Small-Scale Structures and Grazing Intensity in Semi-Natural Pastures - Eff ects on Plants and Insects

Pihlgren, A. 2007. Small-scale structures and grazing intensity in semi-natural pastures effects on plants and insects. Doctor’s dissertation. ISSN 1652-6880, ISBN 91-576-7312-1. Semi-natural grasslands characterized by nutrient poor soils are among the most species-rich habitats in Europe and they harbour many red-listed species. The area of semi-natural grasslands has decreased drastically due to intensifi ed land use or abandonment of farms. Ceased grazing can lead to encroachment of shrubs and trees and reduced plant species richness. The aim of this thesis is to study the effects of spatial structures and grazing intensity on plant assemblages, plant reproduction and phytophagous insects. Pastures with scattered trees and shrubs grazed by cattle in south central Sweden were used as study sites. The studied spatial structures were two trees: Betula pendula, Pinus sylvestris, two shrubs: Juniperus communis, Rosa dumalis, and dung rejects, grazing rejects (ungrazed patches) and grazed patches. Trees and shrubs are long-lived structures and they signifi cantly infl uenced plant assemblages. For example, R. dumalis shrubs had a positive effect on tall, grazing sensitive species and a negative effect on short, less competitive species. Structures that could function as grazing refuges, i.e. shrubs, dung and grazing rejects increased plant reproduction. Grazing intensity also had a strong effect on plant reproduction and intensive grazing decreased the quantity of fl owers and fruits. The effect of spatial structures on four plant species of the Fabaceae family and their seed predators was also studied. The four species varied in abundance between the structures but in shrub rejects and dung rejects reproduction was higher than in grazed patches. The available oviposition places, i.e. pods, for seed predators were considerable higher in shrub and dung rejects than in grazed patches. But no clear patterns in predation rate between structures were found. For two species long pods were more predated than short pods indicating oviposition selection at pod level. In conclusion, regulation of grazing intensity and shrub cover are important tools when managing pastures for both plants and phytophagous insects.