Retha Hoffman

Bio: Retha Hoffman is an academic researcher from North-West University. The author has contributed to research in topics: Species richness. The author has an hindex of 1, co-authored 1 publications receiving 15 citations.

The effects of fire on grassland bird communities of Barberspan, North West Province, South Africa

Abstract: Considering the frequent nature of fires and resultant drastic change in habitat following fire, research on the effects of fire on birds in the grasslands of South Africa is surprisingly scarce. For at least five months after burns we followed the changes in bird species composition, species richness and densities of two controlled burns and one accidental fire at the Barberspan Nature Reserve in grasslands that had not been burned or grazed in 10 years. Compared with the control areas, species richness and densities increased in the burned areas immediately following the burns, with more species and birds recruited to the burned areas than were lost. Immediate post-burn opportunists tended to be larger species, and the biomass increase mirrored the increases in species richness and densities in burned areas. Avian species richness, densities and biomass tended to return to the initial conditions after a number of months. Although the bird communities from two controlled-burns differed before the burns, they converged to a characteristic immediate post-burn composition. Five months after the burns however, the bird communities reflected a pre-burn composition. Indications were that birds in an area larger than that burned were affected. Mosaic burning, with shifting large and small patches, should be considered on a landscape scale. Ostrich 2007, 78(3): 591–608

Organochlorine contaminants in cormorant, darter, egret, and ibis eggs from South Africa.

Abstract: During the last 15 years, no research has been published on the levels of pesticides in bird eggs from South Africa, despite the high levels found previously. We analysed eggs from African darter, cattle egret, reed cormorant, African sacred ibis, as well as single eggs from some other species, and found HCB, DDTs, HCHs, chlordanes and PCBs at detectable levels. The presence of mirex in all species was unexpected, since this compound was never registered in South Africa. It also seemed as if terrestrial feeding birds had higher DDE:PCB ratios when compared with aquatic feeding birds. Except for chlordane, the African darter eggs had the highest levels of all other compounds (mean 370 and 300 ng g −1 ww Σpesticides and ΣPCBs, respectively). Multivariate analysis clearly distinguished the aquatic and terrestrially feeding birds on pollution profile. The African darter (aquatic feeding) and the cattle egret (terrestrial feeding) would be good indicator candidates. Eggshell thinning was detected in the African darter, and was associated with most of the compounds, including DDE and PCBs. We raise a concern that generally longer living birds in warmer climates, laying fewer eggs per clutch, might be at increased risk when compared with trophically similar birds exposed to equivalent levels of pollution in colder climates. Given the scarcity of water and the high biodiversity in Southern Africa, climate change will exert strong pressure, and any additional anthropogenic contamination at levels that can cause subtle behavioural, developmental and reproductive changes, can have serious effects.

Spatial heterogeneity across five rangelands managed with pyric-herbivory.

Abstract: Summary 1. Many rangelands evolved under an interactive disturbance regime in which grazers respond to the spatial pattern of fire and create a patchy, heterogeneous landscape. Spatially heterogeneous fire and grazing create heterogeneity in vegetation structure at the landscape level (patch contrast) and increase rangeland biodiversity. We analyzed five experiments comparing spatially heterogeneous fire treatments to spatially homogeneous fire treatments on grazed rangeland along a precipitation gradient in the North American Great Plains. 2. We predicted that, across the precipitation gradient, management for heterogeneity increases both patch contrast and variance in the composition of plant functional groups. Furthermore, we predicted that patch contrast is positively correlated with variance in plant functional group composition. Because fire spread is important to the fire– grazing interaction, we discuss factors that reduce fire spread and reduce patch contrast despite management for heterogeneity. 3. We compared patch contrast across pastures managed for heterogeneity and pastures managed for homogeneity with a linear mixed effect (LME) regression model. We used the LME model to partition variation in vegetation structure to each sampled scale so that a higher proportion of variation at the patch scale among pastures managed for heterogeneity indicates patch contrast. To examine the relationship between vegetation structure and plant community composition, we used constrained ordination to measure variation in functional group composition along the vegetation structure gradient. We used the meta-analytical statistic, Cohen’s d, to compare effect sizes for patch contrast and plant functional group composition. 4. Management for heterogeneity increased patch contrast and increased the range of plant functional group composition at three of the five experimental locations. 5. Plant functional group composition varied in proportion to the amount of spatial heterogeneity in vegetation structure on pastures managed for heterogeneity. 6. Synthesis and applications. Pyric-herbivory management for heterogeneity created patch contrast in vegetation across a broad range of precipitation and plant community types, provided that fire was the primary driver of grazer site selection. Management for heterogeneity did not universally create patch contrast. Stocking rate and invasive plant species are key regulators of heterogeneity, as they determine the influence of fire on the spatial pattern of fuel, veg etation structure and herbivore patch selection, and therefore also require careful management.

Heterogeneity as the Basis for Rangeland Management

Abstract: Rangeland management, like most disciplines of natural resource management, has been characterized by human efforts to reduce variability and increase predictability in natural systems (steady-state management often applied through a command-and-control paradigm). Examples of applications of traditional command and control in natural resource management include wildfire suppression, fences to control large ungulate movements, predator elimination programs, and watershed engineering for flood control and irrigation. Recently, a robust theoretical foundation has been developed that focuses on our understanding of the importance of variability in nature. This understanding is built upon the concept of heterogeneity, which originated from influential calls to consider spatial and temporal scaling in ecological research. Understanding rangeland ecosystems from a resilience perspective where we recognize that these systems are highly variable in space and time cannot be achieved without a focus on heterogeneity across multiple scales. We highlight the broad importance of heterogeneity to rangelands and focus more specifically on (1) animal populations and production, (2) fire behavior and management, and (3) biodiversity and ecosystem function. Rangelands are complex, dynamic, and depend on the variability that humans often attempt to control to ensure long-term productivity and ecosystem health. We present an ecological perspective that targets variation in rangeland properties—including multiple ecosystem services—as an alternative to the myopic focus on maximizing agricultural output, which may expose managers to greater risk. Globally, rangeland science indicates that heterogeneity and diversity increase stability in ecosystem properties from fine to broad spatial scales and through time.

Organochlorine residual concentrations in cattle egret from the Punjab Province, Pakistan

Abstract: In this study, residual concentration of organochlorine pesticides (OCPs) in the sediments, prey, and eggs of Bubulcus ibis were measured from three breeding heronries from the Punjab province of Pakistan. Pattern of contamination in eggs followed the order: DDTs > HCHs > heptachlor > aldrin. Overall, pesticide residual concentrations were greater in eggs of cattle egrets collected from heronry on the River Ravi. Among HCHs, γ-HCH was more prevalent in eggs, whereas DDTs followed the order: DDD > DDE > p,p ′-DDT > o,p ′-DDT. Eggshell thinning was detected which showed negative relationship with residual concentration of DDE. In prey samples, residual concentration of POPs followed the order: DDTs > HCHs > dicofol > heptachlor; however, contamination pattern in sediments followed a slightly different order: DDTs > heptachlor > dicofol > HCHs > dieldrin > aldrin. Concentration of β-HCH was more prevalent in sediments and comparatively greater concentrations of POPs were measured in sediments collected from the River Ravi. Dicofol was found for the very first time in the biological samples from Pakistan, and its concentration was measured as relatively high in eggs from heronry from the River Chenab. Residual concentrations measured in eggs were below the levels that could affect egret populations. Biomagnification of the total OCPs through the food chain was evident in three breeding heronries. The concentration of DDE measured in eggs of the cattle egret suggests the need for monitoring this contaminant in other bird species at different trophic levels.

The pyrophilic primate hypothesis

Abstract: Members of genus Homo are the only animals known to create and control fire. The adaptive significance of this unique behavior is broadly recognized, but the steps by which our ancestors evolved pyrotechnic abilities remain unknown. Many hypotheses attempting to answer this question attribute hominin fire to serendipitous, even accidental, discovery. Using recent paleoenvironmental reconstructions, we present an alternative scenario in which, 2 to 3 million years ago in tropical Africa, human fire dependence was the result of adapting to progressively fire-prone environments. The extreme and rapid fluctuations between closed canopy forests, woodland, and grasslands that occurred in tropical Africa during that time, in conjunction with reductions in atmospheric carbon dioxide levels, changed the fire regime of the region, increasing the occurrence of natural fires. We use models from optimal foraging theory to hypothesize benefits that this fire-altered landscape provided to ancestral hominins and link these benefits to steps that transformed our ancestors into a genus of active pyrophiles whose dependence on fire for survival contributed to its rapid expansion out of Africa.