M.T. Hoffman

Acocks' Valley Bushveld 50 years on: new perspectives on the delimitation, characterisation and origin of subtropical thicket vegetation

We present the approach and results of an intuitive, expert-based mapping exercise to identify subtropical thicket (including Acocks’ (1953) Valley Bushveld, Noorsveld and Spekboomveld) vegetation types as features for conservation planning. The study area comprised 105 500km 2 in southern and south-eastern South Africa, the planning domain for the Subtropical Thicket Ecosystem Planning (STEP) Project. We developed a four-tier typological hierarchy based on geography, floristics, structure and grain. This yielded 112 unique thicket vegetation types, 78 of which comprised thicket clumps in a matrix of non-thicket vegetation (mosaics). By identifying mosaics, we expanded the subtropical thicket concept and increased its extent in the study area by between 1.8 and 2.8 times that of earlier assessments. We also compiled a list of plant species that yielded a rich flora of 1 558 species, 20% of which are endemic to our expanded thicket biome. Consistent with previous studies, endemics were strongly associated with succulent members of the Aizoaceae, Asphodelaceae, Euphorbiaceae, Apocynaceae and Crassulaceae. We discuss our results in terms of Acocks’ (1953) typology as well as those of more recent treatments, and comment on the evolution of subtropical thicket vegetation. Although some confusion regarding the delimitation and characterisation of thicket was resolved by this study, much more research is required to develop and test hypotheses on the determinants of thicket boundaries and the origins and evolution of thicket species.

The application of state‐and‐transition models to rangeland research and management in arid succulent and semi‐arid grassy Karoo, South Africa

Abstract We evaluate the ability of existing models of vegetation dynamics to explain how drought and herbivory by domestic livestock can change Karoo vegetation, and why the vegetation does not always return to its original composition when livestock are withdrawn. Using published data for the arid southern and semi‐arid eastern Karoo, we provide new conceptual models of the dynamics of these vegetation types and discuss their relevance for research into the conservation and rehabilitation of Karoo rangelands.

The nature, extent and causes of land degradation in South Africa: legacy of the past, lessons for the future?

The aim of this paper is to reassess the land degradation and desertification problem in South Africa. The country has a wide range of interrelated environmental and development challenges existing in a socio-political context recently subject to substantial change. The paper outlines the fundamental environmental constraints and opportunities that underlie degradation phenomena and illustrates the nature, extent and geographical distribution of the major forms of soil and vegetation degradation in the country. The principal causes of degradation are identified and some important land use and land tenure questions that need to be addressed in the near future are posited.

Seed predation and germination of Acacia erioloba in the Kuiseb River Valley, Namib Desert

We investigated some interactions between mammalian herbivores, bruchid seed predators and seeds of Acacia erioloba E. Mey in the Kuiseb River Valley, Namib Desert. Predation by bruchid beetles was significantly lower in canopy-held pods than pods on the ground. Germination success was higher for ingested seed than in an untreated control and almost zero for predated seed. Acid and scarification treatments resulted in almost complete germination success. The study provides some support for a mutualistic relationship between mammalian herbivores and acacias, like A. erioloba , with indehiscent pods.

Communal land use and the ‘tragedy of the commons’: Some problems and development perspectives with specific reference to semi‐arid regions of southern Africa

Traditional communal farming in southern Africa has often been described as unproductive and directly responsible for regional poverty and vegetation degradation. Each aspect of this argument rests on a set of unchallenged assumptions concerning the nature of communal farming. Studies which emphasize the social and cultural significance of communal farming systems are highlighted. These studies also suggest that communal farming systems are logical and pragmatic adaptations to local conditions, especially from the point of view of local communities, and that they often have underlying rules of behaviour. It is concluded that unless full participation of local communities at every level of the decision‐making process is guaranteed, resistance to and failure of agricultural development programmes is likely.

ACKDAT: a digital spatial database of distributions of South African plant species and species assemblages

The fieldwork that underpins John Acocks’ work on the Veld Types of South Africa, was recorded in a set of field notebooks, which were preserved after Acocks’ retirement. These are adequately geo-referenced to allow the construction of a relational database linking more than 7 000 species’ presence, abundance and site location data at more than 3 000 sites throughout much of South Africa. The database is described with reference to data quality, representivity and spatial precision. Some appropriate and inappropriate types of uses of the data are illustrated.

Realized niche spaces and functional types: a framework for prediction of compositional change

High resolution and comprehensive quantitative plant and environmental data bases permit an iterative screen- ing approach to provide greater freedom from a priori classification of plant functional types. Realized niche spaces are generated at various plant densities for species across a wide range of environmental conditions in South Africa. Resultant response surfaces suggest different putative func- tional types. Determination of traits in common within such types provides testable hypotheses for application to other species from the available species pool. The approach pro- vides a potentially powerful tool for grouping and regrouping species according to different functional frameworks at re- gional level. This includes deriving types that are sensitive to

Mapping Grazing-Induced Degradation in a Semi-Arid Environment: A Rapid and Cost Effective Approach for Assessment and Monitoring

Improved techniques for measuring and monitoring the state of biodiversity are required for reporting on national obligations to international and regional conservation institutions. Measuring the extent of grazing-related degradation in semi-arid ecosystems has proved difficult. Here we present an accurate and cost-effective method for doing this, and apply it in a South African semi-arid region that forms part of a globally significant biodiversity hotspot. We grouped structurally and functionally similar vegetation units, which were expert-mapped at the 1:50,000 scale, into four habitat types, and developed habitat-specific degradation models. We quantified degradation into three categories, using differences between dry and wet season values of the Normalized Difference Vegetation Index (NDVI) for the three succulent karoo habitats, and the difference between maximum and mean NDVI values for the subtropical thicket habitat. Field evaluation revealed an accuracy of 86%. Overall, degradation was high: 24% of the study area was modeled as severely degraded, and only 9% as intact. Levels of degradation were highest for bottomland habitats that were most exposed to grazing impacts. In sharp contrast to our methods, a widely used, broad-scale and snapshot assessment of land cover in South Africa was only 33% accurate, and it considerably underestimated the extent of severely degraded habitat in the study area. While our approach requires a multidisciplinary team, and in particular expert knowledge on the characteristics and spatial delimitation of vegetation types, it is repeatable, rapid, and relatively inexpensive. Consequently, it holds great promise for monitoring and evaluation programs in semi-arid ecosystems, in Africa, and beyond.

‘Enough to be considered useful’: John Acocks’ contribution to South African botany

John Acocks died in 1979 after working for nearly 50 years as a botanist in South Africa. His scientific contributions have had a profound influence on southern African ecology and this introductory article which includes a biographical sketch of his life, places his contribution in perspective. The 10 articles which appear in this issue have been prepared not only as a tribute to his contribution but also as an assessment of its current utility. Scientific thought is constantly changing and an attempt has been made to indicate where Acocks’ views are still supported by current evidence and where revision is necessary. His contribution lay in three main areas. Firstly, his description of the vegetation (or Veld Types) of South Africa remains a work of considerable importance and will probably never be repeated at the same level of scale, by a single individual, again. Three articles show that when assessed at a smaller scale his Veld Type concept is robust in some, but not all, cases. Differences between Acocks’ views and current treatments of the data are highlighted for the Bankenveld and for the vegetation of the Subtropical Thicket and Nama-karoo biomes. Acocks’ second important influence concerns his views on precolonial vegetation and the extent of human impact. He could not have presaged the explosion of palaeoenvironmental techniques which have arisen since his death and which have provided a somewhat different view of pre-colonial environments from the one he articulated, particularly with regard to the influence of fire on grasslands, savannas and forests. Although his views on the expansion of the eastern Karoo are not upheld in the light of recent findings there is good evidence for significant changes in the fauna and flora of the Karoo over the last 300 years. Acocks’ final contribution lay in the field of veld restoration and grazing management where he continues to have an influence on a small sector of the farming and range science community today. Not only has John Acocks provided a rich theoretical framework for scholars to test but he has also left a legacy of well-organised data for future generations. The final two papers in this special issue indicate the utility of his archives which will gain in value over the years. Challenges to Acocks’ views will continue apace and much of what he has written will undoubtedly be revised. This doesn’t detract from his contribution in any way but serves to highlight the importance of the foundation that he created during his lifetime and which we acknowledge in this special issue.

Fruit Abortion in Yucca elata and Its Implications for the Mutualistic Association with Yucca Moths

The yucca moth Tegelicula yuccasella is the sole pollinator of Yucca elata, on yucca fruits to complete its life cycle. A high percentage of pollinated flowers aborts, killing the T. yuccasella eggs and larvae in them. We examined patterns of fruit production and abortion in Y. elata, and related these patterns to vegetative characteristics, moth abundance, and environmental conditions. We studied 38 inflorescences throughout their flowering period, during one season in southern New Mexico, USA. Each night we recorded the number of flowers opening, the number of fruit formed, the relative abundance of yucca moths, and climatic conditions. We monitored 11786 flowers, resulting in 699 mature fruit. Large inflorescences produced more, but proportionately fewer fruit than small inflorescences