sampling intensity, relative to the area of each site. Score varies between 0 and 1, where 0 is the minimum (no sampling at all) while 1 is the maximum effort possible to obtain.

Figure 5

Examples of SEAMA endemics. (a) Rhinolophus mabuensis (AM), (b) Chamaetylas choloensis (JB), (c) Nothophryne inagoensis (WC), (d) Atheris mabuensis (WRB), (e) Epamera malaikae (TCEC), f) Rhampholeon maspictus (JB), (g) Nadzikambia baylissi (WRB), (h) Maritonautes namuliensis (JB), (i) Euphorbia mlanjeana (ID), (j) Widdringtonia whytei (JB), (k) Encephalartos gratus (JB).