Introduction

The order Rodentia is the largest mammalian taxon, encompassing 2,552 species representing 42 % of worldwide mammalian biodiversity (n = 6,495; ^{Burgin et al. 2018}). Rodents produce large litters after a short gestation period and adapt to extreme temperatures that extend from the hot desert sand dunes of Death Valley to freezing cold in the Arctic (^{Ingles 1967}; ^{Nowak 1999}; ^{Vaughan et al. 2000}). Rodents are key components in several ecosystems playing a major role in predator-prey relationships and maintaining ecological balance (^{Kotler et al. 1994}; ^{Granjon and Traore 2007}; ^{Tschumi et al. 2018}; ^{Nyirenda et al. 2020}).

Ethiopia is one of the countries with the highest number of rodent species with a total of 104 rodent species. Within the country, the highest rodent diversity is recorded in the Ethiopian highlands where 43 rodent species are endemic to the country (^{Bryja et al. 2019}). In the Ethiopian highlands, rodents feed mainly on forbs and grasses and support a large number of carnivores and birds of prey. The very high density of rodent biomass in the Ethiopian highlands is equivalent to those of ungulates in East African savannas (^{Sillero-Zubiri and Gottelli 1995}). In the Bale Mountains, for instance, the total density of three rodent species, giant mole rat (Tachyoryctes macrocephalus), Blick’s grass rat (Arvicanthis blicki), and the brush-furred mouse (Lophuromys melanonyx), is estimated at 8,000 individuals/km² (^{Vial et al. 2011}). Minimum density estimates ranged between 32 to 89 animals/ha for L. melanonyx, 32 to 127/ha for A. blicki, and 16 to 60/ha for Stenocephalemys albocaudata. The mean monthly biomass of three diurnal rodent species was estimated at 25.7 kg/ha and 23.8 kg/ha in two Afroalpine regions of the Bale Mountains (^{Sillero-Zubiri and Gottelli 1995}).

The Ethiopian highland is the largest Afroalpine habitat on the African continent and part of Conservation International’s Eastern Afro-Montane Biodiversity Hotspot with a high level of endemism (^{Yalden and Largen 1992}). Almost 50 % of the African highlands above 2,000 masl and 80 % of the land above 3,000 masl occur in Ethiopia (^{Siebert and Ramdhani 2004}). The isolated Afroalpine mountains have resulted from the great Ethiopian volcanic eruptions ca. 30 Ma (^{Hofmann et al. 1997}). These unique environmental conditions, complex topography, narrow homothermal elevation zones, and isolated habitats lead to a wide range of speciation processes, making the Ethiopian highland a center of diversification for numerous Ethiopian mammal taxa (^{Lavrenchenko et al. 2014}).

Role of rodents in maintaining a healthy ecosystem in the Ethiopian highlands. Rodents play a major role in maintaining the structure and natural functioning of grassland ecosystems of the Ethiopian highlands, which is an important factor for the long-term survival of several endangered species (^{Hillman 1986}; ^{Sillero-Zubiri et al. 2008}). Through burrowing, rodents increase landscape heterogeneity by creating unique patches of habitat that differ from the surrounding landscape, facilitate resource flow, and enhance soil aeration and component mixing (^{Davidson and Lightfoot 2008}; ^{Galiano et al. 2014}). Herbivorous rodents also shape plant communities and influence their dynamics through seed predation (^{Moorhead 2017}). The endemic giant mole rat (Tachyoryctes macrocephalus) in the Bale Mountains, for instance, digs burrows and maintains the Afroalpine vegetation in a permanent pioneer stage through turning over the soil (^{Yalden 1985}). In the Ethiopian highlands, three rodent species, giant mole rat, Blick’s grass rat, and the brush-furred mouse, perform an important role in the Afroalpine as ecosystem engineers, turning over the soil and maintaining the vegetation in a permanent pioneer stage (^{Yalden 1985}; ^{Tallents 2007}). The focus of our paper, however, is on the role rodents play as the staple prey for large carnivores in this region.

Rodents as main prey of Ethiopia’s endangered carnivorous mammals and birds of prey. Rodents are prey for many carnivores and birds of prey in the Ethiopian highlands. One of the world’s rarest canids with less than 500 individuals, the Ethiopian wolf (Canis simensis), which is endemic to the Ethiopian highlands, relies on rodents as its main food source (^{Johnson et al. 2010}). Currently, the Ethiopian wolf survives in six isolated Ethiopian highland regions (Figure 1). In the Bale Mountains, Tachyoryctes macrocephalus represents an estimated 69.1 % of their diet by occurrence (46.6 % by volume), while the murine species Arvicanthis blicki and Lophuromys melanonyx compose 54.3 % and 39.8 % by occurrence (26.7 % and 15.5 % by volume), respectively (^{Sillero-Zubiri and Gottelli 1995}). In the Guassa Mountains, giant molerats (Tachyoryctes macrocephalus), the main prey for Ethiopian wolves in Bale Mountains, are not found, and hence the Ethiopian wolf population in Guassa feed on nine rodent species that accounted for 88 % of prey volume (^{Ashenafi et al. 2005}). Rodents of the Ethiopian highlands also serve as the main prey base for several birds of prey inhabiting the region. The Afroalpine moorlands of the Bale Mountains alone support a large guild of at least 25 species of diurnal raptors that depend on rodents as their main prey (^{Clouet et al. 2000}). These raptors include the vulnerable tawny eagle (Aquila rapax) and migrant species such as the endangered steppe eagle (Aquila nipalensis) and vulnerable greater-spotted eagle (Clanga clanga) as well as the near-threatened pallid harrier (Circus macrourus) (^{Clouet et al. 2000}; ^{Shimelis 2008}).

Figure 1 Locations of Ethiopian wolf (Canis simensis) populations in the Ethiopian highlands. 

The only breeding population of the golden eagle (Aquila chrysaetos) in sub-Saharan Africa is located in the Bale Mountains, an area representing the world’s largest continuous extent of Afroalpine habitat (^{Shimelis 2008}). While not endangered, there are also several additional raptors of interest including black eagle (Ictinaetus malaiensis), augur buzzard (Buteo augur), common kestrel (Falco tinnunculus), long-legged buzzard (Buteo rufinus), pallid harrier (Circus macrourus), and red-chested sparrow-hawk (Accipter rifiventris; ^{Clouet et al. 2000}).

The conservation implication of declining rodent densities in the Ethiopian highlands. The Ethiopian Afroalpine highland, which used to be less preferred for agriculture due to low productivity, is now increasingly being used for agriculture and livestock grazing, and new villages are increasingly being constructed (^{Stephens et al. 2001}; ^{Alemu et al. 2012}). The increased human activity and livestock grazing affect rodent density, which in turn affects the survival of Ethiopian wolves and other wildlife species that rely on rodents as their main food source (^{Marino 2003}). Afroalpine habitats in Ethiopia are geographically fragmented and facing rapid ecological change due to continuous pressure from human activities and climate change (^{Mezgebu and Workineh 2017}; ^{Kidane et al. 2019}). The loss of ground vegetation leads to loss of cover and food supply for small mammals thereby decreasing rodent diversity but increasing predation risk (^{Hoffmann and Zeller 2005}). Alteration of habitat can also destroy burrows and increase soil compaction, which affects rodents’ reproductive cycle and survival (^{Ashenafi et al. 2012}). Loss of critical prey resources resulting from increased human activities are especially concerning for the Ethiopian wolf in light of the fact that it represents the large carnivore that has experienced the greatest range contraction (^{Wolf and Ripple 2017}).

Ecological specialist carnivores with limited dietary plasticity are sensitive to impacts of competition, which depend on their specific diets, habitats, and activity patterns (^{Segura et al. 2007}; ^{Colles et al. 2009}). The Ethiopian wolf is particularly vulnerable to the effects of habitat alteration, which affects the rodent prey base of the Ethiopian highlands due to their habitat and diet specialization. The Ethiopian wolf is one of five carnivore species reported to depend on prey species declining in density by the IUCN (^{Wolf and Ripple 2017}). The density of diurnal rodents was found to be correlated with the density of Ethiopian wolves revealing the importance of rodents in the survival and reproduction of this species (^{Sillero-Zubiri and Gottelli 1995}). Rodent density in Ethiopian wolf territories is an important predictor of the rate of the survival of pups (^{Tallents 2007}). The Bale Mountains have much higher density of rodents and giant molerats (Tachyoryctes macrocephalus), which is endemic to the Bale Mountains, compared to the Guassa Mountains, Simien Mountains, and mountains in the North and South wollo (Figure 1). This enables the Bale Mountains to be home for the largest Ethiopian wolf population compared to the other respective sites (^{Ashenafie et al. 2005}; ^{Sillero-Zubiri et al. 1995}; ^{Yihune and Bekele 2014}).

Threats to the Ethiopian wolf due to a decline in rodent density may worsen by exploitative and interference competition from the African wolf (Canis lupaster; Guetema et al. 2018, 2019). The African wolf is one of the most recently discovered large mammals of Africa in the central Ethiopian highlands as it was previously classified as the golden jackal (^{Rueness et al. 2011}; ^{Viranta et al. 2017}). The expansion of human settlement intensifies the competitive interaction between the Ethiopian wolf and the African wolf due to two different mechanisms. First, as the Afroalpine area shrinks, and the available space for the two sympatric carnivores diminishes, spatial interactions leading to increased interference competition between the wolves is expected to occur. Second, as human settlement expands towards the Ethiopian wolf range, more African wolves will be drawn to the Ethiopian wolf range. The African wolf is attracted to human settlements as they feed on garbage dumps and sometimes prey on livestock (^{Guetema et al. 2018}, ²⁰¹⁹).

The Ethiopian wolf went extinct in Mount Choqa several decades ago (^{Yalden and Largen 1992}), and about two decades ago in the central highlands of Gosh Meda and Mount Guna (^{IUCN 2011}). The current increasing impact of humans may further lead to extinction of some of the Ethiopian wolf populations. Sixty percent of the landscape of the Ethiopian highlands above 3,200 m has been converted into farmland (^{Marino 2003}). The effect of climate change on the vegetation and animals in the Afroalpine mountains of Ethiopia is also very well documented (^{Chala et al. 2016}; ^{Razgour et al. 2021}). Protecting the Ethiopian highlands’ Afroalpine area will enable rodent populations to maintain their normal cycle of reproduction, a key factor for the conservation of the endangered Ethiopian wolf and several raptors.

Conclusions. The Ethiopian highland is the largest area of alpine habitat remaining in the African continent, representing a center of endemism. The presence of glaciers and the spatially largest Afroalpine ecosystem are important factors promoting the evolution of endemic biodiversity in the landscape. The highlands support 60 percent of Ethiopia’s rodent fauna (n = 104). Rodents are an important component of the Afroalpine community playing a key role as prey for the endangered Ethiopian wolf and many raptors. As one of the world’s rarest canids, the Ethiopian wolf’s survival therefore is directly tied to the persistence of high densities of these highland-adapted rodents. While the dependence on these same rodents by raptors in these areas has not been fully studied, the Afroalpine moorlands of the Bale Mountains alone maintain 25 species of diurnal raptors thought to prey on rodents. The current increasing human population, livestock grazing, and global warming in tandem affect the rodent biomass through reduction of forage availability and habitats of rodents. Alteration of habitat can also destroy burrows and increase soil compaction, which affects the reproductive cycle and survival of rodents. Loss of habitat further threatens the survival of the Ethiopian wolf by intensifying competition with the sympatric African wolf. The future survival of many endangered species including Ethiopian wolves relies in part on efforts to conserve rodent populations of the Ethiopian highlands.