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.

Long-term ecological research from an arid, variable, drought-prone environment

Abstract: Long-term ecological research (LTER) is invaluable for understanding environmental processes, particularly variability and drought, in arid countries and as support for effective natural resource management in the drylands. The Gobabeb Training and Research Centre undertakes various long-term monitoring focused on, inter alia, climate and the biophysical environment. Examples are given of the results of this LTER and how they could be used to serve social and environmental functions in the drylands.

Effects of extreme climatic events on small-scale spatial patterns: a 20-year study of the distribution of a desert spider

Abstract: Individuals of most animal species are non-randomly distributed in space. Extreme climatic events are often ignored as potential drivers of distribution patterns, and the role of such events is difficult to assess. Seothyra henscheli (Araneae, Eresidae) is a sedentary spider found in the Namib dunes in Namibia. The spider constructs a sticky-edged silk web on the sand surface, connected to a vertical, silk-lined burrow. Above-ground web structures can be damaged by strong winds or heavy rainfall, and during dispersal spiders are susceptible to environmental extremes. Locations of burrows were mapped in three field sites in 16 out of 20 years from 1987 to 2007, and these grid-based data were used to identify the relationship between spatial patterns, climatic extremes and sampling year. According to Morisita's index, individuals had an aggregated distribution in most years and field sites, and Geary's C suggests clustering up to scales of 2 m. Individuals were more aggregated in years with high maximum wind speed and low annual precipitation. Our results suggest that clustering is a temporally stable property of populations that holds even under fluctuating burrow densities. Climatic extremes, however, affect the intensity of clustering behaviour: individuals seem to be better protected in field sites with many conspecific neighbours. We suggest that burrow-site selection is driven at least partly by conspecific cuing, and this behaviour may protect populations from collapse during extreme climatic events.

Does territorial behaviour in the desert-living spider Leucorchestris arenicola Lawrence (Aranae: Sparassidae) affect its spatial distribution?

Abstract: The endemic sparassid spider Leucorchestris arenicola Lawrence builds silk-lined burrows in low-lying sections of dunes in the Namib Desert. Individuals defend certain areas around the burrow against conspecifics. The hypothesis was examined whether social spacing through territorial behaviour creates regular spacing patterns. A population of L. arenicola was monitored for four months during the reproductive season, when all inhabited burrows inside a 3,300 m area were recorded. Spatial patterns were analysed in terms of their degree of randomness or aggregation. The observed patterns between burrows did not fit a regular distribution, but early in the season distribution was random, followed by aggregated distribution at distances greater than 3 m, and later reverting to random again. This aggregation is explained in terms of habitat characteristics and population structure.

Identifying the determinants of tree distributions along a large ephemeral river

Abstract: Although ephemeral rivers act as linear oases and play a fundamental role in sustaining regional biodiversity in dryland regions, little is known about these systems or their sensitivity to human impacts. Without such knowledge, it is difficult to manage or conserve them. Here, we conduct the first systematic investigation into the determinants of riparian tree distributions along a large ephemeral river. Adopting a macroecological approach, we test four hypotheses relating to the effects of topography, river flow, climate, and land tenure on three indices of tree distribution: species richness, occupancy, and recruitment. We also consider the effect of upstream damming. Our study site is the Swakop River in Namibia. The most common trees along the river were the invasive Prosopis spp., followed by native Faidherbia albida, Vachellia erioloba, Euclea pseudebenus, and Vachellia tortilis. We found a gradient in tree distributions along the river, with a drier climate westward associated with lower native tree species richness and increased scarcity of the dominant native species (F. albida). These patterns were seen in both pre‐ and post‐dam samples. We also found F. albida was more likely to recruit immediately downstream of tributaries. Our results suggest that water availability (climate and river flow) is a more important determination of tree distribution along this ephemeral river than topography or land tenure and that ephemeral rivers may show a nodal organization.

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.