Doubtless , as understanding of the structure and function of the world's ecosystems increases , and as monitoring of their degree of degradation improves , new worries will arise .sx Since 1973 , the 'oil-crisis' has attracted attention , yet the potentially much more serious problem of soil-loss has generated far less interest in spite of the problems it caused in the mid-west of the USA in the 1930s ( Brown , 1977 :sx 18 ; 1978) .sx Different processes of degradation could act synergistically and some have a cumulative effect , so recognition of the degree of threat may not be easy .sx The management of land degradation is therefore an art , with a huge palette of responses or avoidance procedures , the application of which involves value judgements , wisdom and scientific skill .sx 2 Why is land degradation occurring ?sx CAUSE AND PROCESS .sx LAND DEGRADATION is commonly blamed on 'acts of God' or 'acts of the peasantry' , and there is typically little attempt to assess the real causes .sx Another approach is to argue that , as land degradation is widespread in environments exploited by Man , then human actions must be the main cause .sx A wide range of human activities can trigger or exacerbate land degradation ; there are also environments which are very vulnerable to degradation , and natural catastrophes from time to time degrade virtually all environments .sx Also , it would be rash to assume that human activity always has a negative effect on rate of erosion , species survival , etc. Sometimes the process of land degradation can be followed , but the cause(s ) may be obscure .sx Commonly , a chain-of-causation stretches away in space and/or time from the site where land degradation is manifest ( see Fig. 2.1 ) ( Eckholm , 1976 ; Darkoh , 1987 :sx 25 ; Blaikie & Brookfield , 1987 :sx 4) .sx Sometimes causes are local and relatively simple , sometimes land degradation results from , possibly complex , global changes some of which are at least partly caused by human activity .sx While the scale of global processes may be vast , they may be in a state of dynamic equilibrium , easily upset by human actions .sx Chadwick & Goodman ( 1975 :sx 4 ) recognized three types of bio-geochemical cycle :sx natural ; perturbed ( upset by Man ) and recycling , i.e. managed by Man to be sustainable .sx The second type is increasingly common , the last type is often elusive .sx Land is a 'stage' within , upon , or above , which , a number of resources may be exploited .sx Where there is exploitation of more than one resource , this may be mutually compatible , or there may be damaging interactions .sx Resources vary in character and some are more difficult to manage than others ( Ramade , 1984) .sx The following classification of types of resource is generally accepted and gives some indication of 'manageability' :sx 1 .sx Continuous resources include :sx solar energy , wind , gravity , tidal energy , geothermal energy .sx These continue to be available , and , with the possible exception of solar energy , the receipt of which could be affected by atmospheric pollution , cannot be degraded , even with gross mismanagement .sx 2 .sx Renewable resources ( flow resources ) include :sx clean water , flora , fauna , soil , clean air .sx Rees ( 1985 :sx 224 ) , defined renewable resources as " .sx .. those capable of natural regeneration into useful 'products' within a timespan relevant to man .sx " These resources are potentially renewable and could be indefinitely available , provided their capacity to regenerate is not damaged by natural catastrophe or human activities .sx Once degraded beyond a certain critical point , a renewable resource may never recover .sx table&caption .sx 3 .sx Non-renewable resources ( stock resources ) include :sx many minerals and some groundwaters .sx These are available only in finite quantities , or else the rate of renewal is so slow that they must be regarded as available only in fixed quantities .sx 4 .sx Extrinsic resources include :sx human skills , institutions , management abilities , etc. They can be fickle and prone to breakdown or degradation , yet can be continuous resources if well managed ( Riddell , 1981 :sx 23) .sx In categorization of resources , one should not lose sight of the fact that non-renewable resources and continuous resources are opposite ends of the same continuum and that the above classes overlap .sx The demand for a resource can vary because attitudes , tastes , willingness or ability of people to use or purchase the resource alter , or because a substitute has been found .sx Technological advances and altered world circumstances affect resource demand , people :sx " .sx .. live and act at a local level , but their alternatives and opportunities are shaped significantly by events at the regional , national and international levels " ( Dorner & El-Shafie , 1980 :sx 8) .sx caption&table .sx Land must be described as a non-renewable ( fixed stock ) resource , although it has a renewable capacity to support most forms of biological life ( Rees , 1985 :sx 224) .sx Exploitation of the world's land has been described by some as conforming to a " lollipop model " , in that , with each " lick" , there is less for the future ( Chisholm & Dumsday , 1987 :sx 354) .sx On the other hand , with good management , it might be possible to sustain indefinite usage of land , even improve its utility .sx Alternative ways of classifying Earth resources include :sx 1 .sx a division into - those that can be safely 'stretched' by Man ; .sx - those that can be safely 'stretched' only if carefully managed ; .sx - those that cannot/should not be 'stretched' .sx 2 .sx a division into - resources with actual value ; .sx - resources with option value ( possible use perceived ) ; .sx - resources with intrinsic value ( no obvious practical value , but there is a will to maintain it) .sx The processes of land and other environmental degradation are not adequately understood , so predictive models are mainly poorly developed ( Chisholm & Dumsday , 1987 :sx 322) .sx Table 2.1 groups various theses which seek to explain why land degradation takes place and Figure 2.1 shows causes or combinations of causes that may result in land degradation .sx Natural hazards as a cause of land degradation .sx There are few , if any , regions so blessed that they will not at some point experience a natural disaster .sx However , some environments are less stable than others or are more likely to suffer disruptions .sx Areas predisposed to disaster include :sx - steeply sloping areas ; .sx - easily damaged soils ; .sx - drylands and localities where soils drain fast ; .sx - lowlands close to the sea , particularly on exposed coasts and in areas prone to glacier 'calving' , submarine eruption or submarine mudslides ; .sx - regions where rainfall is intense ; .sx - drought-risk areas where rainfall is mainly due to airmass movements which can be fickle ( 'monsoon' rainfall ) ; .sx - parts of the Earth where hurricanes or similar storms occur ; .sx - areas prone to sudden frost or cold winds ; .sx - areas of earthquake or volcanic activity ; .sx - areas subject to periodic invasion by destructive insects .sx There is evidence of periodic natural catastrophes ( infrequent on the human time-scale , but not if judged by the geological time-scale ) which caused widespread and severe land degradation .sx A number of scientists argue that massive extinctions of organisms at the end of the Permian and Cretaceous Periods reflect such catastrophes .sx Huggett ( 1988 , 1989 ) provided a fascinating study of catastrophic tsunamis , arguing that huge waves have been quite common .sx The UK he suggested has been , and probably will be , struck every 0.8 to 1.4 million years by 13 to 130 m tsunamis .sx Worse , there are indications of waves several hundred metres in height caused in the past by meteorite/comet strike , eruptions or other Earth processes .sx There is evidence of climatic disruption , sufficient to seriously affect human affairs following the Hekla-3 volcanic eruption ( Iceland c. 1120 BC ) and the Santorini volcanic eruption ( Crete c. 1645 BC) .sx Storms , cyclones and hurricanes are common causes of land degradation .sx For example , in 1986 , Guadalcanal in the S. Pacific was devastated by Cyclone Namu , and in 1988 the Atlantic coastlands of Nicaragua were struck by Hurricane Joan destroying around 7000 km 2 of tropical forest ( The Times , 29/11/88 :sx 24) .sx In the short term , these events cause degradation , but , over the longer term , without such damage , the natural regeneration of some tropical forests would be upset , for there would be no clearings and less chance for fresh growth .sx The onset and/or severity of a natural disaster may owe something to human activity - Man may have 'triggered' the disaster or may have 'sensitized' the environment .sx If development has exceeded environmental limits , a natural disaster might speed up what may otherwise have been a virtually inevitable decline .sx Progress sometimes comes out of adversity - in reacting to a disaster Man may develop new strategies for using land or resources which may be less degrading ; indeed , much development has been spurred on by natural and human disasters ; notably scientific advances in times of war .sx The worst possible scenario is for Man to practise a land use which makes land liable to degradation in an area prone to disasters on land that is especially vulnerable .sx At the other extreme , there is good management of land that is not vulnerable in a region seldom subject to disasters .sx It should be possible to map natural hazard risk and so be prepared for , or to avoid , some disasters .sx Population change as a cause of land degradation .sx Population increase has been one of the most frequently cited causes of land degradation since Malthus drew attention to it .sx The Malthusian , or more recently Neo-Malthusian , view is that increasing demographic pressure results in overuse of reasonable quality land and/or the misuse of marginal , often easily degraded land .sx If population increase has that effect , the impact is double edged :sx a simultaneous increase in demand made upon the environment in order to support growing numbers of people , and a destruction of the resource base ( Clark & Munn , 1986 :sx 8-12) .sx There is a need to treat the argument that environmental degradation arises whenever population grows , even when it exceeds a region's 'carrying capacity' , with caution , for there are areas with large numbers of people and relatively little damage and there are areas with very few people , a short settlement history and much damage .sx In sensitive areas , or with certain types of exploitation , population need not be high to cause problems .sx Population stagnation or decline can also be a cause of land degradation .sx Globally , human population has shown three periods of relatively rapid increase , if a logarithmic plot is used , followed by periods of relative stability .sx Arithmetic plotting of the statistics has reinforced the idea that growth has been exponential .sx There are some who hope that there might be another period of reduced growth in the late twenty-first-century .sx Whether or not that happens , the world population will have risen above 6000 million by AD 2000 .sx The world today supports roughly three times the human population and roughly 100 times the industrial activity it did a century ago .sx Although they comprise only about 30% of the world's population , those in developed countries consume roughly five times as much food and commodities as people in developing countries and produce more industrial pollutants .sx A number of researchers have tried to establish where population has exceeded the limits of food production ( Higgins et al. , 1982 ; Mahar et al. , 1985) .sx However , the indications are that population - poverty , and possibly degradation , correlations can be spurious .sx People , on the whole , have children in response to their economic , political and historical situation - a simplistic Malthusian perspective misses this ( Hecht , 1985 :sx 665) .sx Redclift ( 1987 :sx 30 ) suggested that it is not so much net global population increase that matters , but the rate of change in population in critical regions .sx There has been debate on the relationship between population density , growth rate and agricultural development , particularly the intensity of farming ( Boser u p , 1965 ; Carlstein , 1982) .sx The relationship is by no means a simple one ; put crudely it has been claimed that , if a population does not grow enough , or grows too fast for agriculture to respond , then production is likely to remain extensive , and may cause land degradation .sx If population grows , but not too fast , then intensive agriculture , possibly causing less land degradation , may result .sx Marginalization as a cause of land degradation .sx People may become marginalized - forced or attracted onto poor quality , possibly easily degraded land , because better land is settled ; because of unrest ; because large landowners monopolize the better land , and possibly make poor use of it ; because of the creation of reserves .sx It is not always unrest or social differentiation which causes people to settle marginal land .sx The land may be available and people perceive it offers them a chance for a living or a profit ( Reining , 1978 :sx 75) .sx