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.

Costs of dispersal

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

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

Nematode Parasites of Vertebrates: Their Development and Transmission

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.

Human longevity at the cost of reproductive success.

Robertsʾ birds of southern Africa

Three decades ago a locust conference in Kimberley deliberated on the Brown Locust Locustana pardalina (Walker), its biology, feeding ecology, population dynamics, outbreak extent and frequency, swarm movements and artificial control of outbreaks. The current paper evaluates recommendations made at this conference. In contrast to the high levels of effort and funding allocated for over a century to the control of locust swarms by means of insecticides, relatively little has been spent on research of the Brown Locust, not even to verify and characterise the actual problem. Methods of locust control have improved and environmental impacts of current applications are somewhat reduced. A critical, comprehensive cost-benefit analysis, necessary to evaluate whether costly control measures and continuous interventions are warranted, has still not been conducted. Many other knowledge gaps remain, such as the ecological significance of locusts as one of the remaining swarming phenomena of the Karoo. Swarms of locu...

Locust times – monitoring populations and outbreak controls in relation to Karoo natural capital

Impacts of livestock farming include effects on arthropods and a range of related ecological processes. Grazing by sheep may indirectly affect web spider populations by changing vegetation structur...

Web spider abundance is affected by sheep farming in the Karoo

The Karoo is an arid to semi-arid area across the western third of South Africa, comprising the Succulent Karoo and Nama-Karoo biomes. Its environment and people have experienced considerable chang...

https://www.tandfonline.com/doi/pdf/10.2989/10220119.2018.1535214

Introduction to the Karoo Special Issue: Trajectories of Change in the Anthropocene§

1. Circular bare patches occur in high numbers among the vegetation of the Namib Desert margin. There is an ongoing scientific debate on the origin of these so‐called “fairy circles” (FCs). One of the most frequently discussed hypotheses regards the bare patches to be the result of localised herbivory by sand termites of the genus Psammotermes (family Rhinotermitidae).

https://onlinelibrary.wiley.com/doi/pdf/10.1111/een.12996

Largest on earth: Discovery of a new type of fairy circle in Angola supports a termite origin

The dynamics of a small linear dune on the northern margin of the Namib Sand Sea have been monitored using erosion pins placed at the dune tip since 1969. GPS measurements of these pins enabled estimation of the rates of advance and lateral migration of the dune. The average rate of advance of the dune tip over the period 1969–2012 was 1.99 m yr–1 towards 015°. Rates of advance and lateral movement varied over the period of monitoring, with a decrease in rates of advance by a factor of 50%, but an increase in the rate of lateral movement. Changes in dune behavior appear to be related to changes in wind regime and the vegetation cover of the interdune area, as a result of increased rainfall in recent years. This study demonstrates the dynamic nature of the tip of this dune and its sensitivity to changes in winds and sand supply.

Joh R. Henschel

Papers

Locust times – monitoring populations and outbreak controls in relation to Karoo natural capital

Web spider abundance is affected by sheep farming in the Karoo

Introduction to the Karoo Special Issue: Trajectories of Change in the Anthropocene§

Largest on earth: Discovery of a new type of fairy circle in Angola supports a termite origin

Helga's Dune: 40 years of dune dynamics in the Namib Desert