**3.3. Spatial distribution of animal and plant species along the landscape of West Usambara Mountains**

**Table 1** shows that plant and animal species occur across the three major geomorphic units. The diversity of species per landscape varies as has been reported by Meliyo et al. [51] and Njaka et al. [6]. The authors indicated that species specifically vegetation/habitat diversity

**Plate 1.** Morphogenetic processes shaping the landscape: (a) Mass movement where soils are eroded at wide portion of landscape down the slope after rainfall. (b) Shows deposition of varied rocks and boulders falling from upslope, (c) Shows old landslide scars prominent in the study area indicating the role played by landslide in the existing landform, and (d) Shows rock fall in the plateau, the practice very common along steep slopes over 25°, which is the case in the plateau and escarpment.

increases with increasing elevation. **Table 1** depicts that there were differences of abundance and types of vegetation cover/habitat and small mammals species found at different landscapes. Results show that there were fewer small mammals in the plain compared to the plateau. Although the number of small mammals increased with elevation (escarpment) the number in the plateau were more than the plain and the escarpment geomorphic units. Plant species or habitats in the low plain are mainly woody shrubs; thickets and even the scattered trees in some places are those salt tolerant species. This could be attributed to the fact that the plains are characterised by low sporadic rains and long period of droughts, that have led to development of sodic and saline soils, which only plants adapted to it could survive the hard shrubs and thickest.

The soils of the escarpment are mainly shallow, gravelly, stony and rocky and in some places just rocky land without soil. This implies that many plant species grow with difficulties. In few areas with deep soils, there were large trees and dense none thorny shrubs, well established compared to the drier plain. Moisture availability also could be a factor-segregating species distribution. **Table 1** further shows that some animal species are located only in one geomorphic unit and not the other. For instance, *Dwarf Mangoose*, *Genetta genetta* and Squirrels were just found in the plains, although different elevations. This could be attributed to the characteristics of the niche, which encompasses food availability, breeding places and or climate adaptation.


UP, undulating plain; RP, rolling Plain; LE, lower escarpment; UE, upper escarpment; P, plateau; WST, woody shrubs and thickets; WSST, woody shrubs+surface stones; CSST, cropland+shrubs+surface stones.

**Table 1.** Spatial distribution of plant and small mammal species in West Usambara Mountains.

increases with increasing elevation. **Table 1** depicts that there were differences of abundance and types of vegetation cover/habitat and small mammals species found at different landscapes. Results show that there were fewer small mammals in the plain compared to the plateau. Although the number of small mammals increased with elevation (escarpment) the number in the plateau were more than the plain and the escarpment geomorphic units. Plant species or habitats in the low plain are mainly woody shrubs; thickets and even the scattered trees in some places are those salt tolerant species. This could be attributed to the fact that the plains are characterised by low sporadic rains and long period of droughts, that have led to development of sodic and saline soils, which only plants adapted to it could survive the hard

**Plate 1.** Morphogenetic processes shaping the landscape: (a) Mass movement where soils are eroded at wide portion of landscape down the slope after rainfall. (b) Shows deposition of varied rocks and boulders falling from upslope, (c) Shows old landslide scars prominent in the study area indicating the role played by landslide in the existing landform, and (d) Shows rock fall in the plateau, the practice very common along steep slopes over 25°, which is the case in the

The soils of the escarpment are mainly shallow, gravelly, stony and rocky and in some places just rocky land without soil. This implies that many plant species grow with difficulties. In few areas with deep soils, there were large trees and dense none thorny shrubs, well established compared to the drier plain. Moisture availability also could be a factor-segregating species distribution. **Table 1** further shows that some animal species are located only in one geomorphic unit and not the other. For instance, *Dwarf Mangoose*, *Genetta genetta* and Squirrels

shrubs and thickest.

plateau and escarpment.

154 Pure and Applied Biogeography

Plant species in the plateau are mainly plantation forest, where *Pine* spp., *Eucalyptus* spp., and *Camphor* spp., are planted for timber, project. Before 1980 most of the mountains and hills of the study area were treeless, due to deforestation followed after independence 1961, which then cleared thousands of forests for obtain farmland. Deforestation was followed by severe soil erosion, which the Tanzania Government intervened by formulating a project Soil Erosion Control and Agroforestry Project (SECAP) [52], which promoted tree plants and agroforestry. Hence individuals established tree woodlots in places where was stricken by soil erosion and landslides. However, in the plateau there are still few, small pockets of natural forest such Magamba Nature Reserve. That kind of forests remains, are the pockets harbouring the natural rich biodiversity of plant and animal species, west Usambara Mountains.

#### **3.4. Abiotic factors explaining spatial distribution of species along the landscape**

**Table 2** and **Figure 5** present results that indicates that factors influencing spatial distribution of small mammals were elevation (*p* < 0.001), surface stones (*p* < 0.001), rock outcrop and cultivation (%) (*p* < 0.05) and slightly surface curvature (profile and cross) which is negatively influencing species spread across the landscape (*p* < 0.1). The atmospheric temperature and


Significant codes: 0 '\*\*\*' 0.001 '\*\*' 0.01 '\*' 0.05 '.' 0.1 ' ' 1.

Null deviance: 3956.1 on 481 degrees of freedom.

Residual deviance: 3026.9 on 471 degrees of freedom.

One observation deleted due to missingness.

AIC: 2277.5.

**Table 2.** Factors influencing distribution of small mammal species along the landscape of West Usambara Mountains, Tanzania.

Plant species in the plateau are mainly plantation forest, where *Pine* spp., *Eucalyptus* spp., and *Camphor* spp., are planted for timber, project. Before 1980 most of the mountains and hills of the study area were treeless, due to deforestation followed after independence 1961, which then cleared thousands of forests for obtain farmland. Deforestation was followed by severe soil erosion, which the Tanzania Government intervened by formulating a project Soil Erosion Control and Agroforestry Project (SECAP) [52], which promoted tree plants and agroforestry. Hence individuals established tree woodlots in places where was stricken by soil erosion and landslides. However, in the plateau there are still few, small pockets of natural forest such Magamba Nature Reserve. That kind of forests remains, are the pockets harbouring the natural rich biodiversity of plant and animal species, west Usambara

**3.4. Abiotic factors explaining spatial distribution of species along the landscape**

**Coefficients Estimate Std. error t-value Pr(>|t|)** (Intercept) −5.8547295 1.2733596 −4.598 5.49e-06\*\*\* Elevation (m a.s.l.) 0.0024506 0.0006312 3.883 0.000118\*\*\* Rock\_Out\_crop 0.4354978 0.1870275 2.329 0.020307\* Surface\_Stone 1.3789108 0.3826639 3.603 0.000347\*\*\* Slope\_Aspect 0.0011544 0.0011295 1.022 0.307281 Profile\_Curv 0.3181454 0.1881989 1.690 0.091599. Cross\_Curv −0.3193107 0.1882792 −1.696 0.090558. Soil\_Depth1 −0.0175444 0.0158829 −1.105 0.269894 Soil\_Depth3 −0.0055701 0.0150322 −0.371 0.711143 Cultivated\_Area 0.0268368 0.0136358 1.968 0.049641\* Atmospheric T°C 0.0191391 0.0170757 1.121 0.262929

Significant codes: 0 '\*\*\*' 0.001 '\*\*' 0.01 '\*' 0.05 '.' 0.1 ' ' 1. Null deviance: 3956.1 on 481 degrees of freedom. Residual deviance: 3026.9 on 471 degrees of freedom. One observation deleted due to missingness.

AIC: 2277.5.

Tanzania.

**Table 2** and **Figure 5** present results that indicates that factors influencing spatial distribution of small mammals were elevation (*p* < 0.001), surface stones (*p* < 0.001), rock outcrop and cultivation (%) (*p* < 0.05) and slightly surface curvature (profile and cross) which is negatively influencing species spread across the landscape (*p* < 0.1). The atmospheric temperature and

**Table 2.** Factors influencing distribution of small mammal species along the landscape of West Usambara Mountains,

Mountains.

156 Pure and Applied Biogeography

**Figure 5.** Plot of deviance residuals from GLM fitted to establish factors influencing small mammal species distribution along the landscape of West Usambara Mountain, Tanzania.

many other factors that were considered did not work out to be predictors. These results are congruent with field observation presented in **Table 1**, which show that the number of small mammals trapped was high in the plateau (higher elevation) than in the escarpment and the plain.

The results are too supported by those reported by Njaka et al. [6] who indicated that small mammal abundance increased with increasing elevation. The increase of small mammals with elevation could be attributed to favourable climatic conditions such occurrence of rainfall and reduced temperature compared to the low plain characterised by hot temperature and drought. Cultivation of food crops in the higher plateau due to comparably higher rainfall and cool temperatures serves as the habitat and continuous food supply which in short while attracts species but in future lead to extinction of those which do desire human interactions. The above account is in agreement with Ward et al. [53] who indicated that distribution of species is influenced by a large number of abiotic factors like environmental stability, habitat heterogeneity and relevance [54] and ecosystem production. It is also true that surface stones and rock outcrops offer small mammal both safe havens and breeding places. This implies that surface stones and rock outcrops create microhabitats that are hardly accessible to disturbances due to other organisms mostly humans. All predictors had positive coefficients, which show positive correlation of the independent variables to dependent ones. Surface stones and rock outcrops are microhabitats with unique characteristics, which influence spatial distribution of the small mammals particularly in the plateau [51]. The results also show that surfaces stones make microhabitat which influence abundance and hence distribution small mammals along the landscape, and the high population of small species at the plateau may be attributed to microhabitat as well as stable food supply associated to food crop cultivation. The results also agreed well with field data which show that few species were captured per trap station in the plain than the plateau, and also the plant species richness and diversity are greater in the plateau than the plain. Similar results were reported by Meliyo et al. [51] and Njaka et al. [6]. The results are also in agreement with those presented by Hastie et al. [55] who indicated that microhabitat influences local density of species and their spatial distribution, however, our results could not show that temperature and moisture to be important drivers of spatial distribution in the study area.

**Table 3** and **Figure 6** present results depicting abiotic factors influencing spatial distribution of plant species along the landscape of West Usambara Mountains. The results show that elevation (metres above sea level), top soil depth and cultivation practices are major determinants of spatial distribution of plant species and they are statistically significant (*p* < 0.001) predictors, although cultivation practices are negatively influencing plant distribution i.e. vegetation clearing.

Other negatively influencing factors of statistical significance are hillshade (*p* < 0.01) and surface stones (*p* < 0.05) and atmospheric temperature in degrees Celsius (p < 0.05) (**Table 3** and **Figure 6**). Our results are in agreement with findings by Chen et al. [56] who studied factors affecting the distribution of pant species in Hainan Island, China, and reported many factors including elevation, soils, rainfall and human disturbances.

Similarly, our results on spatial distribution of plant species in the west Usambara Mountains are supported finding by Trigas et al. [57] who reported an increase in proportion of plant species endemism with increasing elevation of Cretan Mountain that could only be explained by elevation-driven ecological factors. Ecologically, there are many factors coming into play, including temperature, rainfall, soils, and where human disturbances occur particularly deforestation, these leading to plant species extinctions in some area in the world [56].
