Ecological and phylogenetic patterns of carbon isotope discrimination in the winter-rainfall flora of the Richtersveld,South Africa

Leaf tissues of vascular plant species from the arid Richtersveld of northern Namaqualand (Succulent Karoo), South Africa, show a great range of variation in carbon isotope discrimination. Analyses of values for species with obligate C₃ metabolism demonstrated that the water use efficiency indicated by these values varies between species and within a species population both between seedlings and mature shrubs and between sites along an aridity gradient. An expected pattern of increasing water use efficiency with greater longevity in these species was not present, however. Only among ephemerals were there taxa with high values associated with low water use efficiency, but these high values were balanced by other ephemeral species with low values comparable to those typical of moderate to long-lived shrubs. The extent to which putative CAM species in the succulent flora rely entirely on nocturnal carbon fixation is highly variable. Species of Crassulaceae and Aloe (Asphodelaceae) relied almost exclusively on CAM photosynthesis, although Crassulaceae from more mesic environments may be CAM-flexible. Succulent-leaved species of Senecio and leaves of stem-succulent Euphorbia were CAM-flexible in their values, while shrubby species of Ceraria (Portulacaceae) appeared to rely primarily on C₃ photosynthesis. Variable patterns of reliance on CAM photosynthesis were present in the ecologically dominant Mesembryanthema of the Aizoaceae. Shrubby species of the subfamily Mesembryanthemoideae, although capable of CAM-flexibility in less xeric habitats, relied largely on CAM fixation of carbon in our field sites. Within the subfamily Ruschioideae, there was a wide range of values indicating species with CAM, CAM flexibility, and primary reliance on C₃ photosynthesis. Low values indicative of typical CAM fixation were associated with species with greater longevity, suggesting that a positive correlation may exist between the degree of reliance on CAM photosynthesis and plant life span.     

Remarkable medium-term dynamics of leaf succulent Mesembryanthemaceae shrubs in the winter-rainfall desert of northwestern Namaqualand,South Africa

Populations of shrubs in a winter-rainfall (ca. 70 mm yr_-1 desert community (Succulent Karoo), dominated by leaf succulents, showed remarkable dynamics over a 17-yr period. After a severe drought in 1979, which caused high plant mortality, perennial species number in a permanent 10 m×10 m plot doubled between 1980 and 1996, when the maximum of 41 species was recorded. Numbers of individuals of evergreen, leaf succulent shrubs also doubled over the same period, but showed fluctuations in response to dry years during the monitoring period. Detailed observations on the four dominant leaf succulent shrubs (all members of the Mesembryanthemaceae) between 1983 and 1996, showed species-specific patterns in population turnover. Mortality of all observed individuals ranged between 60% and 85%, and the proportion of the population that was recruited over this period ranged from 62% to 89%. In only one species did individuals persist throughout the monitoring period. Mean ages of individuals, excluding seedlings with lifespans of <1 yr, and individuals observed at the beginning of the monitoring period, ranged from 4.6 yr to 5.6 yr. Patterns of mortality and recruitment showed substantial differences among species and were not all attributable to rainfall patterns. Overall, the turnover of the shrub populations over the 15-yr monitoring period was remarkably high for a system of desert perennials. These unusual population patterns may explain the unique structure of leaf succulent-dominated communities in the Succulent Karoo.     

Plant species richness and composition a long livestock grazing intensity gradients in a Namaqualand (South Africa) protected area

The study described changes in floristic and vegetation structure in relation to livestock grazing intensity in a conservation area in the Succulent Karoo, South Africa. Grazing by goats and sheep is allowed in the Richtersveld National Park (a contractual National Park. which is also an area of high floristic richness and endemism. We used goat faecal pellet density, degree of trampling and percentage bare-ground at distances from the stock posts as surrogates for a gradient in grazing pressure. A stock post is the place where farmers keep, in most cases in an enclosure called a ‘kraal’, their animals at night and to which they return every evening after the day’s herding. Twenty-seven stock posts were located in the Richtersveld National Park; nine stock posts on flats, foot– slopes and mountain each. We measured plant species richness and diversity, and mean percentage cover of the various plant growth forms (including the number of species falling into each growth form category. in each of the five 10 m . 10 m plots (each 200 m apart. demarcated along a transect of one kilometre length from the centre of each stock post. The results showed that distance from the stock post does reflect grazing intensity use because densities in faecal pellets rapidly declined with increasing distances away from the stock post for all habitats studied. Faecal density was positively correlated with stocking density. Plant species richness and diversity was at a minimum near stock posts. Plants able to endure the effects of heavy grazing occurred near stock posts where declines in palatable plant species, assumingly sensitive to heavy grazing and trampling, were recorded. Grazing increased vegetation patchiness up to 800 m from the stock post for all the habitats. The degree to which this change in species composition occurred did not depend on stocking densities, suggesting that both grazing and landscape variability were responsible for vegetation changes in rangelands of that area of the Succulent Karoo biome.     

A fence-line contrast reveals effects of heavy grazing on plant diversity and community composition in Namaqualand,South Africa

Changes in plant species richness and community composition were investigated across a fence separating heavily grazed communal and lightly grazed commercial farming systems in Namaqualand, South Africa. No significant differences in plant species richness between communal and commercial farming systems were detected either locally within individual plots or overall across all plots. Within-plot, richness of species tolerant of grazing, such as annuals and geophytes, has increased, while the richness of large palatable shrub species has decreased on the communal rangeland. In terms of plant cover, species' responses to grazing were strongly associated with growth form. Annuals and geophytes formed the majority of grazing increasers, while large, presumably palatable, shrubs and leaf succulents were characteristic grazing decreasers. An investigation into population processes of five shrub species revealed that heavy grazing on the communal rangeland has resulted in: reduced size of palatable shrub species; reduced flower production and seedling recruitment of palatable species; increased density and recruitment of the unpalatable shrub, Galenia africana. Reductions in shrub volume, reproductive output and seedling recruitment were most marked in the palatable shrub Osteospermum sinuatum and were in the order of 90%. The results are further discussed in terms of their relevance to rangeland dynamics and the current land use practices of the region.     

The Potential of Cephalophyllum inaequale (L. Bolus) for the Restoration of Degraded Arid Landscapes in Namaqualand, South Africa

The potential of Cephalophyllum inaequale was investigated for use in initiating ecosystem restoration in degraded landscapes of Namaqualand, South Africa. Cephalophyllum inaequale, a perennial shrub, is a member of the succulent Mesembryanthemaceae family, typical of the Succulent Karoo Biome and in particular of the Namaqualand area. A bioclimatic envelope was modeled to establish the area in which this species might feasibly be used. The regional bioclimatic potential for C. inaequale proved to be extensive, covering approximately 17,500 km². An examination of the functional role of C. inaequale showed it to facilitate early seedling survival in this community. A nearest‐neighbor study found no evidence of interspecific competition between C. inaequale and its dominant co‐occurring species, possibly due to vertical stratification of rooting structures. Cephalophyllum inaequale significantly reduced wind speed and soil erosion. Experiments to test the feasibility of propagating, reintroducing, and establishing this species showed that it easily germinates from seed, and transplanted cuttings have a high survival rate. This study demonstrates that C. inaequale has potential for use in initiating the restoration of degraded lands in South Africa .     

The role of domestic herbivores in endozoochorous plant dispersal in the arid Knersvlakte,South Africa

Herbivores can act as dispersal vectors by purposely or accidentally ingesting ripe fruits, and thus endozoochory is one determining factor for plant distribution patterns. The objective of our study was to investigate to what extent plants of major taxonomic groups of the Knersvlakte (Succulent Karoo, South Africa) are endozoochorously dispersed. On three different farms in the central Knersvlakte, dung of domestic herbivores was collected and analysed by the seedling-emergence method. The resulting species composition was compared to the standing vegetation of thirty-four 1000-m² plots each recorded on one hundred 400-cm² subplots. Our results show that domestic livestock facilitated the dispersal of taxa characteristic of the Knersvlakte, in particular Aizoaceae. Among the taxa of this family, the local endemic dwarf shrub Drosanthemum schoenlandianum emerged with the highest frequency in dung (14.5% of all seedlings). For the Asteraceae, which are frequent in the standing vegetation of the Knersvlakte, however, endozoochorous dispersal by livestock was only of minor importance. Conservation planning should consider these dispersal patterns on behalf of future population dynamics. The complete exclusion of livestock might change current processes and thus alter vegetation patterns.     

Effects of Multispecies Clumping on Survival of Three Succulent Plant Species Translocated onto Mine Spoil in the Succulent Karoo Desert, South Africa

Salvaging from premining areas and translocation of succulent plants has been investigated for the revegetation of gypsiferous mine spoil in an arid region of South Africa. Given that facilitation effects are thought to outweigh competition effects in harsh environments, we hypothesized that the survival of translocated succulents would be higher when planted in multispecies clumps than alone and that the growth rate (measured as stem extension) and fruit‐set would be greater for plants in clumps than for those planted alone. Two leaf‐succulent species (Aridaria noctiflora ssp. noctiflora and Drosanthemum deciduum) and one stem‐succulent species (Psilocaulon dinteri) were salvaged from the area destined for mining and translocated onto the mine spoil. These plants were planted either in a multispecies clump of the three species together or alone. One year after translocation, 67% of the plants survived. It was also found that the succulents used in these experiments survived in higher numbers when planted alone. Due to the similar root morphology of D. deciduum and P. dinteri, they competed for resources instead of facilitating each other's establishment. The results were variable for each of the species used, and neither growth nor seed‐set was improved by clumping. These findings would also possibly vary from year to year with different abiotic conditions.     

Plant induced fertile islands as possible indicators of desertification in a succulent desert ecosystem in northern Namaqualand,South Africa

Zones of nutrient enrichment beneath shrubs, the so called 'fertile islands', are the result of a range of interacting physical and biotic concentrating mechanisms. These concentrating mechanisms lead to specific spatial patterns of soil properties across a landscape and it has been suggested that disruption of the normal patterns into more aggregated 'fertile islands' might be a useful index of desertification in semi-arid regions. The objective of this paper was to determine whether 'fertile islands' are as characteristic a feature of succulent deserts as they are of shrub dominated deserts. Results indicate that, even in a community dominated by succulent species fertile islands develop under extremely arid conditions and that the degree of development is dependent on plant dynamics. Long-lived species showed greater concentration effects. However, the patterns of nutrient accretion in islands are very different to shrub invaded grasslands in North America. In particular nitrogen accretion dominated the 'fertile islands' in Namaqualand, and we suggest that these islands are more ephemeral than carbon and phosphorus dominated islands reported for other systems. These more dynamic islands are a consequence of rapid community dynamics since the Namaqualand region is characterised by many short-lived perennials which turnover generations very rapidly in response to environmental conditions. Using the development of 'fertile islands' to assess desertification as applied elsewhere might be inappropriate in Namaqualand because of the different patterns of community dynamics underlying the formation and characteristics of these islands.     

Fossil wood charcoal assemblages from Elands Bay Cave, South Africa: implications for Late Quaternary vegetation and climates in the winter-rainfall fynbos biome

Aim The aim of this paper is to analyse fossil charcoal deposits, largely identified to the species level and spanning a sequence from the late Holocene to < 40,000 BP , in order to reconstruct Late Quaternary vegetation and climatic patterns in the western (winter-rainfall) fynbos biome of South Africa. Location The charcoals were excavated from the Elands Bay Cave (32°19S, 18°20E) on the semiarid (200–250 mmyr^?1), winter-rainfall coastline of the western fynbos biome. Methods Patterns in the charcoal data set over time were sought by manual sorting of the charcoal×sample matrix, as well as by subjecting the data to multivariate analysis. Palaeoclimatic reconstruction was attempted by comparing the climatic controls on contemporary vegetation communities that resembled the fossil assemblages. Charcoal diversity was modelled using sample age and number of charcoal fragments as explanatory variables. Results The fossil assemblages ranged from xeric communities (similar to those presently occurring at the site) during the Holocene, to more mesic thicket and fynbos vegetation in the terminal Pleistocene, to Afromontane forest and riverine woodland communities after about 18,000 BP . Diversity of the charcoal samples increased monotonically with increasing sample age. Main conclusions The results suggest that, unlike the eastern fynbos biome, which is under fundamentally different climatic controls, soil moisture conditions in the western part of the biome were higher in the Last Glacial than during the Holocene. This scenario may help to explain the higher regional richness and associated diversification in the western than eastern part of the biome.     

Rodent assemblage in a habitat mosaic within the Valley Thicket vegetation of the Eastern Cape Province, South Africa

We sampled small mammal assemblage in a mosaic of Valley Thicket vegetation in the Great Fish River Reserve during the summers of 2001 and 2002. Assemblage was dominated by seven rodent species with a single nonrodent species (Elephantulus edwardii) in the 357 trapped specimens. Rhabdomys dilectus was the most abundant in both years, followed by Mus minutoides (ranking 2nd in 2001) and Mastomys coucha (2nd in 2002). These three species, which are widespread generalists, made up 85.0% of the total rodent catch in 2001 and 74.3% in 2002. Individual species were captured on 2–14 sites of a total of 17 and M. minutoides was the most widespread. The majority of samples consisted of two (ten sites in a pooled sample for 2 years) or three species (ten sites). Significant morphological structuring was found only in two‐species samples. Species turnover was high both among sites and between the 2 years. We found no evidence of significant nested structure which would suggest hierarchical sets of ecological relationships among the species. Vegetation structure explained better than species interactions, the population variables in the rodent assemblage.     

The distribution and biology of nomadic birds in the Karoo, South Africa

Dryland nomadic bird species, as a proportion of all bird species in a biome in southern Africa, are highest in the arid grassland and arid and semi-arid Karoo in South Africa. Nomadic birds, of which the most widespread species is the greybacked finchlark Eremopterix verticalis (Smith), are most frequently observed in the north-central and north western Nama Karoo. The species richness of nomadic species is inversely correlated with species richness of all bird species in the Karoo. Since the distribution of nomadic birds is in areas where rainfall is patchy, low (<250 mm per year) and aseasonal, this supports the idea that fewer species are able to cope with resources that are patchy in time and space, and that there has been selection for nomadism in the species that are able to use patchy environments. Species richness and abundance of nomadic birds is negatively correlated with rainfall amount but positively correlated with the coefficient of variation of the rainfall and with rainfall in autumn. The frequency of nomadic birds is inversely correlated with altitude range; nomadic species are most often recorded in structurally simple habitats (shrubland and grassland) on open plains. Most nomadic bird species in the Karoo are granivorous. Perennial desert grasses are important components of the habitat and diet of small nomadic granivores, and also provide nest sites and nest material. Nomadic birds can breed throughout the year, without a clearly defined ‘season’ in both the Succulent and Nama Karoo. Average clutch sizes do not differ significantly between resident and all nomadic species in the arid and semi-arid Karoo. Nomadism is an evolutionary stable strategy for individual species only when extremes in environmental conditions are frequent enough, and unpredictable enough, to maintain movements to high resource patches or to maintain dispersal away from low resource patches. If high rainfall years are too regular or infrequent, or peaks in fluctuations of resources in the environment too low, or rainfall patches are randomly distributed, nomadism would not be maintained as part of the individual behaviour pattern.     

Release from native root herbivores and biotic resistance by soil pathogens in a new habitat both affect the alien <Emphasis Type="Italic">Ammophila arenaria</Emphasis> in South Africa

Many native communities contain exotic plants that pose a major threat to indigenous vegetation and ecosystem functioning. Therefore the enemy release hypothesis (ERH) and biotic resistance hypothesis (BRH) were examined in relation to the invasiveness of the introduced dune grass Ammophila arenaria in South Africa. To compare plant–soil feedback from the native habitat in Europe and the new habitat in South Africa, plants were grown in their own soil from both Europe and South Africa, as well as in sterilised and non-sterilised soils from a number of indigenous South African foredune plant species. While the soil feedback of most plant species supports the ERH, the feedback from Sporobolus virginicus soil demonstrates that this plant species may contribute to biotic resistance against the introduced A. arenaria, through negative feedback from the soil community. Not only the local plant species diversity, but also the type of plant species present seemed to be important in determining the potential for biotic resistance. As a result, biotic resistance against invasive plant species may depend not only on plant competition, but also on the presence of plant species that are hosts of potential soil pathogens that may negatively affect the invaders. In conclusion, exotic plant species such as A. arenaria in South Africa that do not become highly invasive, may experience the ERH and BRH simultaneously, with the balance between enemy escape versus biotic resistance determining the invasiveness of a species in a new habitat.Plant nomenclature follows Arnold and De Wet (1993)     

Aspect and slope as determinants of vegetation composition and soil properties in coastal forest backdunes of Eastern Cape,South Africa

Coastal sand dune ecosystems are subjected to many stress and disturbance factors that are particularly high in the foredunes compared to the backdunes. Although a few studies have been conducted on eastern coastline sand dunes of South Africa, none have examined the relationship between aspect and slope on vegetation composition and soil properties of coastal forest backdunes. Vegetation and soil sampling were conducted in 11 transects, each with four plots measuring 10 × 10 m, located on the seaward and landward sides and on middle and lower slopes of backdunes of Bathurst coastal forest. A total of 39 species were identified, of which 23 were trees and shrubs, thirteen were forbs and three were grasses. The data show that both aspect and slope had limited influence on vegetation community assemblage and soil properties, but had significant effects on individual species distribution. There was a grass‐dominated community on the middle slope and a tree‐ and shrub‐ dominated community on the lower slope. These two plant communities act as the required coastal forest ecosystem engineer driving variability in soil properties between the slopes, the most prominent being high soil nutrients and moisture in the lower slopes compared to the middle slopes.     

Live fast,die young: estimating size-age relations and mortality pattern of shrubs species in the semi-arid Karoo,South Africa

We present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets of size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic idea of our technique is to assume general (three parameter) families of size-dependent functions which describe growth and mortality that occurred between the two data gathering events. We apply these growth and mortality functions to the size data of the early data set and construct predicted size-class distributions to compare it, in a systematic way, to the size-class distribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest difference between observed and predicted size-class distribution. Applying this technique to size data of five dominant shrub species at the Tierberg study site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequent measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the life-history attributes evergreen vs. deciduous, succulent vs. woody, and early reproductive vs. late reproductive. The results of our pilot-study suggest a broad applicability of our technique to other shrublands of the world. This requires at least one older record of (individual) shrub-size data and performance of resampling.     

The first record of gynodioecy in a species of Gnidia (Thymelaeaceae) from South Africa

Sexual polymorphism was studied in the shrub Gnidia wikstroemiana (Thunb.) Meisn. from the semiarid Nama Karoo Biome, South Africa. The populations comprised plants bearing either female flowers, or hermaphrodite flowers with variable female function. In two populations, female plants accounted for 36–37% of the flowering plants. Female flowers were smaller and their stamens were reduced to staminodes, but their styles were significantly longer than those of hermaphrodite flowers. Energy investment in flowers and fruits for females and hermaphrodites was measured using bomb calorimetry. Females produce a greater number of less costly flowers than hermaphrodites, and invest less energy per unit in production of flowers and inflorescences. In contrast, females invest more energy per unit in production of fruits and infructescences than hermaphrodites. Females overall invest 7.3% more energy in reproduction than hermaphrodites. Female flowers were obligate out-crossers (xenogamous), with 35% of nonmanipulated, open-pollinated flowers setting fruit, comparable with fruit set among selfed hermaphrodite flowers. The breeding strategy of G. wikstroemiana most closely resembles gynodioecy. This is the first report of sexual dimorphism in Gnidia L. and sub-Saharan Thymelaeaceae.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 152 , 219–233.     

Freshwater rock pools: a review of habitat characteristics,faunal diversity and conservation value

1. In the light of the recent surge of interest in small and often temporary wetlands as model systems for ecological and evolutionary research, this article reviews current knowledge on freshwater rock pools and their fauna. 2. Freshwater rock pools occur all over the globe in all major biomes and depend mainly on precipitation for filling. Rock pool clusters are some of the more persistent and oldest freshwater habitats worldwide. Interactions between climate and geology (e.g. limestone, sandstone, granite) generally determine the morphology and hydrology of rock pool habitats, with hydroperiods ranging from several days up to the whole year. Pool volume is usually small, resulting in strongly fluctuating environmental conditions, low conductivity and wide variations in pH (from 4.0 to 11.0) and temperature (from freezing point to 40 °C) often with well‐marked diel cycles. The highly variable environmental conditions, combined with the unpredictability of the flooding regime, require high stress tolerance of the inhabitants, with adaptations for surviving the dry phase such as the production of resistant stages and active emigration followed by recolonisation. 3. About 460 aquatic animal species have been recorded from freshwater rock pools around the world. Approximately 170 of these are passive dispersers, which mainly disperse as resting stages via wind and overflow of water between pools. Successful long distance dispersal seems limited. This group is composed mainly of rock pool specialists with a high degree of endemicity. The remaining taxa can be considered active dispersers, with migration usually restricted to the adult stage. Often these taxa are broadly distributed and occur in a wide range of temporary habitats in addition to rock pools. 4. The inherent characteristics of freshwater rock pools, such as their simple structure and occurrence on similar substrata all over the world, facilitate comparison of research results and promote rock pools as model systems for ecological and evolutionary research. Nevertheless, despite their potential as model systems, the unique fauna and their importance as sources of freshwater in dry countries, the ecology of freshwater rock pools remain virtually unexplored in large parts of the world.