| Retzer, V: Carrying capacity and forage competition between livestock and a small mammal, the Mongolian Pika (Ochotona pallasi), in a non-equilibrium ecosystem, South-Gobi, Mongolia, Wissenschaft in Dissertationen 762, Görich & Weiershäuser Verlag, Marburg, 272 S. (2004) | |
| Abstract: In Mongolia nomadic pastoralism has a long tradition and is of great economic importance. During the transition from socialism to a free-market economy since 1990 livestock was privatized and livestock numbers have risen dramatically. The carrying capacity for livestock and the impact of potential pests on forage availability therefore are of immediate interest. Against this background this study presents the results of a full-year empirical field study on the forage competition between pika and livestock, on the altitudinal distribution of domestic and wild large herbivores and the consequences for nature conservation, and on the impact of pika in the mountain-steppe. The findings of these field studies were then integrated into a model of forage competition between pika and livestock. The model was used to determine the quality of the understanding of most important ecosystem processes and to generalize the findings. 1. Forage competition between pika and livestock Generally we have to conclude that there indeed is potential for forage competition between pika and livestock. Both groups overlap in habitat and forage plants, and forage is not always sufficient to fulfill the needs of both groups. In a year of drought, such as 2001, forage is scarce and competition can be observed. The results of an exclosure experiment show that in 2001 above-ground net primary productivity was 184 kg/ha. Of this 107-143 kg/ha were consumed by livestock on treatment only livestock, 148-167 kg/ha by pika on treatment only pika, and 150-172 kg/ha on the treatment accessible to both groups. Pika consume a higher percentage of the vegetation than livestock does. Pika also dominate the pattern of consumption on the area accessible to both groups of herbivores. Therefore pika are competitively superior to livestock: probably due to their smaller size they can access forage which is out of reach for livestock. Nevertheless both groups can coexist, as they have access to mutually exclusive forage resources. For pika this is the forage below the biting height of livestock and livestock can reach forage on pastures far away by migrating. 2. The special situation of the mountains The mountains in the Gobi Gurvan Sayhan National Park are the habitat of a number of endangered animals. The mountains are humid islands in an arid environment and therefore provide higher amounts of phytomass than their surroundings. Along the whole transect from the pediment at 2000 m to the mountain top at 2800 m the impact of livestock is much higher than that of wild large herbivores. A clear altitudinal zonation of wild and domestic animals can be observed. Horses are the dominant species along the whole transect (40-52 %), camels and gazelles are preferably found on the pediment (2000-2350 m), while herds of sheep and goats also mainly graze on the pediment but occasionally reach an altitude of 2700 m in summer. Argali (wild sheep) concentrate in the lower mountainous areas and ibexes and yaks are mainly observed in the summit region. In a year of drought the large herbivores argali and ibex are under double stress. Apart from the low forage availability they additionally face increasing competition by livestock which is intruding into the mountains in search of forage. So on one hand the mountains provide an additional forage supply for herders' livestock. But on the other hand this contributes to the stress argali and ibex face. Any additional stress may be critical for the populations as the habitats of both species are isolated biogeographic islands. 3. The function of pika in the ecosystem Pika are found to be ecosystem engineers: by their digging activity and dung collection they enhance nutrient availability in the upper soil layer. Therefore the burrows show higher vegetation cover and a 2.7-times higher productivity than the surrounding steppe-matrix. Below-ground biomass is higher on the burrows, but the difference is not significant. Also important forage plants such as Agropyron cristatum start off earlier on the burrows and provide valuable first forage in spring for pika and livestock alike. Pika's potential as pest is low. Although their burrows a®ect considerable areas (7-12 %) and they compete for forage with livestock, the devastation caused by the animals is low and they are not likely to show mass population outbreaks. They have positive effects on primary productivity on their burrows which at least partially is used by livestock, too. Therefore they cannot be considered a pest. 4. Model simulation of carrying capacity and the impact of transformation The presented model is capable of simulating pika and livestock densities and forage competition between both groups as a reaction to variable precipitation input. The model results show that the drastic increase in livestock numbers after the transformation was a result of changing herding strategies. The sharp increase observed was supported by a series of years of above-average precipitation, but cannot be explained by this fact alone. Herders on average took a greater risk inthe 1990s by stocking the range higher than previously. This risky strategy took its toll in the years 1999-2002, when a large fraction of livestock starved. It can be concluded that the long-term carrying capacity has already been reached in the socialistic period. Distinctively higher stocking rates can only be achieved for short favorable periods, but cannot be maintained in the long-term. |
