Direct and Indirect Effects on Annual Plants in a Sandy Desert Ecosystem
Investigators (most current known information)
Much is known on the sandy desert ecosystems in general, and on the North-Western Negev desert in particular. The goal of this study was to study the effects of granivory and disturbance, of the soil crust, on the richness and density of annual plants and to investigate the existence and the importance of indirect effects in this system. Several hypotheses were generated regarding the above ecosystem key variables :
- Indirect effects are present and important in determining richness and density of annual plants. We 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 that only disturbance or seed predation prevention occurring alone.
- Direct and indirect effects can oppose each other. We predicted that seed predation would affect positively the change in richness and density of annual plants caused by disturbance, while affecting negatively the same plant parameters directly.
- Some key variables are more important than others. We predicted that seed predation would affect richness and density of annual plants more than soil crust disturbance.
We built in the field 48 experimental stations. Each station consisted of three 160X80 cm plots. Each plot consisted of two 80X80 cm subplots. We left one subplot of each plot intact and disturbed the soil crust of its twin subplot every year. From one plot, in each station, we excluded all granivores by installing metal sheets that were buried 25 cm below ground and extended 25 cm above ground. For the second plot we installed the same metal sheet but allowed granivores to enter it via gates. The third plot served as an open control.
We found a strong and significant effect of soil disturbance on richness and density of annual plants. We did not find a significant effects for granivory or for the interaction between soil crust disturbance and granivory. During the study we found a differential effect of the soil crust on rodents and ants. Results suggest that rodents are attracted to disturbances while ants prefer to forage in undisturbed patches.
During the early season of the second year we found a highly significant effect of granivores on annual plant species richness, which disappeared during late season. We suggested that this effect was the result of physical conditions, at the early season that allowed germination of only the last year seed crop that mostly laid exposed to the granivores on the sand surface. Indeed, granivores reduced annul plant density and diversity to zero on plots that they had access to. We concluded that no significant effect of granivory at the late season of the second year, and in other years, was a result of a storage effect that may be the outcome of a large seed bank obscuring small annual fluctuations in number and composition of seed crop. The strong positive interaction between granivory and disturbance, during the early season of the second year, suggests that granivory have a positive indirect effect on the annual seed crop. We integrated our observations and data from this and related studies into an ecosystem model that could be a basis for a predictive simulation model.
Ben-Natan, G., Z. Abramsky and Z. Kotler. 2004. "Sees redistribution in sand dunes: A basis for coexistence of two rodent species." Oikos 105(2):325-335.