**3. Results and discussion**

#### **3.1. Variability of landforms**

The geologic characteristics and folding and faulting of the area have had significant impact on drainage line and river systems all along the plateau and escarpment. The faulting had stronger influence on the scarp formation where there are fault lines that led to vertical scarps, and/or hanging rocks. The dense drainage and rivers network which is identical to dissection of the plateau which mainly has been influenced by hydrological water flows suggesting that the dominant land forming process in the area has been denudation by moving water (erosion) in different forms such as mass wasting (landslides, mass movement), gully, rill and sheet. The different landform components and slope forms links well with landslide and in particular waste movement. The recent and past geomorphic processes influence not only the vegetation establishment but also the habitats and the diverse animals that occupy them. This assumption is similar to the description by Cottle [24] who pointed the relationship between the geology and biodiversity of both animal and plant species.

and Klute [34] respectively. Micronutrients (iron, manganese, copper zinc) were determined using Diethylenetriaminepenta-acetic acid (DTPA) according to Moberg [35]. The field and laboratory data were used to classified soils to level-2 of the FAO World Reference Base [36]. Although chemical soil properties were not used in modelling, it was used for soil classifica-

The data was organised for multiple regression analysis. There were two dependent variables (plant cover (%) and total rodent burrows. The independent variable examined were 25, which were landform types, slope gradient (degrees), slope length (m), slope form (concave, convex, straight, compound), elevation (m a.s.l.), drainage, erosion type, rock outcrops and surface stones (number), slope aspect, hillshade (radians), slope curvature types (radians), soil depth (cm), soil texture (textural class), atmospheric temperature (degrees Celsius), topsoil (10 and 30 cm) temperature and topsoil (10 and 30 cm depth) relative humidity (%) were model input data. There was a total of 487 data entries collected. Categorical data such as

Abiotic factors explaining spatial distribution of plants and animals species were established by inputting 25 factors in a Generalised Linear Model, distribution family 'Gaussian' which is

Yi = β<sup>0</sup> + β<sup>1</sup> X1i + β<sup>2</sup> X2i + β<sup>3</sup> X3i + ε<sup>i</sup> (1)

Where: Yi = respondent (dependent) variables (plant cover, trapped animals/rodent burrows

Using R software the GUI rattle [38]. Model validation was addressed by portioning the data. The 70% of the data was allocated for training while 30% was used to develop the model. Different runs were made first using all predictors then reduced or added examining the model goodness of fit by looking the null and residual deviance and Akaike information criteria (AIC), whereby a model with a smallest AIC and a narrower gap between null and residual deviance was opted as model explaining the factors influencing species distribution along the landscape. Multicollinearity, was tackled by keying or deleting weakly correlated

The geologic characteristics and folding and faulting of the area have had significant impact on drainage line and river systems all along the plateau and escarpment. The faulting had

X3i = predictors or independent variables; ε<sup>i</sup> = error term.

tion. For modelling only topsoil depth and texture were used as input data.

a multinomial for multiple dependent variables [37] applying a formula:

X1i+….β<sup>3</sup>

**2.6. Statistical analysis**

148 Pure and Applied Biogeography

textural class were given dummy number.

as a proxy); β<sup>o</sup> = Intercept; β<sup>1</sup>

variables serially in the model.

**3. Results and discussion**

**3.1. Variability of landforms**

**Figure 2** describes three different geomorphic units: the plain, escarpment and plateau. The units are congruent with the geology, and plateau is the largest and strongly dissected forming a complex landscape dominated by a network of ridges at different altitude levels or terraced ridging. The plateau is characterised into three distinctive terraced plateau levels differentiated by altitude, viz.: Plateau terrace level I (PTI) a landscape situated at the altitude over 2067 m a.s.l. (i.e. characterised by irregular, conical narrow cliffs or rock outcrop narrow (<10 m) summits with limited vegetation mostly due to shallow soil (<30 cm or rockiness). Plateau terrace level II (PTII) is composed of isolated ridges with altitude range of 1862–2067 m a.s.l., (i.e. characterised by narrow ridge summits with scarps, cliff or rock outcrops and or shallow soil depth) and hence limited vegetation. Plateau terrace level III (PTIII), situated at altitude range of 1657–1862 m a.s.l. (i.e. forming a continuum of low ridges

**Figure 2.** Landscape variation in the LEPUS project study area, West Usambara Mountains, Tanzania.

characterised by comparably broad crests with few localised rock outcrops at summits and/or upper slopes) and well covered with diverse dense vegetation where human did not clear for cultivation. There is a strong correction between plant species distribution, landform characteristics and soil characteristics particularly soil depth and quantity of gravel and/or stoniness or rockiness.

The soils in the plateau are diverse but are congruent with the landforms position on which they occur. The soils found on upper slopes and on their ridges' crests of PTI and PTII are dominantly **Regosols,** and **Lithic Leptosols.** The mid and lower slopes of PTI ridge crests are complexes of **Cutanic Acrisols** and **Cutanic Alisols**. The PTII plateau soils are **Ferralic Cambisols** on the upper slopes and **Cutanic Acrisols** and **Ferralic Cambisols** on the mid slopes [39]. The soils on ridge crests, upper and mid slopes of plateau PTIII are dominantly complexes of **Cutanic Alisols** and **Haplic Regosols**. The dominant soils in the lower slopes of PTIII are **Luvic Ferralic Phaeozems** while the dominant soils of the very narrow valley bottoms of plateau are **Mollic Fluvisols, Gleyic Fluvisols** and **Antrosols** [40].

The entire plateau is composed of aggregated micro and macro watershed with high potential for soil loss through erosion. The erosion hazard is attributed to the steep slopes; weak soil structure and poor agronomic practices whereby farmers cultivate at very steep slopes of over 45° without conservation measures. The soils of the area had overall poor fertility. One of the macronutrient phosphorus is very low below 4 mgP/kg soil which may affect uptake of others. Also, Ca, Mg and K are low in most soils. Micronutrients Fe and Mn are in very large quantities whereas Cu and Zn are within recommended critical levels. These soils are good for establishment of most vegetation and habitats. However, for food crops, which most small mammals are depending upon as food, the poor soil fertility which is leading to poor crops and in dry years no crops will soon bring in natural selection especially to animal species whereby those which will not be able to scramble for small amount of food will perish and those which will adapt to smaller amount and new food will survival. From residents of the area, there are already several species of gazelle and wild pigs, which are no longer, found in the Usambara because of poor habitats and possibly availability of food. Furthermore, it is important to note that due to the influence of elevation on temperature the plateau is colder than the low plains. There are even variations between valley bottoms, higher ridges and Mountains in the Plateau, and congruent to soil variation, there are vegetation distribution and hence forest dwellers. The explanation agree well with reported by Cottle [24] and research work by Valencia et al. [25] and Baltzer et al. [41] that soil type have a strong influence on spatial distribution of plant species.

Escarpment geomorphic unit indicates three levels of uplift, indicating tectonic cycles and it's characterised by steep slopes, canyons, cliffs and rocks with slope gradients of over 72°. There are colluvial foothills, and slope complexes with varied slopes from 3 to 60°. In certain locations, steep slopes over 60° with deep, shallow and rock soils were observed. Escarpment rises from the plain at 600 m a.s.l., to over 2000 m a.s.l. (**Figure 2**). Lower escarpment is characterised by colluvial/alluvial foot slopes, scattered foot ridges and talus slopes. Dominant soils in escarpment are complexes of **Mollic Leptosols**, **Lithic Leptosols, Cutanic Luvisols** and **Haplic Cambisols** while the associated vegetation species are shrubs and large trees where soils are deep. In canyons, *Ficus* spp., have been observed and dense shrubs occupied by different animal species including primate, wild pigs and diverse small mammals [6].

The plain is the lowest geomorphic unit in the study area (**Figure 2**) divided into the upper rolling, rolling and gently undulating plain, characterised by hot temperatures, low rainfall and deep soils developed from Neogene/Miocene deposits. The dominant soils are complexes of **Fluvic Cambisols** and **Mollic Fluvisols** on the lower plain and complexes of **Mollic Leptosols**, **Cutanic Luvisols** on the upper rolling plain and **Haplic Umbrisols** on colluvio-alluvial fans [39]. Dominant abiotic factors prevailing in the plain are low rainfall and higher temperatures, which are supporting the sparse vegetation mainly woody shrubs and thickets. The diversity of animals is higher because the plain is an animal corridor from nearby Mkomazi National Park. There is also an extensive influence of humans including over grazing. Generally, climatic and soil factors are major determinant of spatial distribution of animal and plant species, which is similar to reports by Valencia et al. [25] and Baltzer et al. [41] the influence of soil types and landform characteristics on the distribution of trees.
