W. R. J. Dean

TIERBERG KAROO RESEARCH CENTRE: HISTORY, PHYSICAL ENVIRONMENT, FLORA AND FAUNA

SUMMARY The Tierberg Karoo Research Centre near Prince Albert in the southern Karoo is intended for long-term ecological monitoring and manipulative studies. This paper summarizes information on the management history, geology, soils, climate and vegetation and includes preliminary checklists of 189 vascular plant species, 25 ant, 90 bird, 18 reptile and 18 mammal species known to occur on the 1 km2 study site. In comparison with arid shrublands in North America, Australia. Israel and elsewhere in southern Africa, grasses and annual forbs are scarce, succulents abundant and there are few alien taxa.

Effects of animal activity on the absorption rate of soils in the southern Karoo, South Africa.

The rates of absorption into various microsites in Karoo soils were compared. The absorption of water by hard, bare intershrub soils was significantly increased by the presence of emergence holes of adult cicadas (Homoptera: Cicadidae) and near nest‐mounds of the harvester ant Messor capensis. Both these insects play an important role in raising the amount of water that penetrates into the soil after rains. Absorption rates were also high in the soils of heuweltjie mounds but were reduced by sustained use of an area by sheep and were particularly poor on sheep paths.

Preferential utilization of pans by springbok (Antidorcas marsupialis)

Forage utilization by springbok in pans and surrounding habitats was compared at one karroid shrubland site and two desert grassland sites and was found to be greater in pans than on adjacent plains and dunes. Springbok used pan areas for various social interactions and there was noticeably more springbok dung in pans. Pan soils had higher Ca, K, Na and P concentrations and plants remained green for longer in pans than in surrounding areas. The high quality of food in pans together with the suitability of pans as display sites may explain the preferential use of pan areas by springbok.

An underground index of rangeland degradation : cicadas in arid southern Africa

SummaryThe prediction that density of long-lived, underground herbivores (Cicadidae) is a function of rangeland condition was examined in arid shrublands in the Karoo, South Africa. It was found that the density of adult cicadas was correlated with cover of deep-rooted, perennial plants. Differences in perennial plant cover were independent of soil type and rock cover. On degraded rangelands, where perennial plants had been replaced by ephemerals and short-lived plants, cicada densities were significantly depressed. We concluded that vegetation transformation by domestic livestock is likely to affect invertebrate populations and their vertebrate predators.

Rates of wood and dung disintegration in arid South African rangelands

Abstract Dead shrubs lying on the soil surface in an arid shrubland in the southern Karoo have half‐lives of 9 to 18 years depending on wood density which varies among species. Dung pellets of sheep and springbok can remain intact on the soil surface in Karoo shrubland and desert grassland for five years or more. Burial hastens the breakdown of dung, but decay may be slower in fine‐textured soils than sandy soils. This preliminary study highlights the dearth of information on ecosystem processes and their response to landuse and climate in arid southern Africa.

Evidence for rapid faunal changes on islands in a man-made lake

SummaryFew studies of island biogeography have been made on islands in which the time of insularization is precisely known. We tested the effects of island formation on ant species diversity in a man-made lake in South Africa, to determine whether island effects are detectable after only 16 years of insularization. The number of ant species observed at trap-line censuses on islands was significantly correlated with island size (r=0.608; P<0.05) and ant species diversity was generally low compared with similar mainland habitats. Mean species number for all islands, including landbridge islands, was 5.5±3.3 species, and on mainland sites was 7.9±2.85 species. Island effects were more marked on islands <20 ha, which had a mean of 3.3±2.5 species per island. Species number on islands was inversely related to densities of the aggressive Anoplolepis custodiens and A. steingroeveri. These two species were only patchily distributed on mainlands, but these ants were nearly ubiquitous on small islands. Several lines of evidence suggest that this single species domination may be responsible for island effects. Island sites also differed in the number of ant species in different trophic groupings, tending to have fewer granivorous species than the mainland sites, but species in other diet groups were similar in both island and mainland habitats. We conclude that there have been marked changes in the ant faunas on islands smaller than 20 ha apparently due to changes in abundance of the dominant ant species. However, the causes of these changes are unknown.

Adaptations of birds for life in deserts with particular reference to larks (ALAUDIDAE) : animals

Birds inhabit all deserts, an unusual observation because they are mainly diurnal, non-fossorial, and have the highest mass-specific metabolism of all vertebrates. Many of the “adaptations” of birds for life in deserts are thought by some to be intrinsic in all birds, and thus have been considered to be pre-adaptations. Nevertheless, recent evidence suggests that desert birds do differ from birds of more mesic habitats, and physiological adaptations include reductions in basal metabolism and cutaneous water loss. Behavioural mechanisms that permit a desert existence include nomadism, enabling them to move away from low resource patches to more productive patches, and the ability to use features of their environment to escape from high temperatures. Desert birds can prevent overheating by using rodent burrows and other shelters during the heat of the day. Birds in deserts include a high proportion of ants in their diet, a resource not often exploited by birds of more temperate environments, and they also show seasonal or opportunistic shifts in their diet, feeding on seeds when available, and on more succulent green plant matter, such as leaf bases of grasses, after rain. Certain taxa, for example sandgrouse (Pteroclidae) and larks (Alaudidae), are mainly found in deserts. The adaptations in some species of larks are discussed in detail.