The dynamics of desert ecosystems control levels of resources that are essential to the survival of desert biotas. Because precipitation is both low and relatively unpredictable in arid regions, the climates, topographies, and soils, of these areas present formidable constraints to resource availability in space and time. And for the same reason, the processes of production, consumption, decomposition, and nutrient-cycling in deserts are also highly irregular and difficult to predict with accuracy. For example, global models relating actual evapotranspiration to primary production and decomposition apply poorly in arid regions.

Surprisingly great amounts of carbon are stored in desert soils, particularly in caliche deposits which represent a major ‘sink’ of carbon from the atmosphere. In Arizona desert soils, inorganic carbon exceeds organic carbon by a factor of > 10. Direct use of organic carbon is made principally by organisms that break down desert litter and simultaneously cause relatively high rates of nitrogen mineralization. While nitrogen is traditionally considered deficient in arid environments, its flux is considerable because of high rates of gain by fixation and loss by denitrification and volatilization. Nitrogen accumulates in ‘islands of fertility’ beneath desert shrubs where it becomes relatively available because of (i) its high concentration in plant litter, and (ii) reduced activity of any aromatic modifiers that retard decomposition.

It is misleading in deserts to relate nutrient availability to yearly averages, as nutrients may become highly available following pulses of ‘effective’ precipitation. Moreover, mineralization and subsequent availability to plants of phosphorous, the ‘master element’ in nutrient cycling, are moderately independent of nitrogen mineralization and can proceed rapidly. Clearly, the case for nutrient deficiency in deserts may be overstated.

Consumption of primary production has varying effects on rates of resource availability in desert ecosystems. Generally weak regulation of primary production is predicted for consumers of green vegetation, except occasionally during early drought. Carnivores should exert variable controls over their prey, while pollinators, seed-eaters, and detritivores—most of which are strongly soil-associated—should have the greatest impacts on primary production and nutrient cycling.

The effects of controlled motorcycle traffic on a Mojave Desert soil in California were studied in order to describe soil compaction quantitatively. Four experimental trails, representing 1, 10, 100, and 200, passes each with an off-road motorcycle were established in a loamy sand at 6.2% by weight moisture content. Penetration resistance, density, infiltration rate, and response to rainfall, were measured for the undisturbed soil and for the experimental trails immediately after the conclusion of the impact, and soil cores were measured in the laboratory to determine pore-size distributions. The soil density was remeasured again one year after the impact, to determine the amount of recovery.

Immediately after motorcycle use, the average density of the soil in the 0 to 60 mm depth increased from an average of 1.53 metric tons/cubic metre in the undisturbed soil to 1.77 t/m3 in the 200 pass trail, and the average density in all the trails could be expressed empirically as a logarithmic function of the number of motorcycle passes. The soil penetration resistance profile for the 200-passes-trail showed that the maximum compaction occurred between 30 and 60 mm depth, and that changes in penetration resistance could be measured to a depth of 0.21 to 0.25 m. The infiltration rate after 2 hours decreased from an average of 98 mm/hr in the undisturbed soil to 29 mm/hr in the 200 pass trail.

The undisturbed soil infiltrated artificial rainfall up to an intensity of 46 mm/hr before surface ponding occurred in the 100- and 200-passes-trails at intensities greater than 25 mm/hr. Decreases in the soil-pore volume were greatest in pores with effective radii greater than 150 μm, with incremental decreases occurring in pores with radii between 150 and 4.5 μm; the total volumes of smaller pores were similar in the compacted and undisturbed soils. One year after the impact, the density of the soil at a depth of 0–30 mm had decreased significantly in the 10-, 100-, and 200-passes-trails but the densities at 30–60 mm depth remained unchanged.

The utilization of tussock grasslands in the semi-arid and sub-humid tussock grasslands of Central Otago, South Island, New Zealand, by European sheep farmers, began during the 1850s. Many of the sheep farmers and their hired shepherds migrated from the Scottish Highlands, and brought with them the management practices with which they were familiar. The extensive sheep farms or ‘runs’ were rented on government-owned land, and the graziers did not enjoy long-term security of tenure; nor did they receive compensation for any improvements which they might have undertaken during their occupancy.

The combination of frequent burning of the vegetation and overgrazing soon resulted in a perceptible decrease in vegetation cover, such that land deterioration accelerated in the 1870s and 1880s when a rabbit pest irrupted. No effective action was taken to deal with the problem of land deterioration, and by the early part of the twentieth century parts of Central Otago were described as man-made desert. The tussock grassland had given way to almost bare soil interspersed with patches of Scabweed (Raoulia lutescens), and accelerated soil erosion resulted in slope-wash and gullying. Remedial action was delayed by the outbreak of the First World War, but a major Commission of Inquiry at the end of the war concluded that unsuitable forms of land tenure had contributed significantly to the desertification and land deterioration.

Eventually, changes in land tenure were introduced, providing for security of tenure, entitlement to compensation for land improvements, and controls on husbandry and stocking levels.

Maps are presented of the desertification status and degree of hazard in Falcon State, Venezuela, comprising 24,750 sq. km of land, and the criteria employed for their compilation are described.

The information used for the compilation of the maps was obtained from the results of a regional survey recently conducted by the Authors. Degree of slope, landform, and soil texture, were employed to assess the physical fragility of the different landscapes, which were expressed as classes of inherent vulnerability. The classes of desertification hazard were obtained from the combination of degrees of plant cover and of inherent vulnerability.

The results show that more than 42% of the area of Falcon State has already undergone deterioration, and that 41% is under severe or very severe desertification hazard. The subhumid climatic zone is more critical than the semiarid, both because of inherent vulnerability and of its human occupation pattern.

The aim of the present paper is to get Falcon State incorporated in the world inventory of land under desertification hazard, and to call public and governmental attention to the seriousness and extent of the desertification threat—in Venezuela and also in other countries that face similar problems, whether or not they realize their danger.

Reafforestation, reclamation to agriculture, urbanization, and a variety of industrial activities—such as mineral extraction and the laying of gas and oil pipelines—cause loss or severe degradation of heather moorland and lowland heathland in Britain. In recent years the expansion of the gas and oil industries has involved installation of many major pipelines, some of which traverse areas of heather moorland. Heathland vegetation is very sensitive to disturbance, and natural regeneration of its plant communities is relatively slow. Three detailed examples are given of severe disturbance caused by the laying of pipelines, where the original heathland vegetation has not recovered. There have been irreversible changes in the vegetation, to grassland with Juncus spp., and these have been associated with considerable changes in soil properties across the pipeline easement.

Improved techniques have been developed for reinstatement of heather moorland and lowland heathland after pipeline installation. The techniques are based on fundamental ecological principles and have been tested during the installation of a pipeline for Scottish Gas (across the Pentland Hills near Edinburgh) and during the installation of an oil pipeline in Dorset. In both cases recovery of the heathland vegetation was satisfactory. Based on these experiences, recommendations are given for the alleviation of problems associated with the laying of pipelines through heather moorland and lowland heathland.

Coastal habitat loss must be reduced either through conservation or mitigative efforts. Implementation of mitigation depends largely on accurate cost-assessment of the projects that are involved. We centre our discussion here on seagrass transplanting as an exemplary mitigative tool.

The technology of restoring seagrass communities has received increasing attention in recent years. However, the methods used have had varying degrees of success, a largely unknown factor being the cost of the technique that is used.

In this paper is presented a universal format that includes consideration of the essential factors in our costanalysis, such as planning, planting, and monitoring activities, geographic location, tidal influence, labour, and materials (both expendable and non-expendable). Cost per (successfully established) shoot or seed or fruit is recommended as the best indicator of cost effectiveness for a given technique. Incorporation and presentation of this information is urged in future projects to motivate wider application of seagrass and other essential plantcommunity restoration.