Trends in natural resource use and prospects for sustainable resource management in Kenya's Arid and Semiarid Lands

This paper describes the environmental problems facing the development of Kenya's Arid and Semiarid lands (ASAL) today. The problems arise because of a rapidly declining resource base in the ASAL as a result of overexploitation and misuse, following basic land use conflicts. The extent of the overutilization or resource damage is not well known and varies a great deal from one agroecological zone to another and from resource to resource in the ASAL; those most affected include: soil, water, range, forest, livestock and wildlife. As a result of increased population growth in the ASAL, these resources are under stress. This paper argues that many of the environmental problems in the ASAL cannot be solved if the development programme does not address the needs of the people who live in the ASAL. There is a need to shift the objective of ASAL development from a preoccupation with economic growth and the idea that the ASAL should make a contribution to GNP through interregional trade and export to sustainable development that enables the communities in the ASAL to support themselves at as good a standard of living as possible. The last part of this paper outlines a strategy to promote such sustainable development.     

Gully and sheet erosion on subtropical mountain slopes: Their respective roles and the scale effect

As most mountains in tropical and subtropical zones, the Western Sierra Madre suffers active present erosion, which may create some constraints to the social and economic development in the area.The objectives of this study of soil degradation in the Western Sierra Madre, are to determine the respective roles of gully and sheet erosion. This research is based on field observations, field measurements of runoff and, soil losses at the plot, as well as the watershed scales as an analysis of an exhaustive census of the few gullies located in an experimental area.Measured soil losses in the Western Sierra Madre are high although there are few gullies. Most of the sediment yield seems to originate in widespread degraded areas where stoniness is the main evidence of a previous stage of erosion. Previously overgrazing and deforestation were determined as the factors of the appearance of new soil surface characteristics which explain the high runoff and sediment productions. The soil compacted by cattle trampling reduces infiltration. The decrease of the vegetation cover triggers a rise in the splash effect and thus, a soil sealing.These processes induce an increase in runoff and soil losses. The main erosion type has been described as sheet erosion: it is characterised by the removal of fine soil particles and the remains of gravels, pebbles and blocks, which constitute a pavement on the soil. Gullies generally appear on the bottom of wide valleys and depressions, where soils are thick. It is shown that sheet erosion is two orders of magnitude higher than gully erosion at the hillslope scale.Due to the spatial distribution of land use and the geological context such as the heavily degraded areas close to the main rivers, the reduction of runoff and soil loss rates within the extension of a considered area, commonly observed in hydrology, only applies up to the elementary catchments scale (1 to 50 km²). Above this area, runoff coefficient and soil loss rates increase.     

Identifying the impacts of chronic anthropogenic disturbance on two threatened cacti to provide guidelines for population-dynamics restoration

Species frequently have to be preserved in disturbed areas. Restoring their predisturbance population dynamics may increase their population growth rate (λ) while preventing the development of vulnerable genetic- or age-structures in the population. We assessed how population dynamics have been affected by anthropogenic disturbance in the threatened cacti Mammillaria dixanthocentron and Mammillaria hernandezii. We worked in areas exposed to different intensities of disturbance, and applied a retrospective analysis to detect which of the vital rates affected by disturbance were responsible for reductions in λ. M. dixanthocentron had lower λs in more disturbed conditions as a consequence of the reduced survival of adults because of livestock trampling and increased predation by hares. M. hernandezii had significantly higher λs with intermediate disturbance. The reduction in λ under intense disturbance resulted from reduced reproduction and adult plant mortality. The death of these individuals seemingly was associated to the change in soil conditions due to overgrazing. However, the factor with the greatest influence over both species was temporal variability, which interacted with disturbance sometimes increasing its negative effects. The identification of hypothetical mechanisms reducing λ was facilitated by the retrospective analysis, which is then a valuable heuristic tool guiding research. As long as they are not properly tested, these hypotheses should be considered carefully in management plans, but they may provide timely information. Retrospective analysis allows taking initial conservation measures, while suggesting guidelines for future research that may improve such measures.     

The causes of soil alkalinization in the Songnen Plain of Northeast China

The causes of soil alkalinization in the Songnen Plain of Northeast China were mainly analyzed from two aspects, natural and anthropogenic. Natural factors of alkalinization are parent materials, topographic positions, freeze-thaw action, wind conveyance, water properties and semi-arid/sub-humid climate. Some of them were always being neglected, such as freeze-thaw action and wind conveyance. Anthropogenic causes are mainly population pressure, overgrazing and improper agricultural and economic policies. In recent decades, overgrazing played a main role in secondary soil alkalinization, which led to the decline of Leymus chinensis grasslands. Now, the alkalinization is very severe, and more than 3.2 × 10⁶ ha area has been affected by salt, which becomes one of the three largest sodic–saline areas in the world.     

The measurement of chronic disturbance and its effects on the threatened cactus Mammillaria pectinifera

Measuring how anthropogenic disturbance affects biological systems is crucial for conservation and management. However, it is often difficult to quantify disturbance when it occurs in a slow, long-lasting - or chronic - fashion. Because various human activities influence gradually an area, chronic disturbance must be measured on a continuous scale that accounts for different sources of disturbance. Here we propose a method to develop multimetric indices for chronic disturbance. The approach is exemplified by considering the effects of disturbance on the threatened cactus Mammillaria pectinifera. Fourteen indicators of three agents of disturbance (human activities, livestock raising and land degradation) were measured in 10 populations of M. pectinifera, and summarised through principal components analysis (PCA). An index for each agent was also developed. M. pectinifera achieved maximum density at intermediate values of the first PCA axis, which was related to disturbance intensity. Assessing only both extremes of the disturbance gradient, where density is low, would erroneously suggest that disturbance has no effect on this plant. The different disturbance agents act synergistically on M. pectinifera, and their combined effects are detrimental. Land degradation reduces plant density, while livestock enhances it. However, overgrazing promotes degradation, so maintaining appropriate livestock levels is critical for management. Our method allowed us to identify which agents have more impact on threatened species, and sets the basis to manage disturbance agents in a way compatible with conservation. The method proposed here can be easily modified for its use in other environments and to account for different forms of anthropogenic disturbance.     

Overgrazing effects on vegetation cover and properties of volcanic ash soil in the páramo of Llangahua and La Esperanza (Tungurahua, Ecuador)

Abstract. The páramo is a high-altitude ecosystem of the northern Andes. The vegetation is continuous, with grasses as the dominant groundcover. Because of their high water retention, páramos play a fundamental role in water availability for all the population of the inter-Andean valleys. There are many studies of this specific ecosystem, but very few are focussed on overgrazing and its effect on vegetation and soil properties. Intensive grazing started less than 20 years ago and was studied in a representative area in the western Cordillera of central Ecuador covered by recent volcanic ash deposits. Intensive sheep grazing has led to a strong decline in the number of plant species, the replacement of the tussock grass vegetation by a short carpet grass vegetation, and an increase of bare land. In that area, the upper 50 cm of Andisols are deeply affected by a convergent decrease of Al and Fe oxalate and pyrophosphate in soil extracts, carbon contents decrease from 100 g kg^–1 to less than 50 g kg^–1 in the humid zone, from 70 to 40 g kg^–1 in the dry zone and a reduction of water-retention capacity at −33 kPa matrix potential from 800 g kg^–1 to 200 g kg^–1 in humid zones, from 350 to less than 100 g kg^–1 in drier areas. They showed also a decrease in the macrostructure and the development of a highly water repellent microstructure. All these important transformations favour the development of aeolian erosion in dry areas, runoff on bare surfaces, and gully erosion on slopes. The role of the páramo in water-regulation of this ecosystem seems to have been adversely affected for the future.     

Temporal variation of soil organic matter content and potential determinants in Tibet, China

Understanding temporal variations in soil organic matter (SOM) is important because SOM improves soil, sustains soil productivity, and sequesters carbon. The Tibetan Plateau plays a critical role in climate change at both regional and global scales. Thus, the main objective of this study was to investigate temporal variations in SOM content in Tibetan soils as an important indicator of climate change. SOM content data were obtained from soil surveys conducted in the 1980s and resampled in the 2000s. In this study, SOM content data from the A horizon of the soil profile were compared over time, and a paired t-test was conducted to verify differences between the two datasets. The results reveal that SOM content in the A horizon decreased significantly. To determine the causes of SOM content decreases in the A horizon, soil samples were grouped into ten soil types, four land-use types, and seven physiogeographical units. SOM content in four soil types (felty soils, cambisols in the World Reference Base (WRB) for soil resources; cold calcic soils, cambisols in the WRB; bog soils, gleysols in the WRB; and meadow soils, umbric gleysols/haplic phaeozem in the WRB), pasture, and broad valleys and basins of the south Tibetan zone of montane shrubby steppe decreased significantly in the A horizon. Based on the analysis of some known realities, this change should be attributed to pasture degradation resulting from overgrazing and climate warming.     

Trillium grandiflorum height is an indicator of white-tailed deer density at local and regional scales

High densities of white-tailed deer (Odocoileus virginianus) in the U.S.A. and Canada are reducing or preventing forest regeneration, and as a result, deer function as a keystone species in some sites. Management decisions about deer require reliable population density data, which are challenging to obtain at both regional and local scales. We tested the broad-scale applicability of the indicator species approach in which forage plant height was used to estimate deer density. The maximum heights of marked and unmarked plants of the widely distributed, spring-flowering polycarpic herb, white trillium (Trillium grandiflorum) were measured across southern Ontario, Canada, over a 15-year period. A significant negative relationship was found at 10 sites between maximum plant height and estimates of deer population densities, which were derived from counts of live or culled animals and varied from 7 to 40 deers km⁻². Maximum plant height could be reliably measured within a 4-week period. The underlying mechanism driving the negative relationship between plant height and deer density was attributed to deer preferentially selecting taller plants that grew less than ungrazed plants in the subsequent year. In 16 additional sites with locally high deer populations, the mean maximum height of T. grandiflorum appeared to be a more reliable indicator of deer density than estimates based on hunter returns across the broader regional scale of the Wildlife Management Unit. The ability to assess local scale white-tailed deer densities based on measuring heights of an easily identifiable, widespread plant, provides local residents and landowners with a tool for estimating the potential impacts of deer browsing and grazing in local woodlots and forests, improving local knowledge about herbivory pressure.     

放牧对草地生态经济价值的影响——以新疆阿尔泰山两河源自然保护区为例

本文以新疆阿尔泰山两河源自然保护区为研究对象,研究库尔木图不同放牧情况下草地生态服务经济价值的变化过程。结果显示:未放牧条件下,库尔木图草地生态服务价值为0.93亿元。2015年放牧条件下,研究区载畜量为120434.5羊单位,接近理论载畜量;当前库尔木图放牧情况下草地生态服务价值是未放牧的159%,说明适度放牧可以增加草地整体的生态服务价值,对研究区有重要意义。放牧强度为120%,140%,160%和180%时,草地生态服务价值分别为理论载畜量下生态服务价值的0.911,0.847,0.8,0.732,表明超载会造成生态服务价值下降。该研究为库尔木图自然保护区和其他放牧地区科学放牧提供依据。     

Effects of livestock grazing on pollination on a steppe in eastern Mongolia

Widespread degradation of Mongolian grasslands by overgrazing is of global concern. The objective of this study was to reveal the effects of grazing on pollination as an example of interaction biodiversity in Mongolian grasslands. We established three plots according to grazing intensity on the eastern steppe of Mongolia. In each plot, we recorded the numbers of insect-pollinated plants and observed the foraging behavior of pollinators in June and August. The richness of insect-pollinated species was high and these species were most abundant in lightly grazed plots, and formed complex relations with diverse pollinators. But, frequency of flower visitation and pollination index were greater in heavily grazed plots. All pollination properties were poorest in intermediately grazed plots. These results suggest that the forb-biased foraging of sheep and goats reduces the floral diversity of insect-pollinated species, and consequently reduces pollinators in the intermediately grazed plots. In the heavily grazed plots, only limited ruderal species could survive under heavy cattle grazing, and such simple vegetation formed unbalanced but strong bonds with pollinators. Removal simulation showed that the mutual network was more fragile with respect to the extinction of certain species.