Impacts on terrestrial ecosystems: # 1

Habitat Loss and Fragmentation:  

It has been clearly demonstrated that the species richness of ecosystems is closely related to their unbroken (contiguous) surface area (Rosenzweig 1995, Bond et al. 1988). This important theory implies that continued land transformation will reduce species numbers, even without direct utilization pressure on species. Major transformation of land accompanying colonial expansion resulted in the near-destruction of wild ungulate and predator communities (Macdonald 1989) and the replacement of natural grazing and browsing species by domestic stock. Changes in defoliation patterns in turn (forage selection and grazing intensity) have probably caused large-scale vegetation change, though this is poorly quantified (Macdonald 1989).

Conversion of natural habitats (that have a high agricultural production potential) to cultivated areas, has reduced the extent of several vegetation types. These are, most notably, communities of the Grassland biome (Macdonald 1989) and renosterveld shrublands of the western Cape, which presently occupy a fraction of their original range (Heydenrych & Littlewort 1995).

Forest plantations have been a further cause of transformation and fragmentation of natural habitats due to selection of certain sites for afforestation.

Changes in Biodiversity:  

There have been both positive and negative impacts on biodiversity. For example, plantation forestry has contributed to the protection of natural habitats and species on about 300-400 000 hectares of land, as it is a legal requirement of afforestation permits that 25% of the land under plantation be conserved. Many of South Africa's declared Natural Heritage Sites are also located on forest estates.

However, afforestation has led to local loss of biodiversity, and changes in species composition. For example, in Mpumalanga Province, the species diversity of grassland birds in general, and of globally threatened grassland birds in particular, is significantly negatively impacted by afforestation (Allan et al, 1997). Table 2.6 gives details.

Perennial plant diversity and abundance has been shown to decline under sustained grazing pressure (Todd 1997, Todd & Hoffman 1999), as weedy species which are tolerant of grazing become dominant, and other species are out-competed (Hoffman & Cowling 1990). This has "knock-on" effects on other components of the community, such as insects (which respond positively to the availability of seeds of abundant annuals) and small mammals with diurnal activity (which require the cover afforded by perennials). Grazing pressure has been implicated in the extinction of at least three plant species listed as Globally Extinct in the South African Red Data List (Hilton-Taylor 1996). Of the 48 species that are listed in the "Indeterminate" category (potentially extinct) in the Red Data List, six have been impacted by agricultural activities in the major rangeland regions (Hoffman et al. 1999).

Erica Sociorum

Erica verticillata

Five plant species (Erica pyramidalis, E. sociorum, E. verticillata, Asparagus variegata, and Romulea papyracea) are known to have become extinct due to urbanisation in the metropolitan area of Cape Town (Hall & Ashton 1983), and a further 294 species are under immediate threat of extinction (Wood et al. 1994). This situation is critical and contentious, with an urban environment expanding within a floristic hotspot of international repute.

The medicinal and horticultural plant trades also have a negative impact on terrestrial plant diversity. While traditional healers collected and stored plant material according to traditions which may have limited over-harvesting in the past, it appears that increasing demand has created an incentive for less discriminate harvesting methods (van Wyk et al. 1997). Unsustainable extraction rates combined with the fragmentation of natural ecosystems is threatening many species. Traditional medicine is widely practised in South Africa, with healer-to-population ratios roughly 10 to 20 times higher than western doctor-to-population ratios (Cunningham & Davis 1997). It has been estimated that upwards of 75% of the population uses traditional plant treatments (Ellis 1986) (often self-collected and administered) for a wide range of ailments (Hutchings 1989). Some examples of highly desirable medicinal species that are severely threatened with extinction by harvesting in the wild include Warburgia salutaris, the pepper-bark tree (Scott-Shaw et al. 1998), Siphonochilus aethiopicus (wild ginger, now possibly locally extinct), Haworthia limifolia (a succulent species almost extinct), and Boweia volubilis (the climbing lily, locally extinct in parts of its range). Of 31 species featured in a cultivation guide for indigenous medicinal plants of Kwa-Zulu Natal (Mander et al.1995), 26 were described as specially protected, most having declining or depleted populations. Sales of southern African wild plant medicines are significant in Europe, with an unquantified, but likely significant proportion derived from collecting in the wild (Lewington 1993).

Private horticultural collectors are known to have conducted unscrupulous field collection campaigns that have severely reduced certain species populations, e.g. succulent species and cycads. In a recent report, Newton and Chan (1998) have revealed that habitat degradation and plant collectors threaten 128 South African succulent species. Fourie and Boyd (1995) suggest that 12 of 20 cycads of the genus Encephalartos in Mpumulanga and the Northern Province are subject to intense collector activity, illustrated by more than 15 000 possession permits being issued in the Northern Province for more than 300 000 plants (Fourie & Boyd 1995). This results in the removal of thousands of live plants from natural populations. Populations of cycads are usually small, with limited geographic ranges. These plants are also very slow growing, and collection of such large numbers has led to local extinction of populations, and in some cases, extinction of the entire species in the wild.

International trade in Aloe species targets South African and Madagascan rare species, with a focus on dwarf species in South Africa (Oldfield 1993). While commercial utilization of Aloe ferox is relatively heavy, it appears sustainable. Pressure on some dwarf ornamental Aloe species, by contrast, is presently threatening their natural populations with extinction. Aloe peglerae (CITES category II) is a dwarf Aloe form found on the rocky slopes of the Magaliesberg, Gauteng Province, where it may occur in numbers of several hundreds of plants per kilometer. Where this species is accessible in the wild, it has been heavily, illegally collected, and several local populations have been completely wiped out (Newton & Chan 1998).

Alien invasive organisms also contribute to reductions in biodiversity. Hall et al. (1980) estimated that 33 of 70 threatened plant species of the southwestern Cape were the potential extinction victims of invasions of alien woody plants. Alien woody plants also alter animal diversity significantly, for example reducing the densities of insect-feeding birds but increasing seed-feeders. Tree invasion in the southern and western Cape has also allowed tree-dependent bird species to expand their ranges [36 indigenous bird species have invaded the south western Cape, and a further 27 have increased their distribution range subsequent to alien woody plant invasion (Macdonald & Richardson 1986)]. The mite Varroa, which attacks bee colonies, can cause such damage that it has raised concerns that there may be insufficient bee colonies available for pollination of deciduous fruits in a particular season (Allsopp, 1999).

>     Terrestrial Ecosystems: Impacts #2

There is also information about Terrestrial Ecosystems in the following reports:
Metropolitan reports:
	Cape Metropolitan Council (1998 edition)		Durban Pilot Study
	Greater Johannesburg Metropolitan Council (1999 edition)		Greater Pretoria Metropolitan Council (1999 edition)