Conservation biology of the succulent shrub, Euphorbia barnardii, a serpentine endemic of the Northern Province, South Africa

Abstract

Euphorbia barnardii White, Dyer & Sloane is a relatively small, succulent shrub found in the Northern Province of South Africa. In 1994 it was listed as endangered by the Transvaal Threatened Plants Programme, because only three populations, totalling 1150 plants, were found. However, our study found five populations totalling 10 783 plants (9503 were reproductive). One population, found 50 km from the others, in the Bewaarkloof district, differs in terms of habitat type and morphology and may therefore be a different taxon. The other populations occur in Sekhukhuneland. Soil analyses showed that E. barnardii is restricted to ultramafic (serpentine) substrates. A sample of 2015 plants was examined in 1995 for size, stage, new growth, dead branches, reproduction (indices of vigour), damage and disease. Analysis of the results of 10 years’ demographic monitoring (1985–1995) on two of the Sekhukhuneland populations showed that one population, which was vigorous in 1995, showed little change in population size and structure, while the other, which had low vigour in 1995, showed a precipitous decline to local extinction in the monitoring plot. Aerial photographs taken in 1957 (1963 for Bewaarkloof) and 1986 showed large increases in human population density within 1.5 km of the E. barnardii populations except at Bewaarkloof, where it decreased. An increase in human habitation is associated with an increase in livestock (mostly cattle and goats) which trample plants. Trampling damages the terminal segments, which may lead to lowered reproductive output and increase susceptibility to opportunistic bacterial wilt pathogens. Bacterial wilts were prevalent on all populations but were particularly high at Bewaarkloof and on the population that showed a steep decline in numbers. Disease incidence was associated with the level of plant damage. The impact on plants further up the slopes and on the crest of hills (quite far from human settlements) was to a far lesser degree but further increases in human population density could change this situation. While this study has shown that the population size of E. barnardii is much greater than previously thought, the species is still threatened by several different processes and should be listed as ‘Vulnerable (A1a + c, B1, B2b + e, C1, D2)’ according to World Conservation Union categories.

People, species richness and human population growth

“…Shackleton, 1993; Harcourt et al ., 2001; Liu et al ., 2001). While the negative correlation between human population size and the percentage of protected land suggests that small protected areas will be more vulnerable to the effects of large human populations (Knowles & Witkowski, 2000; van Rensburg et al ., 2004), it also indicates that larger protected areas may be less prone to the direct effects of large populations or fast growth rates (although the majority of protected areas in South Africa are generally small — 70% < 5000 ha).…”

“…Therefore, there is little evidence to suggest that for frogs and birds at this spatial scale, the magnitude of conservation conflicts likely to arise from positive species richness–human relationships have increased. Of course, these results do not mean that landscape transformation driven by economic activity and other threats to biodiversity are not ongoing (see Czech et al ., 2000; Fairbanks et al ., 2000; Kemper et al ., 1999; Knowles & Witkowski, 2000; Leseberg et al ., 2000; Driver et al ., 2005; Reed & Czech, 2005). Economic policies are generally developed to a greater extent than those concerning human populations and it may be easier, at least in the short term, to address such impacts through adjustments to economic policies rather than those concerning human populations.…”

Remove or retain: ecosystem effects of woody encroachment and removal are linked to plant structural and functional traits

“…Thus, management decisions on whether to retain or remove woody plants will depend on whether the end game is to increase diversity, for example, under a conservation management setting where small stature plant species are favoured (e.g. Euphorbia barnardii , Knowles & Witkowski, 2000). Similarly, the management objective might be to enhance biodiversity (e.g.…”

Translocation trials confirm specific factors affecting the establishment of three endangered plant species

“…They also trample on surface microflora and seedlings, thus damaging populations even in the absence of any obvious grazing. Trampling by livestock led to lowered reproductive output and increased susceptibility of Euphorbia barnardii to disease (Knowles & Witkowski 2000).…”