Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Effects of disturbance on beetle and spider assemblages in a Mediterranean woodland

Auteur : Angel, Noa

Université de soutenance : Ben-Gurion University of the Negev

Grade : Master of Science 2009

Résumé
In Mt. Meron Nature Reserve landscape is becoming homogeneous following tree encroachment. Understory vegetation encompasses lower herbaceous plant species richness than adjacent open patches which are scarcely shaded or modulated by the canopy. This study aimed to examine the canopy effects on higher trophic groups. The research tested whether beetle and spider species diversity was higher in the open patches, following herbaceous species richness, or if it is positively affected by the filtering of the trees, and therefore higher in areas with higher tree cover. I tested two main hypotheses in which the spider and beetle richness in the woody patches are expected to increase with a decrease in canopy area. The Landscape Modulator-filtering hypothesis postulated that trees modulate their environment thus allowing the existence of woody-affiliated species. With the decrease in tree cover, beetle and spider species richness in the woody patch was predicted to decrease. The taxonomic diversity hypothesis postulates a direct relationship between plant species richness and arthropod species richness thus predicting an increase in beetle and spider species richness as a function of herbaceous species richness. Open patches-affiliated species were predicted to have a positive relation with both herbaceous plant richness and increased open area. . I examined these hypotheses through the use of two disturbance forms of management affecting the woody patch. Tree removal was used to decrease the tree cover, or increase the open area and cattle grazing was introduced in order to test its disturbance effects through grazing and browsing on regenerating trees. The results of this study partially supported the landscape modulator-filtering hypothesis. Ground-dwelling predator and detritivore species diversity decreased in their diversity with decreased tree cover. The taxonomic diversity hypothesis was also only partially supported. Herbivore species richness increased only in open patches as a function of herbaceous richness. Disturbance effects were attributed mostly to tree removal which had a large effect on beetle and spider diversity patterns and composition while grazing had little to no effect. The interaction between grazing and removal decreased diversity of grounddwelling spiders and beetles in the woody patches. Removal with no grazing generally increased vegetation-dwelling beetle and spider richness and abundance. Removal changed the assemblage composition of ground-dwelling spiders and beetles and removed woody patches were more similar in their composition to open patches than to woody unremoved patches. Vegetation-dwelling beetles exhibited different communities in the two patch types with and without removal. My study shows that beetle and spider diversity and composition is influenced more through the effect of the tree as a dominant landscape modulator than the change in herbaceous plant richness. Removal can be used as an effective land-use management tool in order to manipulate the landscape patch mosaic.

Mots Clés : Beetles — Ecology — Merron(Israel). — Grazing — Environmental aspects — Spiders — Ecology — Tree felling

Présentation (BGU)

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