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sur le développement des zones arides et semi-arides

Titre : Estimating the direct and indirect effects of microbiotic soil crust and granivore activity on the annual plant guild in a sandy Negev desert ecosystem

Auteur : Ben-Natan, Gil

Université de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2006

Résumé partiel
The accumulation of information about desert sandy dunes demands synthesis and large scale analysis, leading to further understanding and direction for future research. The established knowledge of the pair-wise interaction and effects of granivorous (seed eating) animals with annual plants and biological soil crusts with annual plants suggests the possibility of indirect effects of each of these guilds on the annual plants through their interactions with the other guild. However, no one showed such an indirect interaction previously. At the guild level of organization of this study, I measured the size of the indirect effect and its relative importance to the direct effect on annual plants, for both granivore activity and soil crust integrity. I also searched for evidence for or against the possible interaction between granivores and soil crust at the species level. At the guild level I hypothesized thus : 1. Indirect effects are present and important in determining richness and density of annual plants. I predicted that annual plants would consider patches where the microbiotic soil crust was disturbed and seed predation was prevented, to be of higher quality than patches where only disturbance or seed predation prevention alone occurred. 2. Direct and indirect effects can oppose each other. I predicted that seed predator activity would affect positively the change in richness and density of annual plants through disturbance, while affecting negatively the same plant parameters directly. 3. Some key variables are more important than others. I predicted that seed predation would affect richness and density of annual plants more than soil crust disturbance. I built in the field 48 experimental stations. Each station consisted of three 160x80 cm plots. Each plot consisted of two 80x80 cm subplots. I left one subplot of each plot intact and mechanically disturbed the soil crust of its twin subplot every year. From one plot, in each station, I excluded all granivores by installing metal sheets that were buried 25 cm below ground and extended 25 cm above ground. For the second plot I installed the same metal sheet but allowed granivores to enter it through gates. The third plot served as an open control. Disturbance of the soil crust had a positive strong and constant effect on annual plants, while the effects of granivore activity were found to be very weak. No evidence for indirect effect was found on the annual plant guild. However, detailed ה observations suggest that annual plant seed crop is affected strongly by granivores, in contrast to the weaker effect on the annual plant seed bank. Furthermore, a strong positive interaction between granivory and disturbance suggest that granivory indeed has a positive indirect effect on the annual seed crop, but very little effect on the annual plant seed bank. The overall lack of a significant granivore activity effect suggests the existence of a storage effect resulting from a large seed bank obscuring small annual fluctuations in number and composition of seed crop.

Mots Clés : Desert plants — Israel — Negev — Utilization. ; Desert resources — Israel — Negev ; Granivores — Israel — Negev. ; Plant ecology — Israel — Negev.

Présentation (BGU)

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