Broken bridges: The isolation of Kilimanjaro's ecosystem

Abstract

Biodiversity studies of global change mainly focus on direct impacts such as losses in species numbers or ecosystem functions. In this study, we focus on the long-term effects of recent land-cover conversion and subsequent ecological isolation of Kilimanjaro on biodiversity in a paleobiogeographical context, linking our findings with the long-standing question whether colonization of African mountains mainly depended on long-distance dispersal, or whether gradual migration has been possible through habitat bridges under colder climates. For this, we used Orthoptera as bioindicators, whose patterns of endemism and habitat demands we studied on about 500 vegetation plots on Kilimanjaro and Mt. Meru (Tanzania) since 1996. Land-cover changes in the same area were revealed using a supervised classification of Landsat images from 1976 to 2000. In 1976, there was a corridor of submontane forest vegetation linking Kilimanjaro with Mt. Meru, replaced by human settlements and agriculture after 2000. Until recently, this submontane forest bridge facilitated the dispersal of forest animals, illustrated by the large number of endemic submontane forest Orthoptera shared by both mountains. Furthermore, the occurrence of common montane endemics suggests the existence of a former forest corridor with montane vegetation during much earlier times under climatic conditions 2-7°C cooler and 400-1,700 mm wetter than today. Based on the endemicity patterns of forest Orthoptera, negative consequences are predicted due to the effects of isolation, in particular for larger forest animals. Kilimanjaro is becoming an increasingly isolated ecosystem with far reaching consequences for diversity and endemism. Forest bridges between East African mountains acted as important migratory corridors and are not only a prehistoric phenomenon during periods with other climatic conditions but also disappeared in some places recently due to increasing and direct anthropogenic impact.

Diversity of Leptogium (Collemataceae, Ascomycota) in East African Montane Ecosystems

“…Tropical mountains and especially the montane forests are hot spots of biodiversity and endemism and may represent true evolutionary cradles especially for neoendemics [1][2][3][4][5]. The tropical rainforests of eastern Africa originated approximately 30 million years ago and have persisted through climatic fluctuations, mainly due to the atmospheric moisture supplied by remarkably stable Indian Ocean currents [6].…”

“…The tropical rainforests of eastern Africa originated approximately 30 million years ago and have persisted through climatic fluctuations, mainly due to the atmospheric moisture supplied by remarkably stable Indian Ocean currents [6]. There, a very complex climatic history has fragmented a once extensive ancient forest ecosystem and given rise to many unique habitats with high levels of local endemism [1,2,[5][6][7]. The windward slopes of many East African mountains benefit from moisture brought by the trade winds and sustain the last remaining fragments of East African montane rain forests, surrounded by more extensive arid forests and woodlands [6].…”

“…On Mt. Kilimanjaro, forest corridors to nearby mountains have vanished due to deforestation, leaving the mountain isolated [5]. At the higher altitudes, the montane ecosystems are influenced for example by climate change-driven forest fires [20] and altogether, Kilimanjaro has lost about 50% of its forest cover since the beginning of the last century [13].…”

Dispersal and speciation of East African mountain bushcrickets of the genus Phlesirtes (Orthoptera: Tettigoniidae: Conocephalinae, Conocephalini) related to climate fluctuations

“…As microclimatically dependent organisms, Orthoptera have the potential to reflect the climatic past and to indicate migration corridors, especially in flightless species (Hemp & Hemp, ). Arrays of closely related species of various genera are conspicuous in East Africa, with distinct species on many of the high mountains or mountain ranges.…”

A 3000-year record of vegetation changes and fire at a high-elevation wetland on Kilimanjaro, Tanzania

“…Biodiversity is high and vegetation distribution patterns are complex owing to intermittent habitat connectivity between Kilimanjaro, Mount Meru, and the older and highly endemic Eastern Arc Mountains (Burgess et al, 1998, 2007; Platts et al, 2011; Jump et al, 2014; Hemp and Hemp, 2018; Burger et al, 2019). The alpine zone (>3900 m asl) of the southern flank has a mean annual temperature ranging from 0.7 to 4.2°C and a mean precipitation ranging from 500 to 700 mm year -1 (Schüler et al, 2014a; Schüler and Hemp, 2016), where short alpine plants have patchy coverage (Hedberg, 1951; Hemp and Beck, 2001).…”