Ragnhild Mobæk defended her PhD thesis on January 27th 2012

Ane Gro Siri Skjelfjord

Density dependent foraging ecology and performance of domestic sheep on alpine ranges

Abstract from the thesis - PhD thesis 2012:02

Author: Ragnhild Mobæk

Populations of large herbivores are strongly influenced by density dependent and independent processes such as climatic variability, which have common effects on individual performance and demographic rates. However, little is known about proximate mechanisms involved in these processes, such as how foraging behavior vary as a function of population density. Whether density-climate interactions during summer may affect ungulate performance is also little explored. Domestic sheep is the most abundant large herbivore in alpine pastures during the Norwegian summer grazing season. Increasing numbers of sheep foraging in many alpine areas have been heavily debated. Despite this, knowledge about their foraging ecology and thus their impact on alpine ecosystems is scarce. In a landscape scale experiment, I collected long term data from sheep kept at two densities and tested whether density dependent foraging patterns, alone or in interaction with climate, could explain annual variation in autumn body mass by lambs. I predicted an inclusion of habitats and plant species with lower quality at high density. Sheep generally selected high productive meadow vegetation. The most selected plants were the grass Avenella flexuosa, herbs, and Salix spp. Both habitat selection and diet choice was density dependent. Ewes at high density selected less meadow vegetation than ewes at low density and consumed a diet of average lower quality. Further, activity patterns differed depending on density, and ewes at high density spent more time grazing. I documented a negative effect of high sheep density on individual performance. Contrasting foraging patterns at the two density levels signify that reduced abundance of high quality forage plants due to food competition is the main mechanism promoting density dependence in this system. Temporal variation also affected selection and performance patterns, and especially diet choice was strongly influenced by seasonal and annual variation in plant phenology. I found limited evidence of interactions between density and temporal variation, except that density dependent performance patterns varied between years, likely reflecting annual variation in diet composition. A long term temporal trend (nine years) in body mass development indicated that processes related to grazing effects, rather than annual climatic variation, was most important for sheep's performance pattern. Sheep at both densities likely altered their foraging habitat with long term opposite consequences for performance. This was most evident at high density, and the performance pattern suggests that lamb growth were restricted by delayed vegetation responses to high grazing pressure. At low density, lamb body mass tended to increase over years. This could be indicative of a grazing facilitation within the most selected meadow vegetation types. In conclusion, density dependent foraging patterns affecting performance as documented in this study may provide one behavioural mechanism explaining density dependent variation in vital rates.

Updated: 01.02.12
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