Life history strategy and ecosystem impact of a small mammal herbivore in a mountain steppe
Pikas (genus Ochotona) were suspected to possibly behave as „small mammal pests" in the mountain steppes of the Gobi Gurvan Saikhan National Conservation Park (GGS) in the Mongolian Gobi Altai. The present study shows that the Mongolian pika (Ochotona pallasi pricei ), a subspecies of the Pal...
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|Zusammenfassung:||Pikas (genus Ochotona) were suspected to possibly behave as „small mammal pests" in the mountain steppes of the Gobi Gurvan Saikhan National Conservation Park (GGS) in the Mongolian Gobi Altai. The present study shows that the Mongolian pika (Ochotona pallasi pricei ), a subspecies of the Pallas pika, is the dominant small mammal species in the mountain steppes of the GGS. It outnumbered other small mammals by one order of magnitude and outweighed them by two of them. Data on capture and observation are available for three summers and one winter from a trapping grid of 100x100m2, including a summer of drought. Data based on observation was more reliable in terms of encounter success than data based on capture. Captured juveniles were discriminated from adults using information on the development of their weight. An average adult weight was estimated to range between 180 and 200 g. Possible scenarios for dynamics of population densities were simulated using a system of Leslie-matrices based on estimates for survival and reproduction of the observed individuals. Survival rates were estimated using maximum likelihood techniques with competing models for survival and recapture probabilities. The most parsimonious model included effects of population density, age, and sex on survival, while there was no effect of cohort affiliation nor climatic factors season and drought. Generally, adults showed higher survival rates than juveniles, females showed higher survival rates than males, and survival declined with density. Estimates for reproduction were based on the observations of litter number and size, resulting in a maximum of 13 juveniles per female pika. Simulated population densities were similar to the measured population densities, but did not reflect the effects of the year of drought. Population densities were measured using pooled data from capture and observation sessions. In the study period pika densities varied between 14.6 and 49.8 individuals per ha. Median density was 21.4 animals, which is less than the 28 burrows on the trapping site. Lowest densities were reached one year after the summer of drought, indicating a time lag of one year in the response to the drought conditions. Comparing reproductive effort, factors influencing survival rates, and density dynamics of the Mongolian pika with other species of the genus shows that this species exhibits traits of the group of non-burrowing pikas, which are closer to a K-type of life history strategy than the group of burrowing pikas. Density dependent survival indicates that burrow possession may be crucial for survival. Ecosystem impact of the pikas was assessed studying the productivity of the burrow habitat in comparison to steppe habitat together with the effect of grazing by pikas and by larger herbivores. Productivity and biomass removal was measured using exclosure plots on burrow and steppe habitat. Burrows showed higher productivity than steppe habitats when water availability was higher. The grass Agropyron cristatum profited most from the burrow habitat in terms of biomass and quality. This grass is an important fodder plant for livestock. An effect of livestock grazing was missed by the experiment. The productivity of the vegetation was controlled by the availability of moisture, not by pika grazing, although individual pikas removed more biomass, when it was available. This biomass is probably stored in the burrows. Pikas preferentially grazed Agropyron cristatum on burrow plots. Burrows were estimated to last 120 years at least. The studied system behaved according to the prediction based on the non-equilibrium theory of rangeland dynamics: plant productivity was controlled by climatic conditions, as was the density of the herbivores. However, although pika densities varied, there were upper and lower limits to this variation made possible by the territorial behaviour of the animals. Possession of territories together with harvesting plant material enables the species to mitigate climatic inter-annual variability. Pika burrow densities were controlled by altitude and thus probably by the longterm availability of plant biomass. Livestock densities had only a small effect on burrow densities. However, this effect changed at the pediment angle separating pediments from the mountain ranges, indicating a change of system behaviour. The present study shows that the Mongolian pika cannot be seen as „small mammal pest" species, since its densities are controlled by the availability of burrows and it has a positive influence on pasture quality.|