Habitat preferences, seasonal abundance and diets of rodents in Alage, Southern Ethiopia

Abstract

To understand habitat preferences, seasonal abundance and diets of rodents in wet and dry season surveys were conducted in Alage, Southern Ethiopia. Sherman and snap traps were used to capture rodents from the four habitats: bushland, Acacia woodland, maize and wheat farmlands. A total of 3312 trap nights, from the four trapping habitats, yielded 776 individuals that represented 11 species of rodents. The distribution of rodents varied between habitats and seasons. Wet season rodent abundance was 52.3% while in the dry season it was 47.7%. Seasonal differences in species abundance were insignificant. Bushland habitat had high wet and dry season abundances with 137 and 211 individuals, respectively. Abundance was low in maize farm (57 individuals) in the wet season and wheat farm (10 individuals) in the dry season. Stomach content composition analysis of snap-trapped rodents from different habitats showed differences between species and across seasons. Six rodent species were recorded as pests on the farmlands in this study area. In conclusion, variation in habitat preferences and diet of rodents in different habitats and across seasons might be due to the role of ground cover and food sources. Pour bien comprendre les préférences des rongeurs en matière d'habitat, leur abondance saisonnière et leur régime alimentaire, en saison sèche et en saison des pluies, des études furent menées à Alage, dans le sud de l'Éthiopie. Nous avons employé des tapettes et des pièges de Sherman pour attraper des rongeurs dans les quatre habitats suivants: brousse, forêt d'Acacias, cultures de maïs et de blé. En 3 312 nuits-pièges, dans ces quatre habitats, 776 individus furent collectés, représentants 11 espèces de rongeurs. Leur distribution variait selon l'habitat et la saison. En saison des pluies, l'abondance des rongeurs était de 52,3% alors qu'elle était de 47,7% en saison sèche. Les différences saisonnières de l'abondance des espèces étaient non significatives. L'habitat brousse connaissait de fortes abondances en saison sèche comme en saison des pluies avec respectivement 211 et 137 individus. L'abondance était faible dans les cultures de maïs (57 individus) en saison des pluies, et dans les cultures de blé en saison sèche (10 individus). L'analyse de la composition du contenu stomacal des rongeurs capturés par les tapettes dans différents habitats révélait des différences selon les espèces et les saisons. Six espèces étaient rapportées comme nuisibles dans les exploitations agricoles de la zone d'étude. En conclusion, les variations des préférences de rongeurs en matière d'habitat et de régime alimentaire, selon les habitats et les saisons pourraient être dues au rôle de la couverture du sol et aux sources de nourriture. Globally, small mammals form a major proportion of the mammalian fauna. Among mammals, rodents comprise 42% and represent the largest order with over 2000 species (Nowak, 1999; Singleton et al., 2003). The abundance and distribution of rodents depend mainly upon the nature of vegetation for food and shelter (Kotler, 1984). Temperature, energy and nutrition are prime important factors in determining the timing of reproduction of rodents (Magige & Senzota, 2006). Habitat selection and dietary habits of rodents are considered to be important factors shaping in community dynamics of small mammals (Oguge, 1995; Shurchfiesd, 1997). Rodents are known to have ecological, eco-social and cultural values and provide major benefits to our environments as biological engineers. Despite their values, some rodent species play a role as pests, and human and animal disease transmitters in the world (Delany, 1986). However, the economic and ecological benefits of rodents offset their negative effects. To date, 5% of rodent species have been identified as pests in agricultural areas causing significant damages to agricultural crops (Singleton et al., 2003). In Ethiopia, 84 species of rodents have been recorded (Bekele, 1996a), and from these, 11 have been identified as pests (Bekele & Leirs, 1997). In spite of having high biodiversity of small mammals (Yalden & Largen, 1992; Bekele, Leirs & Verhagen, 2003), Ethiopia is among the least studied in terms of faunal diversity and documentation, habitat characterization, seasonal variation and their diet preferences. Few studies on the small mammal diversity, abundance and habitat associations were conducted, but focused in some central, south-eastern and northern localities of Ethiopia. Studies on habitat preference, seasonal abundance and diets of the rodent community are still poorly known for many regions of Ethiopia. Countrywide studies on diets and habitat preferences of rodents in different habitats are important to understand relationships between species and between rodents and the environment. This study was on ecological surveys in Alage and its environs to determine habitat and diet preferences, seasonal variation of rodent abundance and to identify pest rodents that cause damage on agricultural products. The study was carried out in Alage and its environs, southern Ethiopia in 2006 and 2007. The study area is located 215 km South of Addis Ababa along the Rift Valley and borders the two Rift Valley Lake Abijata (10 km east) and Lake Shalla (8 km north). Alage is located at 38°27′ east longitudes and 7°36′ north latitudes, with a range of altitudinal range of 1580–1650 metre above sea level (a.s.l.). It receives bimodal rainfalls (from June to September and from March to April) with mean annual rainfall of 860 mm. Temperature ranges between 16 and 29°C with a mean daily temperature of 21°C (Fig. 1). Four permanent 70 × 70 (4900 m2) live-trapping grids were set-up at randomly selected habitat types. Grids 1 and 2 were in a natural habitat (acacia woodland and bushland). Grids 3 and 4 were in an agricultural area (maize and wheat farms). Trapping was made in four sessions covering the two different seasons. The first wet season data gathering was carried out from end of August to the middle of September and the second from end of September up to October 2006. The first dry season data were collected in December 2006 and the second was from February up to March 2007. In each trapping grid and session, 49 Sherman live traps were set at 10-m intervals for three consecutive days. Traps were covered by hay and plant leaves to protect trapped animals against the strong heat and cold. A capture–mark–recapture (CMR) method was employed by toe-clip marking, and animals were released at the point of capture. In addition, snap-trapping was carried out to obtain information on stomach content composition/diet only. Snap-trapping grids were chosen at least 200 m away from live-trapping grids to avoid interfering with live-trapping study. A total of 20 snap traps at an interval of 20 m were set. Traps were baited with peanut butter and crushed white onions, and traps were set between 05:00 PM and 6:30 PM in the evening. Traps were checked twice a day in early morning and late afternoon. Trapped animals were removed from the trap, identified and recorded as for grid and trap station number. Subsequently, animals were weighed, toe-clipped for coding, sexed and aged, observed for sexual condition and released at the point of capture (Alpine et al., 2003). All the external attributes such as fur colour and texture, back colour of fore and hind foot, whisker and other physical features (size and shape) were also recorded. Sampled specimens were identified to the species level and mounted voucher specimens were deposited in the Natural History Museum of Addis Ababa University. For the specimen identification, museum specimens and taxonomic characters in Yalden & Largen (1992), Bekele (1996a), Nowak (1999) and Alpine et al. (2003) were used. Snap-trapped animals were dissected for stomach content analysis, and the content preserved in 10% formalin until further microscopic examination. Stomach contents were weighed using a Pesola spring balance after drying the moisture content at 50°C for 24 h. Then, the stomach samples were mixed thoroughly and washed with distilled water to remove fine particles for proper identification. Five slides were prepared for each stomach sample and observed under a compound microscope to identify the type as well as the proportions of the content compositions (Leirs, 1995). Food items were grouped into plant seeds (monocot and dicot), plant leaves (monocot and dicot), plant roots, earthworms and arthropods. Identification of arthropods was carried out based on the presence of head, legs and wings from the contents. Then, the proportion of different food fragments in the stomach content was determined by counting the fragments from the entire slide. The results were expressed in terms of the mean percentage of occurrence of different food fragment. Data were stored and managed in an excel spreadsheet. SPSS (version 14.0) software was used to analyse the data. Habitat preferences of rodents were determined by comparing the number of individuals capture in a specific habitat. Relative abundance of small mammals was calculated by dividing number of each species by total number of capture and multiplied by hundred. Chi-square and Fisher's exact tests were employed to evaluate the significance of differences for the stated parameters. A total of 790 individuals of small mammals were captured in 3312 trap nights from four trapping sessions. Seven hundred seventy-six individuals (98.2%) represented 11 species of rodents while the remaining 14 individuals (1.8%) represented 2 species of insectivores/shrews. Trapped small mammal species and their relative abundance are referred in Table 1. Mastomys natalensis was the dominant (31.6%) species trapped, followed by Arvicanthis dembeensis (23%) and Mastomys erythroleucus (18%). Graphiurus murinus was the least in abundance (0.1%) among the trapped small mammals. Insectivores were trapped using live traps from three of the habitats, but not in the maize plantation. Six hundred and eighty-four (88.1%) of the rodents were captured by Sherman live traps and 92 (11.9%) by snap traps in 2352 and 960 trap nights, respectively. Eight species were captured commonly by both traps; but three species (Lophuromys flavopunctatus, Mus musculus and Graphiurus murinus) were caught only by live traps. Mastomys natalensis was the most commonly trapped species using both trap types, followed by A. dembeensis and M. erythroleucus (Table 1). The distribution of species varied among habitats. The highest percentage (50.9%) of individuals was trapped from bushland, followed by 26.6% from acacia woodland habitat. The lowest percentage (11%) of individuals was captured from maize farm, followed by 11.8% from wheat farm. Mastomys natalensis was the most widespread and abundant species in most habitats, followed by A. dembeensis and M. erythroleucus. Lophuromys flavopunctatus and G. murinus had restricted distribution and were found only in bushland and acacia woodland, respectively. Arvicanthis dembeensis was the most abundant rodent in the maize farm followed by M. natalensis. Habitat choice was statistically significant (χ2 = 149.0, df = 30, P < 0.001). Mastomys natalensis, A. dembeensis M. erythroleucus and P. harringtoni were present in all habitats (Table 2). Variation on species richness and abundance was insignificant seasonally. The abundance of rodents between the wet and dry seasons was 52.3% and 47.7%, respectively; the difference was statistically insignificant (Table 3). More individuals were caught in bushland during wet and dry seasons with 137 and 211 individuals, respectively. Much fewer were caught in maize (57) and wheat (10) farms during wet and dry season, respectively (Table 3). A variety of food items were eaten by the rodents (Table 4); plant aerial parts were the most frequently eaten. Earthworms and arthropods were more eaten in the wet season whereas plant roots were eaten more in the dry season. However, earthworms were recorded only from M. natalensis and M. erythroleucus. During the dry season, there was a high dependence on eating seeds of both monocots and dicots. All examined rodents species consumed plant matters and arthropods during both seasons (Table 4). Based on stomach contents, a total of six rodent species were recorded as pests of wheat and maize crops on the farmlands including M. natalensis, A. dembeensis, M. erythroleucus, M. mahomet, S. albipes and T. robusta. Abundance of rodent pests was in the order of A. dembeensis (33) followed by M. natalensis (21), M. erythroleucus (11), M. mahomet (10) and S. albipes (10) in maize farmland and M. natalensis (30) followed by A. dembeensis (27), M. erythroleucus (15), M. mahomet (9) and T. robusta (12) in the wheat farm. Rodents were classed as pests basing on their stomach contents. In most pest rodents, the percentage of maize seed fragments was higher than that of other food items. In this study, eleven species of rodents and two species of insectivores were trapped. This is in accordance with other reports elsewhere in Ethiopia (Bekele, 1996b; Bekele & Leirs, 1997; Bekele, Leirs & Verhagen, 2003; Demeke, Afework & Gurja, 2007), which showed similar representation of rodents and insectivores. Among rodents, M. natalensis was the most widely distributed and abundant species in all habitats in this survey. Its abundance was very high in natural habitats (bushland and acacia woodland). Similarly, Yalden, Largen & Kock (1976)reported that it has wide distribution in different parts of Ethiopia. Demeke, Afework & Gurja (2007) also affirmed that M. natalensis was the most abundant rodent species in natural habitat (bushland) in southern Ethiopia. Arvicanthis dembeensis, unstriped grass rat, was recorded as the second most abundant species and occurred in all habitats. Similar findings were reported by Capula et al. (1997) and Bekele, Leirs & Verhagen (2003). Ducroz, Granjon & Chevret (1997) also explained that it is the most common species of rodent in Africa. M. erythroleucus, the third most common and abundant species in the present study area, is widely distributed and closely associated with A. dembeensis in bushland and acacia woodland. His species was recorded in different areas of Ethiopia by Bekele & Leirs (1997). It has also been recorded as widespread and most common species in natural habitat and farmlands from central Ethiopia (Bekele, Leirs & Verhagen, 2003). Lophuromys flavopunctatus, the brush-furred rat, was recorded only in the bushland in the present study area. This might be due to the open nature of the habitats as well as the peculiar feature of this species that specializes on arthropod feeding, which has facilitated its dominance in the forest, bush scrub or dense vegetation cover. As Kingdon (1997) and Nowak (1999) stated, it is common in areas with dense vegetation cover and scarce in open habitat types. In the present study, habitat preference and abundance of rodents varied with habitat types. This agrees with reports with similar studies in other localities of Ethiopia (Bekele, 1996b; Bekele, Leirs & Verhagen, 2003). This might be due to vegetation cover and food as indicated elsewhere (Taylor & Green, 1976). Species richness and abundance of rodents were higher in the natural habitats whereas in farmlands few numbers of rodents was recorded, this suggesting that most species in the area preferred natural habitats over farmlands. Of the 11 trapped rodent species, all were recorded in natural habitats. This might be due to the habitat heterogeneity as well as suitable vegetation cover for shelter and continuous food supply in the natural habitats. Moreover, suitable moisture content of soil and moderate temperature in the habitat might also favour rodents to prefer such habitat. Farmlands contained homogenous and low vegetation cover, particularly in maize farms which were mostly clean from weeds and supplied low amounts of food and shelter resulting lowest species composition and abundance. As Magige & Senzota (2006) indicated, habitat heterogeneity and seasonal variations of the area are known to influence species richness, diversity and abundance of rodents. Kotler (1984) also noted that habitats with enough ground cover provide sufficient food and reduce predation risk, which in turn would enhance the species richness and abundance of rodents. Similarly, Adler (1995) and Shanker (2001) revealed the positive role of ground cover and habitat heterogeneity in structuring rodent population and habitat preferences. Although seasonal variation in species richness and abundance of rodents in the present study was insignificant, more individuals were recorded during the wet season than dry season. This is not consistent with the findings of Happold & Happold (1991) and Demeke, Afework & Gurja (2007) who have recorded more individuals during the dry season in their respective study areas. Different factors such as climatic condition, availability and quality of food, ground cover, habitat disturbances and predation would contribute for abundance fluctuation of rodents, particularly in areas with intense human and large animal interactions. In the present study area, crop harvesting and grazing were perhaps the considerable factors for the reduction in rodents abundance during the dry season. Information obtained from stomach content analysis on rodents is the main ecological parameter to study the food preference, habitat association, seasonal fluctuation and the pest status (Campos et al., 2001). In present study, results of the stomach content analysis showed that all examined rodents consumed plant matters and arthropods during both seasons. However, frequency of each food item was varied with species. Unlike others studied rodent species, M. natalensis and M. erythroleucus fed on all types of described food items. This might be an indication for their success in having wide distribution. Arvicanthis dembeensis usually preferred dicot plant matters during the wet season and depended more on monocot plants during the dry season. This agrees with the finding of Taylor & Green (1976), which stated that Arvicanthis switched its diet dramatically first to dicotyledonous plants and then almost completely to grasses during the wet and dry seasons, respectively. Gebresilassie et al. (2004) also found that A. dembeensis was more frequent in habitats with monocot plants. Tatera robusta ate more seed during the wet season than the dry season. Consumption of arthropods by T. robusta in the wet season was in relatively high proportion compared with other prey species. This was in agreement with the findings of Oguge (1995) from Kenya. Plant root was absent only from the stomach content of M. mahomet. However, food item composition in M. mahomet was not varied in both seasons. In the present survey, the common pest rodents identified in maize and wheat crops were A. dembeensis, M. natalensis, M. erythroleucus, M. mahomet, S. albipes and T. robusta. The stomach content analysis showed that A. dembeensis and M. natalensis consumed the highest quantity of grain followed by M. erythroleucus and M. mahomet. The dietary results and their abundance revealed that A. dembeensis and M. natalensis were the major pest rodents in the study area. This is similar with the reports of Bekele & Leirs (1997), Bekele, Leirs & Verhagen (2003) and Odhiambo et al. (2005), which indicated that they are important pest rodents in farmlands elsewhere. Moreover, Gebresilassie et al. (2004) also reported them as major pest rodents in Maynugus irrigation field. In general, variation in the habitat preferences and food items of rodents in different habitats and seasons might be due to the attribution of ground cover and food sources. Bushland had a relatively better ground cover, habitat heterogeneity and food resource; it contained the highest number of rodents whereas farmlands due to the homogenous nature of the habitat contained less number of rodents. In farmlands, A. dembeensis, M. natalensis, M. erythroleucus, M. mahomet, T. robusta and S. albipes were recorded as pest rodents of maize and wheat crops. We acknowledge the Developmental Initiative Fund (DIF) and Addis Ababa University for their financial support.