Sekhukhuneland grasslands: a treasure house of biodiversity
Janine Victor1, Stefan Siebert2, David Hoare3 and Braam Van Wyk2
1Ms Janine E. Victor,
Biodiversity, Policy and Planning, National Botanical Institute, Private
Bag X101 Pretoria, 0001 South Africa
The Sekhukhuneland region of South Africa has rich biodiversity and a high level of endemism. Some parts of the region are over-utilized whereas others are well managed, so making it an ideal system for investigating the effects of over-utilization on biodiversity and the implications of loss of biodiversity. Increasingly it is becoming apparent that indigenous plants have a role in agricultural ecosystems for ensuring food security and sustainable agricultural production and maintaining key ecosystem functions. This paper examines the role of biodiversity in the grassland ecosystems of Sekhukhuneland and, specifically, compares the level of goods and services between a pristine and a degraded system. The case study illustrates and highlights the importance of maintaining biodiversity and focuses particularly on various grasses and medicinal and food plants.
Sekhukhuneland is a small area in north-eastern South Africa, in the provinces of Limpopo and Mpumalanga. The region was named after Sekhukhune I, who was the chief of the Pedi tribe when land rights were granted to them in 1885 (Raper 1987). Thereafter it became part of the former self-governing homeland of Lebowa (Botha 1983) until the democratisation of South Africa in 1994.
South Africa has a rich vascular plant flora and harbours prominent foci of plant diversity and endemism (Van Wyk and Smith 2001). Sekhukhuneland is a micro-regional centre of plant endemism (Van Wyk and Van Wyk 1997) because of its exceptionally rich biodiversity and high degree of species endemism (Siebert et al. 2001). This natural biodiversity provides diverse important goods and services to rural communities, such as traditional medicine, grazing and browse, fuel, food and housing materials. Its grassland ecosystems play an important role in the preservation of agricultural biodiversity. The area is however under threat from factors such as mining for heavy metals, inappropriate land management, rural sprawl and unsustainable use of natural resources. This affects the level of goods and services provided by the ecosystem. The implications of loss of biodiversity are discussed.
Increasing international attention is being given to the role of biological diversity in the maintenance of agricultural production. In many academic institutions the emphasis is now shifting towards the loss of biodiversity, as its implications become more apparent and the existence of baseline data increases because of our rapidly increasing knowledge of the use and importance of maintaining a diversity of plant species. The services provided by biological diversity are the key to meeting future food needs while maintaining and enhancing other goods and services provided by ecosystems (Waliyar et al. 2002). Agricultural biodiversity is a term which has come into wide use in recent years. It is the genetic, specific, ecosystem and cultural diversity used by people in agriculture and the production of goods such as food, fodder, fibre, fuel and pharmaceuticals. Wild biodiversity is considered to be a component of agricultural biodiversity and consists of non-domesticated resources within systems, such as grazing lands. The importance of maintaining agricultural and wild biodiversity lies in the preservation of genetic diversity, which becomes eroded in the mass-scale production of genetically uniform crops for commercial use.
Many rural people, whether predominantly pastoral or crop-based, deliberately incorporate wild resources into their livelihood strategies (FAO 1999). Wild fruits may be relied upon in some poor households as an alternative to cultivated grain, thus supplying vital nutritional supplements and are a crucial source of vitamins and minerals for children (FAO 1999). Other than the provision of resources, biodiversity may contribute to key ecosystem functions, such as breakdown of organic matter, recycling of nutrients, breakdown of pollutants, maintenance of productive animal populations and the protection of soil and water resources through vegetative cover. Ecosystem integrity (i.e. resilience and inertia) is linked to the level of biodiversity. An assessment of biodiversity can therefore provide insight into ecosystem processes at a functional level (Golliscio and Sala 1993) and consequently the ability of the ecosystem to supply ecological goods and services. This paper focuses particularly on various grasses and medicinal and food plants.
The Sekhukhuneland Centre of Floristic Endemism (Figure 1) is defined as the area bordered by the Highveld Escarpment to the south, Strydpoort Mountains to the north, the Steenkampsberg and Drakensberg to the east and the Springbok Flats to the west (Figure 2) (Van Wyk and Smith 2001). Geologically the area is dominated by ultramafic substrates of the Rustenburg Layered Suite (Wilson and Anhaeusser 1998); topographically it is characterised by undulating hills, from the Steelpoort River valley lying at about 900 metres, the Leolo Mountains rise to 1,932 m, the highest point of the study area (Figure 3).
The climate is fairly typical of the Savanna Biome: warm, moist summers and cool, dry winters. Mean annual rainfall ranges from 400 mm in the valleys to 600 mm on the mountain slopes (Erasmus 1985) and mean summer temperatures from 25 °C in the north to 20 °C in the south. Fire is an important factor in the mountains and helps to maintain the strong grassy component of the vegetation.
The human population of Sekhukhuneland is mostly rural and is highly reliant on wild plant biodiversity. The grasslands are mostly managed as extensive grazing in a low external input production system. There are four main vegetation types: Sekhukhune Plains Bushveld (Mixed Bushveld), Sekhukhune Mountain Bushveld (Sourish Mixed Bushveld), Sekhukhune Montane Grassland (Bankenveld) and Leolo Summit Sourveld (North-eastern Sandy Highveld) (Acocks 1953). The first two are semi-open woodland with a strong grass component, whereas the latter two are pure grassland. The vegetation is mainly used for grazing and browsing domestic livestock, primarily cattle and goats, although crops (mainly sorghum) play a very important role in subsistence agricultural production in some areas (Figure 4). The long-term ecological sustainability of these natural ecosystems is therefore critical to the maintenance of livelihoods and for sustaining rural development.
So far more than 2,200 species of vascular plants have been reported in the natural vegetation of the region (Siebert et al. 2002). Close to 70% of the plants in Sekhukhuneland are herbaceous species of which a large number are used directly for medicinal or cultural purposes, or are edible. Because this has until recently been a botanically under-explored area, new endemic plant taxa and biogeographically important species are still being discovered regularly (Hurter and Van Wyk 2004; Siebert and Van Wyk 2005). Some of these undescribed taxa are already threatened with extinction due to overgrazing and housing development, one example being a new genus closely related to Tulbaghia (Figure 5) (Craib and Siebert 2003).
The Sekhukhuneland region is rich in ultramafic-induced endemic plant species (Siebert 1998), which makes it a treasure house for biodiversity. Unfortunately, the substrate to which these plants are restricted is being used for mining. At present mining activities occupy approximately 15% of Sekhukhuneland, causing some endemic species such as Melhania randii (Figure 6) to be threatened with extinction. There are 58 endemic and approximately another 70 near-endemic plant taxa in Sekhukhuneland, mostly belonging to the two families Liliaceae (sensu lato) and Euphorbiaceae which have immense importance in traditional medicine.
Communal lands occupy at least 35% of Sekhukhuneland and belong to a population of mainly impoverished people, with subsistence farming and use of natural resources as their only means of survival. Cash income is predominantly from work in mines and on farms. Many decades of land mismanagement have caused immense erosion problems (Figure 7) and species composition changes, rendering the land under-productive relative to its potential. Heavy grazing leads to loss of palatable species and an increase in unpalatable ones e.g. Senecio microglossus (Figure 8). This often exacerbates erosion because the unpalatable plants tend to be dwarf shrubs which, unlike grasses, are not good soil-binders.
Commercial farming is restricted to about 30% of Sekhukhuneland, while residential areas (towns and informal settlements) occupy some 15% (Figure 9). These areas are expanding rapidly and the plant diversity will come under increasing pressure from human impact.
From the human viewpoint, the role of biodiversity in agricultural and natural ecosystems is to ensure food security and sustainable agricultural production through direct or indirect provision of food for humans and their livestock, provision of raw materials and services, such as fibre, fuel and pharmaceuticals and the maintenance of ecosystem functions. However, there is the risk in over-utilized (whether through overgrazing, mismanagement, increased cropping, mining etc.) systems that the provision of these natural products and services becomes compromised due to unsustainable harvesting of resources. To compare the level of goods and services between a pristine and degraded system, data from a quantitative vegetation study in the Sekhukhuneland region were analysed. Vegetation plots within degraded grasslands were compared to plots from adjacent pristine grasslands which were matched in terms of environmental conditions; both areas were in the Roossenekal area. A summary of the data is presented in Table 1, which shows that species composition and abundance differ between the areas: trees and shrubs are more dominant in terms of aerial cover and number of species in degraded grasslands, whereas grasses and forbs are more dominant in pristine areas. Whether these differences are significant in terms of the key functions of maintaining stable, robust, productive and sustainable ecosystems is discussed in more detail in the following sections. This has direct implications for future food security in these rural areas.
Table 1: Relative proportions of aerial cover and species richness in degraded and pristine grasslands of Sekhukhuneland. Statistical tests (chi-square) indicated significant differences in species richness (p < 0.0001) and percentage cover (p = 0.087). Cells in bold indicate adjusted chi-square residuals > 2 (approximate 5% point of normal distribution) and may be regarded as significant at a=0.05.
The major source of plant production in grasslands, for grazing and grain for human consumption, is natural grasses. In many natural grasslands, biomass is attributed to relatively few species, even though many more may be present and the identity of the dominant species affects the quality of grazing. Heavily utilized grasslands had much lower species richness than pristine grassland. There were far more grasses which had a greater aerial cover in the pristine areas and most were of good grazing value; over-utilized areas were characterised by unpalatable grasses with lower cover (Table 2).
Table 2: Number and abundance of important species in pristine and over-utilized grasslands of Sekhukhuneland. Statistical tests (chi-square) indicated significant differences in species richness (p = 0.0025) and percentage cover (p = 0.037). Cells in bold indicate adjusted chi-square residuals > 2 (approximate 5% point of normal distribution) and may be regarded as significant at a=0.05.
Many grasses have good grazing value and others perform a multitude of useful ecological functions, for example to bind soil. Grass species composition is also an indicator of vegetation condition. From the data in Tables 2 and 3 it can be seen that the aerial cover of grasses with good grazing value is higher in pristine grasslands. Some examples that best illustrate the nature of the differences in composition between well-managed and over-utilized grassland are discussed in further detail as follows.
Aristida adscensionis is very unpalatable, grows in disturbed areas and is one of the commonest grasses in degraded parts of Sekhukhuneland. It performs a useful ecological function as a pioneer which colonises degraded areas thereby reducing erosion.
Bothriochloa insculpta (Figure 10) is another common grass of over-utilized acres; it is not well grazed although it has good leaf production but its aromatic taste deters animals. It colonises open areas quickly and is therefore useful for controlling erosion, but is a strongly competitive subclimax grass and an indicator of disturbance.
Elionurus muticus (Figure 11), or wire grass, is a competitive grass that is particularly common in overgrazed grassland and also increases through regular burning. It is one of the first grasses to disappear when it is not defoliated in some manner, so light grazing would stimulate it further. It is thus only possible to remove this species by allowing the grassland to become undergrazed. It is unpalatable, partly due to wiry leaves, but it is bitter owing to the presence of essential oils.
Eragrostis gummiflua (Figure 12), or gum grass, occurs more commonly in overgrazed parts, where it increases due to excessive grazing. It produces much seed and can therefore establish rapidly on ground exposed by mining or erosion. Because of its hard culms, only donkeys graze it.
Brachiaria nigropedata is an extremely valuable grazing grass which is very susceptible to overgrazing and is a good indicator of the of grassland condition.
Digitaria eriantha (Figure 13) is endemic to southern Africa where it grows in undisturbed grasslands. It is one of the best natural and cultivated pastures in southern Africa and its dominance indicates good grassland conditions. It can be grazed after the growth period as standing hay.
Eustachys paspaloides (Figure 14) is very palatable and grows in undisturbed open grassland or mixed bushveld. It is a climax grass and is one of the first grasses to disappear during overgrazing.
Hyparrhenia hirta (Figure 15) is the most popular thatching grass in South Africa. It is well grazed by livestock early in the growing season and after fires before the plants become old and hard (Figure 16); it is also drought resistant. It becomes dominant in under-grazed areas. When mature, it is tall and hard and can be cut for thatch (Figure 17), providing a well-insulated and attractive roof.
Table 3: Grass species composition and aerial cover in degraded and pristine grasslands of Sekhukhuneland.
Melinis nerviglumis (Figure 18) is a densely tufted grass which is moderately palatable but is a good indicator of grassland condition since it grows in underutilized pastures. It is extremely common on shallow soils in rocky areas.
Panicum maximum (Figure 19) has high palatability, leaf production and seed production and can be used to make hay. It is an extremely valuable grazing grass in open woodland, but is very susceptible to heavy grazing, disappearing when grazing is intense.
Setaria sphacelata (Figure 20) is an economically important grass which is sometimes sown in pastures and used for making hay. A closely related species, Setaria lindenbergiana, grows in shade, where it can form dominant stands. It is also a palatable grass that makes good hay. It was utilized as a famine food in the past (National Research Council 1996) and the grain has been used to make bread (Fox and Norwood Young 1982). Since it is a decreaser species it does not survive heavy grazing and over-utilization.
Themeda triandra (Figure 21) is the most important grazing grass in open grassland regions of southern Africa, its true value being that it is abundant and can form dense stands. This grass resists fire and will increase through regular burning, so long as it is not overgrazed. It is therefore probably the best indicator in the region of the health of the ecosystem in terms of grazing.
Tristachya biseriata (Figures 22 and 23) is a rare grass, endemic to South Africa. It is important for sheep and goats but is well grazed by all animals early in the season, becoming less palatable later. It mostly occurs in underutilized and infrequently burnt places.
For grazing capacity to be sustained it is clear that introduction of appropriate land management practices to certain areas is required, to avoid the further loss of biodiversity and subsequently valuable grazing species. Eventual dominance of unpalatable “increaser” species as well as alien invasive plants would render more of the grassland unpalatable.
In addition to decreased grazing capacity, many important plants are lost because of inappropriate management and overuse of the grassland. Traditional plant use in the “pharmacopoeia” of rural areas and its economic value, although difficult to estimate, is certainly significant and entirely dependent on natural biodiversity. Many plants used in traditional medicine are slow-growing and, once lost, are unlikely to return to an area. Their presence depends on sustainable harvesting as well as on the maintenance of vegetation condition. These plants often have a low incidence of occurrence and are the component of vegetation that declines under heavy utilization. Comparison of the pristine system with the degraded system (Table 4) shows that there is a far greater abundance of important medicinal plant species in the pristine grasslands, because species loss has taken place during degradation. The medicinal species in degraded grasslands also occur in pristine grasslands.
Table 4: Medicinal species found in degraded and pristine grasslands of Sekhukhuneland.
The following examples illustrate the value of species being lost from over-utilized grasslands. These are selected since they represent the most prominent medicinally used taxa in Sekhukhuneland, none of which are found in degraded areas.
Agapanthus inapertus subsp. inapertus (Figure 24) has roots and rhizomes that are widely used medicinally because of the saponins they contain. These have anti-inflammatory, anti-oedema, antitussive and immunoregulatory properties (Van Wyk et al. 1997) and are used in so-called “isicakathi”, a decoction given as an antenatal and postnatal medicine. It is mildly purgative and may also be used to ease a difficult labour and to ensure that the placenta is expelled (Watt and Breyer-Brandwijk 1962).
Boophane disticha is an extremely attractive plant (Figures 25) with horticultural potential. The alkaloids of this and other members of the Amaryllidaceae are extremely toxic, but are also analgesic (Van Wyk et al. 1997). The bulb scales are used for various boils or septic wounds to alleviate pain and draw out pus, as well as for complaints such as headaches, abdominal pain and eye conditions (Watt and Breyer-Brandwijk 1962). This plant is much too toxic for therapeutic use, but had an important traditional use of the so-called gifbol as a source of arrow poison.
The scientific name of Elephantorrhiza elephantina (Figure 26) alludes to its enormous underground rhizomes, up to eight metres long, which are a traditional remedy for a wide range of ailments, including diarrhoea and other stomach disorders and are also popular for the treatment of skin diseases (Watt and Breyer-Brandwijk 1962; Van Wyk et al. 1997). The bright red colour of the rhizome indicates the presence of tannins known to have antidiarrhoeal, antiseptic, antibacterial and antifungal properties.
Eucomis autumnalis subsp. clavata (Figure 27) is one of the most popular traditional medicines in the country. Because it is so sought after for its medicinal use, it has been widely harvested and is becoming increasingly rare in the wild. This has lead to its conservation listing as “near threatened”. The plant has beneficial anti-inflammatory and antispasmodic effects due to the flavonoids it contains.
Hypoxis hemerocallidea (Figure 28) is possibly one of the most important traditional medicines. This widespread grassland plant is receiving international attention because it has remarkable anti-cancer and anti-HIV activity. It has been used traditionally for the treatment of, amongst other ailments, tumours and also acts as an anti-inflammatory.
Leonotis leonurus is an attractive plant (Figure 29) that is also known as “wild dagga” because of the mistaken belief that it is smoked for narcotic effects. It is in fact smoked to alleviate epilepsy and taken orally for skin ailments as well as to relieve colds and influenza. Its pharmacological effects are attributed to a diterpenoid lactone, a chemical which is also found in a traditional European phytomedicine, present in white horehound (Marrubium vulgare), used for the symptomatic treatment of coughs in acute bronchial disease (Bruneton 1995).
Lippia javanica (Figure 30) is used mainly as a tisane for its pain-relieving and fever reducing effects, for various chest ailments and other sicknesses, but also as a general health tonic. It is rich in volatile oils that have decongestant and antiseptic effects and contains numerous monoterpenoids.
Pelargonium luridum (Figure 31) tubers contain tannins and other phenolic compounds. Infusions of the tubers are used to treat stomach ailments because of their antidiarrhoetic action.
Pellaea calomelanos leaves (Figure 32) are smoked for colds and asthma. Decoctions of the rhizomes are traditionally used to treat abscesses and intestinal parasites. An interesting comparison is that a European fern, Adiantum capillus-veneris, is a traditional remedy in Europe for coughs and asthma (Grieve 1967). Many ferns contain triterpenoids and flavonoids that could possibly have potential as analgesics and anti-inflammatory agents.
Pentanisia prunelloides (Figure 33) has a fleshy, tuberous root that is used in decoctions for burns, swellings, sore joints and rheumatism. The plant is also used to treat heartburn, vomiting, fever, chest pain and haemorrhoids. Antibiotic activity has been reported but otherwise, nothing is known about its chemistry or pharmoacological effects.
Rhoicissus tridentata (Figure 34) is a shrubby creeper with a tuberous rootstock that has important medicinal properties. The uses suggest analgesic effects, but the chemical components and activity is unknown. In Europe vine leaf is a traditional medicine and contains various phenolic compounds that may similarly be present in Rhoicissus.
Scabiosa columbaria (Figure 35) has fleshy roots which are dried and roasted to make a wound-healing ointment. Powdered roots are also used as a pleasant-smelling baby powder. Both roots and leaves can be chewed fresh or taken in dried form as a remedy for colic and heartburn (Watt and Breyer-Brandwijk 1962).
Typha capensis (Figure 36) or bulrush is very common in wet places. The thick fleshy rhizomes are harvested to make decoctions for many uses such as treatment of venereal diseases, or during pregnancy to ensure an easy delivery, as well as for stomach ailments and to promote fertility in women and libido in men. The presence of steroid-like constituents is very interesting and there are indications that this group of phytosteroids can be metabolised to either androgen or oestrogen-like substances (Sandermann 1994).
Xerophyta retinervis is a peculiar and attractive plant which survives fire through a protective coat of fibrous leaf bases persisting on the stem (Figure 37). The dried roots are smoked for asthma relief, or the whole plant is smoked for relief of nose-bleeds. This effect could be because the plant contains flavonoids, which are known to reduce capillary fragility and can also be antispasmodic and anti-allergic.
Many plants which are used medicinally have similar pharmacological functions, e.g. most members of the Amaryllidaceae contain alkaloids and therefore have the same effects (in this case analgesic activity) and perform similar functions when used in traditional medicine. Medicinally important plants are usually widely distributed, because species with restricted distribution are unlikely to have been encountered enough for their use to become popular. However medicinally important plants, though widely distributed, are usually naturally rare in occurrence and plants with similar medicinal functions do not usually co-occur as they often compete for the same ecological niche. There are also many species that are unique in their pharmacological behaviour, e.g. Hypoxis hemerocallidea. No other species, not even of Hypoxis, are yet known to have the same medicinal properties. Therefore, unsustainable utilization of this plant would result in disappearance of the medication, which demonstrates the importance of maintaining biodiversity in grasslands.
Rural populations often rely on a small number of staple crops as their major food source which they have to supplement with indigenous plants which are harvested to provide the additional dietary requirements of a balanced intake. A large amount of biomass is harvested green and cooked for everyday requirements. Many indigenous species have potential as food, but they have to be sufficiently abundant as well as accessible to come into everyday use. Table 2 indicates the number of species of plants that can be used as food and their relative abundance. The species and their use are listed in Table 5.
The enormous natural biodiversity of the study area provides people with the opportunity to use of different species from one area to another. Indigenous species that are a source of
Table 5: Edible species in pristine and degraded grasslands
food were more abundant in the pristine grasslands (Table 5). Although some of these are only used in times of famine, higher biodiversity provides culinary choice as well as ensuring a greater diversity of essential nutrients in the diets of the rural population. The following examples are a small sample of available species from checklists of Sekhukhuneland (Siebert et al. 2002), but are the most important ones encountered in the present study.
Amaranthus hybridus (Figure 38) and other Amaranthus species are either grown for their seeds (Inca wheat) or harvested as spinach (marog). Most species used in these ways are naturalised weeds in this country and are easily cultivated. The leaves of all species are edible and are a valuable source of protein and vitamin A. The leaves of Amaranthus are more valuable than any other leaf vegetables in terms of their food value, being exceptionally high in iron and calcium.
Cleome gynandra and C. monophylla (Figure 39), both of which grow in Sekhukhuneland, are popularly used as vegetables and are cultivated on a small scale elsewhere in Africa. In terms of food value these species are particularly rich in magnesium and iron.
The large tuber of Ipomoea ommannei (Figure 40) has been traditionally used as food by Zulu people in times of famine, often mixed with curdled milk to increase its bulk (Fox and Norwood Young 1982). Medicinal use is also reported, being used as a decoction to treat convulsions and as an aphrodisiac (Van Wyk and Gericke 2000).
Portulaca oleracea (Figure 41), an indigenous succulent weed, is a favourite vegetable in all parts of southern Africa. The leaves can be eaten raw or cooked.
Rhoicissus tridentata (Figure 34) bears grape-like fruit with greenish sweet pulp. These fruits are often eaten, especially by children, although it has been reported that the skins can “cause considerable discomfort when eaten” (Fox and Norwood Young 1982).
Apart from its important medicinal uses mentioned previously, Typha capensis (Figure 36) also has value as a source of starch. The spongy rhizomes may be pounded to a meal; furthermore the pollen may be used as a high-protein food. Pancakes can be made from flour and bulrush pollen. Leaves are also used to make hand brooms and are sometimes used in weaving and thatching.
Sorghum bicolor subsp. arundinaceum is probably the most important indigenous crop in the Sekhukhuneland area. Improved forms of common wild sorghum are the staple diet of more than 500 million people worldwide and sorghum has been cultivated from 4,000 years ago. In Sekhukhuneland, selective breeding has lead to the development of a drought-resistant form of sorghum that is cultivated in many arid areas (Figure 4).
Two species of Setaria that occur in Sekhukhuneland have been used for making grain and bread in the past, but this use is restricted to times of famine (Fox and Norwood Young 1982) because Setaria species are suspected to contain a toxic principle which is removed by boiling the seeds before grinding into meal (Watt and Breyer-Brandwijk 1962). The two species of Setaria that in Sekhukhuneland are S. lindenbergiana, usually found in the shade or forested areas and S. sphacelata var. sphacelata in open savanna or grassland (Figure 20).
Vigna unguiculata, or cowpea (Figure 42), is an indigenous legume that has been cultivated for its seeds in Asia and Africa for many centuries (Fox and Norwood Young 1982). The Pedi in Sekhukhuneland grow it and throughout southern Africa the dry beans are a favourite food and commonly form part of other dishes. The young shoots, pods and leaves are sold in markets and are very popular vegetables, both fresh and dried.
Biodiversity has further economic roles to play in grassland systems. Rich biodiversity can provide species with horticultural value, such as the vulnerable Zantedeschia jucunda (Figure 43), which is endemic to the Leolo Mountains in Sekhukhuneland. This beautiful plant is exceedingly rare and is threatened with extinction because of housing development in the area as well as illegal removal for the horticultural trade (Siebert and Van Wyk 2001). Maintaining good vegetation condition and a rich diversity has attractions from an ecotourism perspective, in that loss of biodiversity leads to loss of plants with aesthetic value such as Erythrina zeyheri (Figure 44).
Loss of biodiversity has numerous effects on the function of an ecosystem as a whole. Landscape function includes the ecosystem processes, for example, nutrient cycling, water holding and infiltration capacity that are essential for landscape sustainability. According to Tilman (1999) biodiversity can, amongst other factors, govern the stability, productivity, nutrient dynamics and invasibility of ecosystems. This implies that biodiversity and the resilience and sustainability of ecosystems (natural and modified) are inextricably linked (Golliscio and Sala 1993). The degraded systems in Sekhukhuneland were characterised by lower species richness. This has resulted in the ecosystem becoming more susceptible to invasion and having a lower cover of productive herbaceous species and a concurrent increase in number and abundance of invasive woody species (Table 1). Comparative data from pristine and degraded grasslands indicate that there are more woody species and their aerial cover is greater in degraded areas. However, although more species are available for timber and firewood in degraded areas, there are more woody species in the pristine grasslands that can be used for crafts (Table 2), because the quality of the wood is higher as the species are slow growing rather than the fast growing invasive woody species which encroach degraded areas. Bush encroachment has an effect on the ecology, creating a state of secondary or sub- climax that can only be altered by intervention.
Biodiversity has multiple roles in agricultural ecosystems and has ecological and economical links with rural livelihoods. It contributes to food and livelihood security, agricultural production and environmental sustainability. The roles of biodiversity in the Sekhukhuneland grassland ecosystem have been described through comparison of the data from heavily utilized grasslands (Figure 45) in close proximity to cultivated lands and pristine grassland (Figure 46) in rural agricultural systems. Changes in biodiversity and composition are demonstrated to have an effect on the quality of products derived from natural ecosystems in these rural areas and this demonstrates the role of biodiversity in these areas. The comparison between these systems illustrates that loss of biodiversity can lead to loss of goods and services to the community and the eventual loss of both ecosystem integrity and agricultural potential of the land. Figure 47 shows a simple graphical synthesis of some of the causes and effects directly related to the risk of loss of biodiversity in grasslands.
Figure 47. Drivers - effects framework:
Biomass production is an important ecological function of natural biodiversity and directly influences, amongst other things, both forage quality and quantity. The effect of over-utilization on grass species, as shown by the comparison between the two systems (Tables 1 and 2), was a reduction in the number of species, a change towards less palatable species and a reduction in the total cover of grasses, which clearly has an effect on the grazing potential and carrying capacity of grasslands. There is also a trend in the study area towards there being a small number of abundant species that account for a large proportion of the cover of grazed species in the degraded areas relative to the pristine areas. This dominance has an effect on ecosystem function, because there are a few species that account for most of the ecosystem function (Golluscio and Sala 1993).
Biological diversity conservation ensures security of food, fibre, building materials and medicinal products. The effect of over-utilization on medicinally important species in grasslands was demonstrated to be a reduction in the number of species as well as a reduction in the relative cover and abundance of useful species. This has the effect of reducing availability of medicinal species as well as reducing the number of medicinal options available. Similarly, the effect of over-utilization on edible species is a reduction in the number of species as well as a reduction in the range of uses that can be derived from them. Better management of these resources would ensure regeneration of localised food resources and greater stability in food supply through times of environmental stress. It is clear that a loss of these resources leads to instability and an insufficient supply which compromises the dietary needs and food preferences that would ensure an active and healthy life (FAO 1999). Pristine grasslands are most certainly better able to provide the resources to meet these needs than the degraded grasslands and this demonstrates the important role of biological diversity.
The role of biodiversity in the grassland ecosystem has also been illustrated to be ecologically important in maintaining key ecosystem processes, which may become compromised in degraded areas. Over-utilization of grasslands in this part of southern Africa usually leads to encroachment by woody species which leads to an increase in cover, the introduction of woody species which did not previously occur in the area and the loss of those that would naturally occur there. The change from grassland to woodland through the process of woody encroachment affects several key ecosystem functions, including decomposition and nutrient cycling, biomass production and soil and water conservation. The dynamics of rainfall interception, overland flow and water penetration into the soil under wooded areas is such that water from rainfall events is quickly lost to drainage systems with a concomitant increase in soil erosion. Pristine grassland intercepts water more efficiently and therefore prevents loss of the soil resources that form the basis of the entire ecological and agricultural production system.
Loss of traditional methods and values in the rural population in Sekhukhuneland has been a large contributor to the demise of the biodiversity of this grassland system. In former times traditional healers or their assistants would have been solely responsible for collecting medicinal plants in accordance with traditions and taboos. One such example is taboo dictates that only a portion of the tuber of Elephantorrhiza elephantina (Figure 26) may be removed and the remainder must be covered. Failure to do so would result in patients treated with this plant not recovering. Bark used for treating kidney diseases should be harvested from the eastern and western sides of the tree, symbolising the kidneys, a taboo which prevented ring-barking. A further example is a tradition of only allowing collection of certain plants for treatment of cold and influenza in winter, thereby ensuring that plants remain in the field to set seed in summer. However, in modern times traditional healers have been less rigorously trained than their ancestors and economic incentives result in the destructive harvesting of vulnerable medicinal plants. This neglect of indigenous knowledge is a prime underlying cause of agricultural biodiversity loss (FAO 1999).
Intensive agriculture as well as inappropriately managed livestock production can lead to ecological degradation that has consequences in terms of the conservation of species diversity. High rainfall grasslands have evolved without major disturbance over millions of years. This is commonly thought to have rendered pioneer species redundant and explains the noticeable lack of non-grassy annuals in the pristine areas. Vast disturbance such as ploughing is completely incompatible with the Sekhukhuneland grassland system. Thus the pioneers that colonise disturbed areas are mainly alien annuals, along with indigenous annual grasses. One of the most noticeable losses in degraded areas is that of non-grassy perennial herbs. These plants include resprouting and clonal plant species. Many of these plants live a very long time and have seed dispersal mechanisms adapted to spreading across short distances, one such example being Erythrina zeyheri (Figure 44). Clonal species such as Elephantorrhiza elephantina (Figure 26) seem to have lost the ability to reproduce by seed and consequently the clones are estimated to be thousands of years old. Thus most non-grassy plants in this high rainfall grassland area are characterised by underground storage organs especially large tubers (e.g. Ipomoea ommannei, Figure 40) or rhizomes (e.g. Elephantorrhiza elephantina, Figure 26). Some of these plants do not propagate efficiently by seed and those that do are characterised by a dispersal mechanism that only enables dispersal across short distances. There is a noticeable lack of species with long-range seed dispersal mechanisms e.g. wind dispersal. The removal of these plants and disturbance of their environment has, therefore, severe consequences as they are unlikely to return. One of the implications is a degradation of the unique floristic composition that forms the basis for ecotourism and has a high potential for contributing to rural economies.
A further aspect affecting biodiversity is soil disturbance through erosion or ploughing. Many species in this area are adapted to a specific soil structure and composition. This explains why attempts to rehabilitate disturbed areas with non-grassy herbs are unsuccessful in Sekhukhuneland. Thus one can rehabilitate an area that has been destroyed by inappropriate land management practices, mining and so forth, but one can never recover the exact species composition, i.e. the natural biodiversity.
Preservation of biodiversity is essential for continued utilization of the resources in traditional ways. Education about sustainable utilization needs to be promoted, to prevent plants from becoming increasingly scarce. Decline of plant populations could lead to local, or even global, extinction, resulting in irreversible loss of important grazing species, medicinally utilized species and other economically important plants. This could in turn lead to further impoverishment of the rural people and a knock-on economic impact on adjacent communities. However South Africa has excellent environmental legislation and a genuine commitment to preservation of biodiversity that encourages hope for the future. The issues of inequitable land tenure and control over resources are also being addressed, which is likely to lead to an improvement in the way in which biodiversity is managed.
[paper edited September 2005]