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Ben-Gurion University of the Negev (2009)

Woody vegetation as landscape modulators : Sarcopoterium spinosum as a tangible model

Moran Segoli

Titre : Woody vegetation as landscape modulators : Sarcopoterium spinosum as a tangible model

Auteur : Moran Segoli

Etablissement de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2009

Résumé partiel
The concept of ecosystem engineer was proposed in order to understand abiotic environmental modulation by organisms and its effects on the distribution and abundance of the biota. The ecosystem engineering concept was incorporated with patch formation theory in order to better understand the drivers of biodiversity dynamics. An organism that functions as an ecosystem engineer in the context of patch formation was termed a landscape modulator (LM). A crucial element of the modulation process is the creation of different resource patches having different spatial patterns, resulting in multiple-layer landscape diversity. Local species assemblages are controlled by the multi layer landscape diversity and filtering processes. In this view, the term biodiversity encompasses two components : landscape diversity and species diversity, and the LM concept serves to understand the functional link between these two types of diversities. Disturbances to the LM, be they natural or anthropogenic, affect landscape diversity, filtering of species and species assemblages. The coupled processes of patch formation, maturation and decay, on the one hand, and species filtering and assemblage response on the other, was termed ’biodiversity cycling’. It has been proposed that the dynamics of this cycling is a hysteresis phenomenon in which the decay and renewal processes follow different trajectories. Against this theoretical background, the main goal of my study was to test empirically the utility of the LM concept in understanding the structure and function of a two-phase mosaic in a semi-arid ecosystem. I used Sarcopoterium spinosum as a model for an LM. I tested the following specific hypotheses : 1) S. spinosum modulates the environment by creating multi layer landscape diversity ; 2) S. spinosum resource modulation has both positive and negative effects on herbaceous vegetation ; 3) landscape modulation by S. spinosum enhances resilience and stability of the two-phase mosaic. My experimental design for testing these three hypotheses was as follows. A full factorial design experiment was set up on four slopes in the Negev semi-arid ecosystem, which is dominated by S. spinosum shrubs. On each slope, four 1000 m2 plots were designated to one of the following treatments : 1) grazed with no removal of the canopy of the woody vegetation, 2) grazed with canopy removal, 3) ungrazed with no canopy removal, and 4) ungrazed with canopy removal. I recorded herbaceous vegetation biomass, species density and richness during 2005-2007 in three patch types (open patch, core of shrub patch and boundary of shrub patch) in each treatment. Soil nutrients were measured in 2006 in each patch type and treatment. The effect of S. spinosum on runoff and depth of water infiltration was measured in 2005. The effect of S. spinosum on seed dispersal was measured in 2006.

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