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

agravel plain (GP), three dune habitats [an interdune plain (IP), a dune slope (DS) and a dune slipface (SF)], and in the Kuiseb River riparian corridor (RE) separating the dune fields from the gravel plain. Beetles were identified to species in the field or laboratory, then released near the capture site. Monitoring was continuous in the relatively stable substrates of RE, GP and IP, but intermittent in the mobile DS and SF habitats. Traps were maintained at their original location throughout the study. The slipface disappeared in 1998, affecting subsequent SF data. In the slipface, avalanching sand often filled traps, necessitating hourly processing. The contents of traps were counted and the traps emptied twice daily on the DS, less frequently at IP and GP. Pit traps in the RE were maintained for 19 years including the four comparison years, 1992–95. We analysed data for 27 focal species from desert habitats, defined as each species captured during the first three study years and at least 120 times during 25 years. There were five to 11 focal species per habitat. Some species were sufficiently abundant to qualify as focal species in two or more habitats but we confine our analysis here to the demographic response of species in the habitat where they were most common. These desert species were compared with 12 species from the ephemeral Kuiseb River. Results

http://reference.sabinet.co.za/webx/access/electronic_journals/sajsci/sajsci_v101_n11_a22.pdf

Long-term data show behavioural fog collection adaptations determine Namib Desert beetle abundance : research letter

Plant growth forms likely respond differently to disturbances such as trampling. We investigated the trampling effect of 1 600 sheep encamped at night in temporary enclosures (kraals, corrals or pe...

Effects of short-term intensive trampling on Karoo vegetation

Namib Ecological Restoration and Monitoring Unit, Gobabeb Research and Training Centre, Walvis Bay, Namibia Arid Lands Node, South African Environmental Observation Network, Kimberley, South Africa Centre for Environmental Management, University of the Free State, Bloemfontein, South Africa Safety Health and Environmental Risks, Swakop Uranium, Swakopmund, Namibia Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey Geochemistry, S.S. Papadopulos & Associates, Inc., Bethesda, Maryland Correspondence Joh R. Henschel, South African Environmental Observation Network, PO Box 11040 Hadison Park, Kimberley 8306, South Africa. Email: joh.henschel@saeon.ac.za

Roots point to water sources of Welwitschia mirabilis in a hyperarid desert

Tierberg-LTER is a research site established in 1987 in the semi-arid Succulent Karoo near Prince Albert in the Western Cape province, South Africa. The original purpose for its establishment was t...

Reflections, applications and future directions of Long-Term Ecological Research at Tierberg§

CITATION: Krug, C. B., et al. 2006. North-south cooperation through BIOTA : an interdisciplinary monitoring programme in arid and semi-arid southern Africa. South African Journal of Science, 102(5-6):187-190.

Joh R. Henschel

Papers

Long-term data show behavioural fog collection adaptations determine Namib Desert beetle abundance : research letter

Effects of short-term intensive trampling on Karoo vegetation

Roots point to water sources of Welwitschia mirabilis in a hyperarid desert

Reflections, applications and future directions of Long-Term Ecological Research at Tierberg§

North-South cooperation through BIOTA: an interdisciplinary monitoring programme in arid and semi-arid southern Africa