Human Impact on Deserts

As all ecosystems with low productivity, deserts are fragile to disturbance. Some ecologists go as far as to state that no direct succession occurs at all after disturbance but it is at least obvious that regeneration times after perturbations can be very long. The few long-term studies following disturbance, as for instance the vegetation recovery of ghost towns in the American West, demonstrate these long recovery times that often exceed many decades. It can be generalized that any human impact that changes the soil structure will last very long. Unlike in mesic environments, abandoned agricultural fields in deserts will recover only very slowly (if at all) to natural desert vegetation. Additionally, formerly irrigated fields will have elevated salt concentrations for long periods of time. Soil surface disturbance caused by off-road vehicles inflict severe changes in hydrological characteristics of soils, which might remain permanently. The increase in off-road vehicles in the North American deserts, and increasingly also in the Middle East, is a serious threat to deserts and desert biotas.

Desertification, largely the human-caused extension of the desert, is one the most serious problems facing the globe. Causes of the growth of the desert regions are multifaceted and are a combination of natural long-term variation in the weather, climate destabilization, and human mismanagement due to overpopulation and land-use change. Under the UN Convention to Combat Desertification, desertification is defined as land degradation in arid, semiarid, and dry subhumid areas resulting from various factors, including climatic variations and human activities. The effects of desertification promote poverty among rural people, and by placing stronger pressure on natural resources, such poverty tends to reinforce existing trends toward desertification. It is now clear that in several regions, desert environments are expanding. This process includes general land degradation in arid, semiarid, and also in dry and subhumid areas. Clearly in areas where the vegetation is already under stress from natural or anthropogenic factors, periods of drier-than-average weather may cause degradation of the vegetation. If such pressures are maintained, soil loss and irreversible change in the ecosystem may ensue, so that areas that were formerly savanna or scrubland vegetation are reduced to human-made desert. To counter this process that will increasingly endanger lives and livelihood of millions of people (not to speak of drastic effects on the biodiversity of the planet), synoptic management approaches are needed that combine understanding of the process and investigation into the regional causes of the process, in order to comprehend the effects on the Earth's overall system. It is important to emphasize that desert border areas that undergo desertification will not simply convert into natural deserts. Disturbed and overused semiarid zones are characterized by lower biodiversity than original, natural deserts. Therefore desertification will not simply increase the global area of deserts; it will create large tracts of devastated lands.

Human activity and human-caused climate change will facilitate the migration of ruderal (disturbance-adapted) plant species into locally favorable microsites within the desert. This has been shown for the vegetation along roadsides in the Middle East and in the North American southwest. Even though this might enhance local, small-scale species richness, an overall reduction in regional diversity and a loss of desert-adapted species might follow. Such a strong mixing of former distinct biotic zones has been observed along the edges of deserts in the context of human-caused disturbances and climate change. A wide variety of 'extrazonal' plants are crossing zonal borderlines, a process that will potentially lead to a marked decrease in large-scale species diversity. This migration by species that are native to the general geographic area but are now spreading into new climatic or biogeographic zones is an overlooked aspect of species invasion.

Due to typically strong abiotic stress, desert areas have been in the past remarkably resistant to invasions by non-native organisms. Notable exceptions have been biological invasions by deliberately introduced organisms in Australian deserts (e.g., rabbits and Opuntia species). However, invasion seems to increase rapidly worldwide and many desert areas today show a dramatic increase in the arrival and spread of non-native species. At present, the deserts of the American South West seem to be affected most. Plants originated from the Old World, mostly grasses (e.g., annual Bromus species, some perennial grasses), but increasingly members of other plant families also have invaded many desert communities and can have strong impacts on native desert communities. Among the detrimental effects are dramatic changes in fire regimes and direct competition with recruiting shrub seedlings and native annual plants, and even negative effects on adult desert perennials have been demonstrated. The main reason for these trends is due to general land-use changes in desert and desert margins. In the Southwestern US, disturbances due to increasing suburbanization of deserts, besides increases in nutrient depositions, seem to be central agents of these changes.

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