Joh R. Henschel

Bio: Joh R. Henschel is an academic researcher from University of the Free State. The author has contributed to research in topics: Arid & Welwitschia. The author has an hindex of 15, co-authored 34 publications receiving 915 citations.

Reflections on the Karoo Special Issue: towards an interdisciplinary research agenda for South Africa’s drylands§

Abstract: We begin this essay with reflections on major research themes highlighted by the Karoo Special Issue (KSI). These include concerns over land-use change, long-term monitoring, climate change, govern...

Sulphur isotopes in the central Namib Desert ecosystem

Abstract: The Namib Desert is hyper-arid in terms of rainfall, but its ecology is influenced by frequent fog events. Fog utilisation by Namib biota has been well studied, but its role in nutrient deposition and cycling, particularly with respect to soil processes, still has open questions. Given its potential for distinguishing between various ecosystem components and fluxes, sulphur isotopic composition (δ34S) is evaluated here as a passive tracer of aerosol deposition and plant water sources in the Namib. Measurements of δ34S in Namib fog, groundwater, soils, plants and aerosols are presented and are consistent with the previously described system of sulphur cycling: primary marine sulphur accumulates as gypsum in the gravel plains and is redistributed by wind. Kuiseb River sediments had a wide range of δ34S values, with several samples that were quite depleted relative to soils, plants, groundwater and gypsum of the gravel plains. This depleted signal appears more commonly in the fine (0.5, 1.0 µm) rather than i...

Mary Seely, visionary scientist and dedicated teacher, turns 70

Abstract: ecology, physiology, geology, geomorphology, archaeology and sociology What, during this first phase of Mary’s career, were the keys to her success? It is clear that a convergence of fate, fortune and Mary’s own special brand of energy, focus and self-motivation, all worked in her favour The locality and research function of Gobabeb provided the perfect springboard to her career, together with the support of several colleagues and friends who helped to open doors, which she had no hesitation in entering! Much of the success of the Gobabeb research programme related to Mary’s commitment to facilitating other people’s research and becoming directly involved with research projects that were out of her own field, but that added to understanding of the Namib ecology Gobabeb became a place where all were welcome She has always worked tirelessly and without complaint, immersing herself fearlessly in the big issues, ever seeking out (and implementing) ways to do things better She will be heard saying, ‘Make use of what you have, then build on it’ She demonstrated this from the beginning, making minimal funds go a long way, while doggedly building up an increasingly viable and professional institution An empathic human being, she is quick to offer a helping hand whether on a personal level, or to facilitate study and career opportunities for her proteges ‘Out of the box’ thinking is Mary’s challenge to all, be they aspirant scientists, peers and colleagues, or young people who are ‘finding themselves in the desert’ Her favourite (and famous) comment on reviewing first drafts of publications is ‘So what?’, forcing the author to go back and find the ‘gee whiz’ aspects of the subject, and cut the waffle Mary’s characteristic focus on her goals, and her perseverance in meeting them against all odds, paid off at the time of Namibia’s independence in 1990 The second phase of her career began at this time, with a crisis: South African funding which had underpinned Gobabeb was withdrawn, and closure was imminent But Mary saw beyond Gobabeb, to the potential for applying science in the development arena Her establishment of the Desert Research Foundation of Namibia (DRFN) opened a gateway in Namibia that served to connect science to development, translating desert knowledge into policy, training and capacity, awareness and sustainable development She simultaneously activated Gobabeb as a site for integrated training, as well as maintaining its research and long-term monitoring functions, while dedicating enormous energy to securing funding for the research station Mary’s work pioneered desertification

Costs of dispersal

Abstract: Dispersal costs can be classified into energetic, time, risk and opportunity costs and may be levied directly or deferred during departure, transfer and settlement. They may equally be incurred during life stages before the actual dispersal event through investments in special morphologies. Because costs will eventually determine the performance of dispersing individuals and the evolution of dispersal, we here provide an extensive review on the different cost types that occur during dispersal in a wide array of organisms, ranging from micro-organisms to plants, invertebrates and vertebrates. In general, costs of transfer have been more widely documented in actively dispersing organisms, in contrast to a greater focus on costs during departure and settlement in plants and animals with a passive transfer phase. Costs related to the development of specific dispersal attributes appear to be much more prominent than previously accepted. Because costs induce trade-offs, they give rise to covariation between dispersal and other life-history traits at different scales of organismal organisation. The consequences of (i) the presence and magnitude of different costs during different phases of the dispersal process, and (ii) their internal organisation through covariation with other life-history traits, are synthesised with respect to potential consequences for species conservation and the need for development of a new generation of spatial simulation models.

Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones

Abstract: SUMMARY 1. Streams and their adjacent riparian zones are closely linked by reciprocal flows of invertebrate prey. We review characteristics of these prey subsidies and their strong direct and indirect effects on consumers and recipient food webs. 2. Fluxes of terrestrial invertebrates to streams can provide up to half the annual energy budget for drift-feeding fishes such as salmonids, despite the fact that input occurs principally in summer. Inputs appear highest from closed-canopy riparian zones with deciduous vegetation and vary markedly with invertebrate phenology and weather. Two field experiments that manipulated this prey subsidy showed that it affected both foraging and local abundance of stream fishes. 3. Emergence of adult insects from streams can constitute a substantial export of benthic production to riparian consumers such as birds, bats, lizards, and spiders, and contributes 25–100% of the energy or carbon to such species. Emergence typically peaks in early summer in the temperate zone, but also provides a low-level flux from autumn to spring in ice-free streams. This flux varies with in-stream productivity, and declines exponentially with distance from the stream edge. Some predators aggregate near streams and forage on these prey during periods of peak emergence, whereas others rely on the lower subsidy from autumn through spring when terrestrial prey are scarce. Several field experiments that manipulated this subsidy showed that it affected the short-term behaviour, growth, and abundance of terrestrial consumers. 4. Reciprocal prey subsidies also have important indirect effects on both stream and riparian food webs. Theory predicts that allochthonous prey should increase density of subsidised predators, thereby increasing predation on in situ prey and causing a negative indirect effect via apparent competition. However, short-term experiments have produced either positive or negative indirect effects. These contrasting results may be due to characteristics of the subsidies and individual consumers, but could also result from differences in experimental designs. 5. New study approaches are needed to better determine the direct and indirect effects of reciprocal prey subsidies. Experiments coupled with comparative research will be required to measure their effects on individual consumer fitness and population demographics. Future work should investigate whether reciprocal prey fluxes stabilise linked stream–riparian ecosystems, explore how landscape context affects the magnitude and importance of subsidies, and determine how impacts of human disturbance can propagate between streams and riparian zones via these trophic linkages. Study of these

Human longevity at the cost of reproductive success.

Abstract: The disposable soma theory on the evolution of ageing states that longevity requires investments in somatic maintenance that reduce the resources available for reproduction. Experiments in Drosophila melanogaster indicate that trade-offs of this kind exist in non-human species. We have determined the interrelationship between longevity and reproductive success in Homo sapiens using a historical data set from the British aristocracy. The number of progeny was small when women died at an early age, increased with the age of death, reaching a plateau through the sixth, seventh and eighth decades of life, but decreased again in women who died at an age of 80 years or over. Age at first childbirth was lowest in women who died early and highest for women who died at the oldest ages. When account was taken only of women who had reached menopause, who were aged 60 years and over, female longevity was negatively correlated with number of progeny and positively correlated with age at first childbirth. The findings show that human life histories involve a trade-off between longevity and reproduction.