Plant Diversity in Agro-Pastoral Grasslands of Tanzania

1. Introduction

There are a variety of natural biological resources in grasslands that human beings rely on for their livelihood, survival, and development. People living in grassland ecosystems tend to believe that cultivating crops and maintaining pastures offer more immediate benefits than conserving natural resources and biodiversity [1]. This implies conflicting goals between agro-pastoralism and biodiversity conservation as a tendency to maximize services from agro-pastoralism results in overutilization and removal of plants that consequently affect plant diversity. This challenges the management of plant resources in grassland ecosystems since agro-pastoralism and plant diversity are not independent [2]. There is a progressive growth in human population and conversion of grassland to agriculture [3]. However, little is known about the situation’s impact on the diversity of plants that form basal feed resources for grazing livestock and wild animals in grasslands.

The diversity of species in grasslands has functional implications, as the quantity and variety of species in the ecosystem dictate the organismal traits that impact ecosystem processes [4]. Species traits can directly influence energy and material flows or modify abiotic conditions, such as restricting resources, affecting disturbance, and regulating climate processes [5].

The elements of species diversity influencing trait expression encompass species richness, evenness, and composition, and their interactions and variations in time and space [6]. Plant diversity and the complementarity of plant species are important regulators of grassland ecosystem productivity [7]. This happens as a result of plant species’ response to nutrient availability and resource use efficiency in a diverse plant community. However, the availability of growth-limiting resources changes along plant diversity gradients under normal conditions. High plant diversity communities tend to contain species that are able to access scarce resources during periods of stress, such as accessing water from deeper soil profiles or plant nutrients from different soil depths in the rhizosphere [8, 9]. It is, therefore, plausible to determine plant diversity in grasslands to get clues about ecosystem health.

2. Methodology

2.1 Study sites

The study was conducted in Western Serengeti and Ugalla ecosystems in Western Tanzania (Figure 1). Western Serengeti is characterized by savannah vegetation that involves extensive grassland with scattered trees and shrubs. Ugalla ecosystem is found in miombo woodlands where grasslands mainly exist in seasonally flooded areas.

3. Importance of plant diversity for agro-pastoralism

Grasslands serve as a crucial feed resource for grazing animals, and beyond that, grasses play significant roles in water catchments, biodiversity reserves, and cultural and recreational aspects [13]. Additionally, they have the potential to act as a carbon sink, mitigating greenhouse gas emissions [13]. The importance of grasses lies in their content of linoleic acid, with over half of the total fatty acids consisting of ω-3 linoleic acid, including the potent anticarcinogen conjugated linoleic acid (CLA; [14]). Research by Dewhurst and King [15] revealed substantial variations in ω-3 linoleic acid content among grass species and cultivars. CLA is linked to significant fat and protein metabolism regulation, leading to increased muscle formation and decreased fat content [14]. Consequently, milk and meat containing CLA offer higher nutritional value for humans, underscoring the importance of conserving the biodiversity of grass species in grasslands.

The diversity of plants in terms of perennial and annual plants is important in grasslands for ensuring year-round feed resource availability. Perennial grasses, for example, play a crucial role in maintaining grassland health by being more productive, supporting extended grazing periods, and enhancing soil quality [16]. Nevertheless, their extended root zones enable the recapture of leached nutrients and water, contributing to overall soil improvement [16]. Grasslands dominated by annual grasses are of poorer quality during the dry season because the plants mature faster and lose nutritive value faster as well. Perennial grasses are observed to be vulnerable to agro-pastoral activities, and they are replaced by annual grasses [17]. This situation causes changes in plant diversity in grasslands subjected to anthropogenic activities.

4. Impacts of agro-pastoralism on plant diversity

In Tanzania, agro-pastoral livelihood involves a combination of traditional and modern practices to fulfill production needs. The choice of agro-pastoralism practices significantly impacts both herbaceous plant biodiversity and the livelihood of the local population. Practices such as keeping large herds of grazing animals within confined areas or grazing continuously on the same range throughout the year lead to land degradation. This degradation results from the high pressure exerted on plant species and soil disturbance due to trampling. Additionally, the unrestricted expansion of cultivated land negatively affects herbaceous plant species, reducing the feed resource base for grazing animals. Animal trampling causes soil compaction, impacting soil density and porosity resulting in poor water infiltration. The removal of plants due to the large number of grazing animals leaves the land bare, contributing to surface water runoff during the rainy season leading to soil erosion.

Clearing land and cultivating for crop production fundamentally alter and disturb a previously stable ecosystem. The cultivated area undergoes immediate succession, with plant species adapted to bare land conditions and disrupted soil invading and establishing themselves. This process leads to shifts in plant species composition and, consequently, changes in plant biodiversity. Some herbaceous plant species within the community may become more prevalent, while new species may invade the community from neighboring ecosystems.

Erosion of soil from exposed cultivated land during the rainy season leads to the loss of the top fertile soil, resulting in decreased soil fertility. This diminished fertility contributes to the establishment of a limited number of plant species, leading to a reduction in herbaceous plant composition. The decline in herbaceous plant composition results in lower above-ground biomass production, creating an inadequate feed resource base for grazing animals. The high number of grazing animals in a shrinking grazing land, coupled with insufficient feed resources in terms of both quantity and quality, increases grazing pressure. This elevated grazing pressure on palatable herbaceous plant species leads to the disappearance of these plants and, subsequently, the loss of herbaceous plant biodiversity. The sequence of events described in this scenario can be termed as “herbaceous plant biodiversity detrimental scenario.”

On the flip side, agro-pastoralism, as a livelihood strategy, encompasses various traditional and contemporary best-bet practices, such as deferred grazing (Ngitiri or Alalili), grass band cultivation, zay pit cultivation (Ngoro system), and controlled grazing based on proper stocking rates. These optimal agro-pastoralism practices contribute to soil conservation by minimizing disturbances to the soil and native plants, resulting in the availability of diverse plant species that contribute to high primary productivity. Sustained high primary productivity ensures a stable feed resource base in terms of both quantity and quality to support grazing animals. This situation allows grazing animals to selectively remove herbaceous plants through grazing while providing an opportunity for the regeneration of plants. This, in turn, contributes to the biodiversity conservation of herbaceous plants. The sequence of events described in this scenario can be termed as “herbaceous plant biodiversity win-win scenario.” Both the “herbaceous plant biodiversity detrimental scenario” and the “herbaceous plant biodiversity win-win scenario” impact the livelihood of agro-pastoralists, either negatively or positively. This conceptual framework can be represented by a schematic diagram as follows (Figure 2).

5. Effect of agro-pastoralism on plant diversity in Western Serengeti and Ugalla grasslands

Changes in plant diversity are attributed to human activities and their effects are exacerbated by the climatic condition of the site. Vegetation changes in many countries are associated with climate change, human population growth, and extensive agriculture [19]. The decline in plant diversity affects negatively ecosystem functions and services because many species are needed to maintain ecosystem functioning and services [20]. Anthropogenic activities (Figure 3) contribute to the reduction of plant diversity [17]. However, their effects differ between different ecosystems due to differences in plant species composition and environmental factors such as soil and precipitation.

This study showed relatively higher plant diversity (p < 0.001) in Western Serengeti than Ugalla ecosystem (Figure 4). This could be attributed to differences in physiognomic aspects of savannah in Western Serengeti and Miombo woodlands in Ugalla ecosystem.

Grasses form a basal diet for wild and domestic grazing animals, implying that a grassland ecosystem with a high diversity of grass species supports many grazing animal species. The current study showed a higher diversity of grass species (p < 0.001) in Western Serengeti than in Ugalla ecosystem (Figure 5). This concurs with the fact that many wildlife species are found in Serengeti grassland while the Ugalla ecosystem harbors few species, particularly specialized feeders such as sable antelope.

This study showed 24 plant species common to grasslands in Ugalla and Western Serengeti, while 84 plant species were exclusively found in the Ugalla ecosystem and Western Serengeti (Figure 6).

Plant species that were common to the Ugalla ecosystem and Western Serengeti include Brachiaria brizantha Cynodon dactylon, Cynodon plectostachyus, Dactyloctenium aegyptium, Digitaria macroblephara, Digitaria milanjiana, Digitaria ternate, Gutenbergia cordifolia, Heliotropium steudneri, Hygrophila auriculate, Hyparrhenia hirta, Justicia exigua, Justicia matammensis, Panicum coloratum, Panicum maximum, Senna occidentalis, Sesbania sesban, Setaria sphacelate, Setaria verticillate, Sida acuta, Solanum incanum, Sporobolus festivus, Sporobolus pyramidalis, Tephrosia pumila, and Triumfetta rhomboidea. Plant species that were exclusively found in each ecosystem are shown in Table 1.

The unique plant species in each ecosystem need to be conserved because they mostly support specialized feeders. Local extinction of these species might affect the existence of specialized feeders in the ecosystem.

Perennial grasses provide a better indication of the health status of grasslands than annual grasses. Perennials tend to yield higher dry matter and provide better soil protection than annuals [21]. In that manner, grasslands with a low diversity of grass species are prone to land degradation by water runoff. In this study, perennial plant diversity was higher in Western Serengeti than Ugalla ecosystem (Figure 7).

The low diversity of perennial grasses in Ugalla ecosystem makes the ecosystem prone to soil erosion under cultivation. This compelled smallholder farmers in Ugalla ecosystem to adopt ridge cultivation to limit water runoff on cultivated land during the rainy season. The functional role of perennial grasses is both biomass production and soil important for a stable grassland ecosystem.

Grasslands sustain grazing animals by providing edible plants that are not harmful and have nutritional values. A high diversity of edible plants ensures the availability of a balanced ration for the grazing animals to enable their production and reproduction functions. The current study showed higher diversity of edible plants in Western Serengeti than Ugalla ecosystem (Figure 8).

The superiority of Western Serengeti in terms of edible plant diversity enables the area to support a large number of different herbivore species. Inedible plant species diversity was not significantly different (p > 0.05) in the Ugalla ecosystem and Western Serengeti (Figure 9). However, the uneven size of the boxplot in Ugalla indicates more variation of inedible plant diversity in different locations within the ecosystem. This suggests variations in the phenology of plants in different locations in the Ugalla ecosystem. This situation provides an opportunity for non-herbivores such as birds and bees to get resources for surviving in the ecosystem.

Generally, this study showed that plant diversity increased with an increase in altitude and the optimum altitude for plant diversity lies between 1600 and 1800 m above sea level (Figure 10).

Results shown in Figure 10 suggest that plant diversity in lowlands is affected negatively by agro-pastoral activities. In most cases, lowlands are considered suitable for cultivation and livestock grazing in the Ugalla ecosystem and Western Serengeti. On the other hand, steep areas are subject to a decline in plant diversity due to soil erosion especially when disturbed by agro-pastoral activities.

6. Prospects of plant diversity in agro-pastoral grasslands of Serengeti and Ugalla ecosystems

The increase in human population causes an increase in food demand that consequently increases agro-pastoral activities in lowlands of the Ugalla ecosystem and Western Serengeti. This situation might negatively affect ecosystem functions due to the decline of plant diversity. Some of the effects have been manifested by an increase in siltation of water bodies found within the ecosystems with consequent decline of biodiversity in lakes [22].

7. Conclusions

• The magnitude of agro-pastoral activities’ effect on plant diversity in grasslands differs between ecosystems. The grassland with a low diversity of perennial grass species is much more affected when subjected to cultivation.

• Poor maintenance of plant diversity in grasslands negatively affects ecosystem services provided by plants, and consequent effects are manifested in water bodies by the decline of biodiversity.

• The impact of agro-pastoral activities on plant diversity is conspicuous in lowlands but much more negative when practiced on steep slopes.

• Variations in the phenology of inedible plants within an ecosystem provide temporal and spatial opportunities for non-herbivore animals and invertebrates to access plant resources for their survival.

Way forward

Plant diversity conservation strategies in agro-pastoral grasslands are inevitable to alleviate negative effects on ecosystem services and consequences manifested in water bodies.

Declaration

Some parts of this chapter are based on an unpublished part of the main author’s Ph.D. thesis on “influence of agro-pastoralism on herbaceous plants diversity and livelihood of communities in Western Serengeti” submitted to the Senate of Sokoine University of Agriculture in Tanzania.