Dynamics and spatial organization of plant communities in water-limited systems.

A mathematical model for plant communities in water-limited systems is introduced and applied to a mixed woody-herbaceous community. Two feedbacks between biomass and water are found to be of crucial importance for understanding woody-herbaceous interactions: water uptake by plants' roots and increased water infiltration at vegetation patches. The former acts to increase interspecific competition while the latter favors facilitation. The net interspecific interaction is determined by the relative strength of the two feedbacks. The model is used to highlight new mechanisms of plant-interaction change by studying factors that tilt the balance between the two feedbacks. Factors addressed in this study include environmental stresses and patch dynamics of the woody species. The model is further used to study mechanisms of species-diversity change by taking into consideration tradeoffs in species traits and conditions giving rise to irregular patch patterns.

A mathematical model of plants as ecosystem engineers.

Understanding the structure and dynamics of plant communities in water-limited systems often calls for the identification of ecosystem engineers--key species that modify the landscape, redistribute resources and facilitate the growth of other species. Shrubs are excellent examples; they self-organize to form patterns of mesic patches which provide habitats for herbaceous species. In this paper we present a mathematical model for studying ecosystem engineering by woody plant species in drylands. The model captures various feedbacks between biomass and water including water uptake by plants' roots and increased water infiltration at vegetation patches. Both the uptake and the infiltration feedbacks act as mechanisms for vegetation pattern formation, but have opposite effects on the water resource; the former depletes the soil-water content under a vegetation patch, whereas the latter acts to increase it. Varying the relative strength of the two feedbacks we find a trade-off between the engineering capacity of a plant species and its resilience to disturbances. We further identify two basic soil-water distributions associated with engineering at the single patch level, hump-shaped and ring-shaped, and discuss the niches they form for herbaceous species. Finally, we study how pattern transitions at the landscape level feedback to the single patch level by affecting engineering strength.

Ecosystem engineers: from pattern formation to habitat creation.

Habitat and species richness in drylands are affected by the dynamics of a few key species, termed "ecosystem engineers." These species modulate the landscape and redistribute the water resources so as to allow the introduction of other species. A mathematical model is developed for a pair of ecosystem engineers commonly found in drylands: plants forming vegetation patterns and cyanobacteria forming soil crusts. The model highlights conditions for habitat creation and for high habitat richness, and suggests a novel mechanism for species loss events as a result of environmental changes.

Diversity of vegetation patterns and desertification.

A new model for vegetation patterns is introduced. The model reproduces a wide range of patterns observed in water-limited regions, including drifting bands, spots, and labyrinths. It predicts transitions from bare soil at low precipitation to homogeneous vegetation at high precipitation, through intermediate states of spot, stripe, and hole patterns. It also predicts wide precipitation ranges where different stable states coexist. Using these predictions we propose a novel explanation of desertification phenomena and a new approach to classifying aridity.

Positive feedback of consumer population density on resource supply.

Recent studies demonstrate positive density-dependent feedbacks between animal populations and their resource supply that result in increased individual fitness at high densities. Such feedbacks occur in both terrestrial and aquatic organisms not showing strong social organization. A number of different mechanisms are involved. Detecting positive feedbacks in natural populations may not be possible from simple correlations between resource abundance and animal population density in space or time, but experimental manipulation of resource supply or animal density can reveal their presence. Positive feedbacks may result in higher equilibrium densities of animal populations, alter the density range over which intraspecific competition is detectable, and offer a resource-based explanation for the evolution of gregariousness and social organization.

Herbivory in rocks and the weathering of a desert.

Two species of snail, Euchondrus albulus and Euchondrus desertorum, eat endolithic lichens growing under the surface of limestone rocks in the Negev Desert, Israel. This unusual type of herbivory has the unexpected and major impact of weathering this rocky desert at a rate of 0.7 to 1.1 metric tons per hectare per year. The biotic weathering contributes to the process of soil formation at a rate that is similar to wind-borne dust deposition. These findings demonstrate that herbivores can have a significant regulatory impact on ecosystem processes, even in cases where the total amount of primary production consumed is small.

A case study of energy, water and soil flow chains in an arid ecosystem.

Little attention has been directed to the study of soil flow and the complex relationships among energy water and soil flow in terrestrial ecosystems. Soil plays an important role in arid ecosystems. After water soil is the second key factor in the development of an arid ecosystem since soil is the only part of the system capable of absorbing and storing water and nutrients during the hot and long summer period. The present work presents a case study of an ecological soil flow chain in an arid environment and analyses the relationship between this chain and the energy and water flow chains. The study was conducted at the Sde Boqer experiment site located in the northern Negev of Israel where average annual rainfall is 92 mn. Data collected during five consecutive years show that the soil movement process within the ecosystem studied cannot be considered as a purely physical phenomenon, but rather as a part of a complex system in which the burrowing and digging activity of Isopods and Porcupines plays an important role by providing disaggregated soil particles easy to remove by shallow flows. Although controlled by the spatial distribution of soil moisture the biological activity acts as a regulator of soil depth and thus of soil moisture. If this regulating role is deleted from the system a new ecosystem, more arid, can be expected to develop. It is therefore concluded that the study of state and flow variables of an arid ecosystem should consider altogether the water, soil, energy and mineral chains.

Phenology, activity and regulation of radiation load in the desert isopod, Hemilepistus reaumuri.

The phenology and activity of the isopod H. reaumuri has been studied since 1972, in the Negev desert; five distinct phenophases were identified in the annual life cycle of the isopod. From the relationship between activity pattern, phenophases, and microclimatic data it was concluded that H. reaumuri's principal strategy of compensating for its' physiological constraints is through behavioural regulation of energy inflow. (850-1000 cal. cm-2 day-1).

Feeding, energy flow and soil turnover in the desert isopod, Hemilepistus reaumuri.

The desert woodlouse Hemilepistus reaumuri is one of the most abundant macroscopic invertebrates in North Africa, the Arabian steppes, semi-desert and desert, and the Negev desert Israel.The main purposes of our study were: 1. To investigate the feeding behaviour of H. reaumuri in the field and the laboratory, with special attention to the importance of soil and perennials in the isopods' diet. 2. To estimate annual energy flow and soil turnover by the isopods and to relate it to the role of isopods in the desert ecosystem. The results of field observations and feeding experiments in the laboratory show: a) That desert isopods are saprovores, herbivores and microbivores. b) That in order to survive the isopod should include soil particles and or soil minerals when feeding on vegetation. c) Annual ingestion was 10.3-38.6 kcals/m2, 3-12% of the available dead organic matter, and soil turnover 28.5-105.7 g/m2. It was hypothesized that by ingestion and defaecation of organic matter and inorganic soil particles, H. reaumuri alters the structure of the decomposition substrate and increase the rate of decomposition in the desert ecosystem.