Bruce Riddoch

Temporary pools are not `enemy-free'

Temporary pools are traditionally considered as refuges where the conspicuous anostracans are protected from predation. While this is true for the size-selective predation by fish, there is compelling evidence that invertebrate predation is an important biotic stress regulating temporary pool communities. In rock pools in southeastern Botswana, we studied the impact of some suspected invertebrate predators on populations of the freshwater anostracan Branchipodopsis wolfi by means of observations and manipulative experiments. In a survey of 45 pools, the relationship between B. wolfi natural population sizes and the abundance of suspected predators were never negative for turbellarians and mosquito larvae. When dragonfly larvae, notonectids or tadpoles were present, the anostracan populations were generally non-existent or very small. In enclosure experiments with turbellarians, there was a significant effect of predation within one hour of the start; the average daily predation rate was about 1/4 anostracan per turbellarian. Anostracans from a pool with few turbellarians were slightly less vulnerable than those from a turbellarian-rich pool. Furthermore, there was an indication of males being predated on more than females. With dragonfly larvae and notonectids, the predation effect was marked with all six anostracans in an experiment eaten in less than one day by a single predator (predation rate: about one anostracan every 2 h per predator). In a behavioral study, both sexes of B. wolfi avoided swimming above sediment that held more turbellarians than the open patches; there was no evidence for chemical communication with respect to this behavior.

Regional structuring of genetic variation in short-lived rock pool populations of Branchipodopsis wolfi (Crustacea: Anostraca)

Abstract The genetic structure of three metapopulations of the southern African anostracan Branchipodopsis wolfi was compared by analysing allozyme variation at four loci (PGM, GPI, APK, AAT). In total, 17 local populations from three sites (metapopulations) were analysed from rock pools in south-eastern Botswana ranging from 0.2 to 21 m2 in surface area. In three populations we found significant deviations from Hardy-Weinberg (H-W) equilibrium at one or more loci due to heterozygote deficiencies. Genetic variability at one site was significantly lower than at the other sites, which may be linked to a greater incidence of extinction and recolonisation, as the basins at this site are shallower and have shorter hydrocycles. Across all local populations, a significant level of population differentiation was revealed. More than 90% of this variation was explained by differentiation among sites (metapopulations), although this differentiation did not correlate with geographic distance, or with environmental variables. Genetic differentiation among populations within metapopulations was low, but significant at all sites. At only one of the sites was a significantly positive association measured between genetic and geographic distance among local populations. Our data suggest that persistent stochastic events and limited effective long-range dispersal appear to dominate genetic differentiation among populations of B. wolfi inhabiting desert rock pools. The lack of association between geographic distance and genetic or ecological differences between rock pool sites is indicative of historical stochastic events. Low heterozygosity, the significant deviations from H-W equilibrium, and the large inter- but low intra-site differentiation are suggestive of the importance of short-range dispersal. Gene flow between metapopulations of B. wolfi appears to be seriously constrained by distances of 2 km or even less.

Quantifying the Hydroregime of a Temporary Pool Habitat: A Modelling Approach for Ephemeral Rock Pools in SE Botswana

Ecological and evolutionary processes in temporary rock pools operate within constraints imposed by their hydrologic regimes. These shallow pools flood when seasonal rains accumulate on impermeable substrates. Despite the ecological importance of hydrologic conditions for these ecosystems, we typically lack tools and empirical data required to understand the implications of hydrologic variability and climate change for biotic populations and communities in these habitats. In this study, we developed a hydrologic model to simulate rock pool hydrologic regimes based on rainfall, evapotranspiration, and basin geometry. The model was used to investigate long-term patterns of seasonal and inter-annual variation in hydroregime. In addition, hydrologic conditions associated with potential climate change scenarios were simulated and evaluated with respect to the biological requirements of the anostracan Branchipodopsis wolfi. The model’s output for daily inundation matched with field observations with an overall accuracy of 85% and correctly estimated complete hydroperiods with an overall accuracy of 70%. Simulations indicate large variation in individual hydroperiods (76–115%) as well as in the number of hydroperiods per year (19–23%). Furthermore, this study suggests that climate change may significantly alter the rock pool hydroregime. These findings confirm the hydrologic sensitivity of these ephemeral habitats to precipitation patterns, and their potential sensitivity to future climate change. Modelling indicates that the suitability of average inundation conditions for B. wolfi deteriorates significantly under future climate predictions. High levels of spatial and temporal variation in hydrologic conditions are dominant features of these habitats and an essential consideration for understanding population and community-level ecological processes.

Branchipodopsis species — specialists of ephemeral rock pools

The anostracan Branchipodopsis genus is widespread throughout southern Africa and is the second most speciose anostracan taxon in this sub-continent. Branchipodopsis species are particularly dominant in small short-lived and clear rock pools, to the vagaries of which they are extremely well adapted. Such rock pools were studied in the Drakensberg region, in the eastern Free State and in south-eastern Botswana. Common features of Branchipodopsis-inhabited rock pools are the transparency of the water and the very low conductivity values (generally below 50μScm−1, often less than 10μScm−1). These shallow (usually less than 50cm) water bodies with limited buffering capacity show major fluctuations in pH values (often between about pH 4 and 11), depending on the time of the day and the stage in the hydrocycle. Rock pools also closely follow ambient air temperatures with resulting fluctuations of values between 10 and 40°C. Depending on local climatic conditions, small rock pools are usually short-lived and have several wet/dry cycles during one rainy season. Branchipodopsis species are the record-holders in this race against time with maturation usually being reached within the first week after inundation. Broods of resting eggs are generally small (less than 80 eggs) but are produced almost daily. Often (in 30% of cases in south-eastern Botswana), erratic rainfall does not allow sufficient time for successful maturation and reproduction. As a safe-guarding mechanism in B. wolfi in south-eastern Botswana, only some of the eggs hatch on each occasion, depending on environmental conditions, thus extending the chances for successful recruitment. While short-range dispersal of floating eggs by overflows is common, long-range dispersal (e.g. by wind) seems to be rare and to be restricted to shallow pools with little vegetation. Limited effective dispersal may explain the high number of endemic species (e.g. four in the Drakensberg region) and the large morphological variation in some widespread species such as B. wolfi.

Faunistics of ephemeral rock pools in southeastern Botswana

Abstract: Rock pools on granite outcrops occur worldwide and are poorly studied,despite their intrinsic biological interest. In semi-arid Botswana, such habitats occurmainly on the granite outcrops in the southwestern Hardveld zone. To date, studies onthese systems have focused mainly on individual species or particular interactions. Bymeans of frequent sampling (every other day) during an entire wet phase (hydrocycle),we attempted to get a time integrated overview of invertebrate species composition ina set of 18 rock pools from two clusters (meta-communities).A faunal list is presentedand described. Rock pool species were separated in permanent and ephemeral inhabi-tants, based on their strategy to survive or escape the frequent dry phases of their habi-tat, respectively. A new chydorid species, four new turbellarian taxa and two new ost-racod species were discovered. All new taxa were permanent inhabitants, illustratingthe need for more intense and time-integrated studies of these ephemeral systems andespecially the permanent residents with specific adaptations to the vagaries of theirvariable habitat. The best sampling strategy to assess species richness in these rockpool systems is to randomly sample three to four pools in a cluster, each in the finalphase of their hydrocycle.Key words: rock pool, ephemeral, fauna, sampling.