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.     

Patchiness and disturbance: plant community responses to porcupine diggings in the central Negev

We tested the hypothesis that diversity and productivity of herbaceous plant communities in disturbed soil are related to the physical and biological heterogeneity of the landscape Our study was earned out on vegetation responses in porcupine diggings on a rocky slope in the central Negev desert in Israel We measured aboveground bio-mass and plant density per species in 150 porcupine diggings (15 cm deep and 15 to 20 cm wide) and in equally sized adjacent control samples in the undisturbed soil matrix We calculated mean annual biomass production, plant density and species richness for 10 sample areas along the slope In addition, we divided the plants into groups according to propagule size and dispersal mode We denoted two types of landscape heterogeneity, which we called physical and biological patchiness Physical patchiness was measured as the ratio of bare rock to soil surface Biological patchiness was the area of the soil covered by shrubs with associated soil mound and under-story relative to the total soil surface We also measured disturbance density, as the long term (17 yr) average density of newly made porcupine diggings We found that 1) the physical patchiness explained 30% of the variation of biological patchiness along the slope, while 2) the patterns of disturbance intensity and biological patchiness were similar (R-=0 386) 3) Biomass, density and species richness were significantly higher in diggings than m the soil matrix 4) Plant density in the matrix, but not m the diggings, was significantly correlated with physical patchiness, 5) species richness in diggings was significantly correlated with biological patchiness, but 6) biomass production in diggings and matrix was not affected by either physical or biological patchiness of the landscape 7) Disturbance density did not affect vegetation responses in diggings and matrix 8) A shift in the plant communities in the matrix towards plants with smaller seeds was associated with increasing physical patchiness, while m diggings there was an opposite shift 9) The proportion of wind dispersers was higher in diggings than outside, while the proportion of runoff dispersers was lower, 10) the densities of runoff dispersers in diggings and matrix were positively correlated with physical and biological patchiness 11) Physical and biological patchiness formed the two major gradients of species composition, explaining 30 and 25% respectively We conclude that the network of physical and biological patchiness and soil disturbance are important in the redistribution of resources and seeds, which control plant biomass, density, species richness and diversity The bare rock surface is the main source for runoff flow with associated soil, organic matter and nutrients The understory vegetation of shrubs provides seeds for creating and maintaining diversity The soil matrix absorbs runoff flow, and disturbances absorb runoff and trap seeds Thus, differences in landscape heterogeneity and their effects on resource and seed movement interact in controlling plant community productivity and diversity in the landscape     

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:     

How landscape modulators function: woody plant impact on seed dispersal and abiotic filtering

The Mediterranean landscape is characterized by a heterogeneous structure: a mosaic of woody plants (trees or shrubs) with scattered patches of herbaceous vegetation. Although the herbaceous and woody patches are adjacent to each other, plant species composition in them is substantially different. This could be attributed to either differences in environmental conditions between patch types (i.e., abiotic filters), or to dispersal limitations caused by the woody plants acting as dispersal filters. In this article, we focus on the relative impact of woody plants, applying these two filter types, in determining plant species composition in Mediterranean woodland. We experimentally manipulated shade and litter cover and examined the effect of each of these factors on plant species composition. We used seed-traps to evaluate seed arrival in the patches, and experimentally removed the shrub canopy to study the effect of the shrub as a physical barrier to seed entry. Results showed that plant species number and composition were not significantly affected by shade and litter manipulation. The number of trapped seeds were significantly higher in the open patches than in the woody patches, and removal of woody plants increased the number of trapped seeds in both open and woody patches, as a result of eliminating the physical obstacle to free seed movement. Our findings show that woody plants affect the herbaceous plant community by influencing seed dispersal, and highlight that they affect other organisms not only by modifying resource availability but also through the creation of a new landscape structure.     

The influence of scale and patchiness on spider diversity in a semi-arid environment

Semi-arid scrubland in the Middle East consists of a soil crust matrix overlain with patches of perennial shrubs. To understand factors influencing biodiversity in this vulnerable landscape we need to understand how this mosaic of habitats influences associated fauna. Spiders are particularly abundant in this habitat so we asked if spider diversity differed between habitat patches and if different patch types contained either a subset of the regional species pool or specific species guilds. We also asked whether changes in the fractal nature of the microphytic and macrophytic patch mosaic altered spider diversity in this habitat. We found that the semi-arid scrubland at Sayeret Shaked Park (Israel) contains different spider communities that require patches of a certain quality to develop fully. Different patch types contain communities of different species, but the community structure of the patches is similar. We suggest that large-scale environmental factors typical of the site as a whole influence coarse-grained community structure, while small-scale differences between patch types result in the specialisation of species to different patch types.     

Energy allocation and life history strategy of the desert isopod H. reaumuri

Summary Population characteristics of the desert isopod H. reaumuri and its energy allocation during each penophase were investigated in the Negev desert, Israel.It was found that on the individual level, 52% of the energy is allocated to the prereproductive, and 48% to the reproductive phenophases. On the population level, only 10–23% is devoted to isopods which are involved in reproduction. Assuming that the energy allocation pattern in H. reaumuri is the consequence of its life history strategy, two unique evolutionary lines were suggested. One suggestion is the evolution of the family mode of life which increases the probability of the individual's survival. This is due to parental investment and co-operation between siblings. The other suggestion is the evolution of high energy utilization for burrowing, in order to solve the low and unpredictable nature of water resources in the desert.     

An algae — Desert snail food chain: Energy flow and soil turnover

Summary The feeding ecology of adult desert snails (Sphincterochila zonata), inhabiting a loess plain in the Negev Desert, was studied during a 7-year period. Energy flow and soil turnover determinations were made at the individual and population level on the basis of field observations and data derived from laboratory simulations. Sphincterochila zonata were only active on 8–27 winter days annually, otherwise they were dormant. The snails fed exclusively on algae that grew on the soil surface following rain. The mean annual dry-weight biomass of algae was 197.4(±118.1) gr·m^-2. The dry weight biomass of snails ranged from 0.02–0.14 gr·m^-2. Annual production in the food chain varied substantially from year to year (1–1539 Kj·m^-2), but the net annual energy balance of the snails was always positive. Soil crust turnover, resulting from grazing of snails on algae, was estimated at 142 kg·hectare^-1 during the study period.The annual magnitude of energy flow and soil turnover is determined by the soil surface moisture regime which in turn, is a function of rainfall patterns. Sphincterochila zonata may serve an important role in Negev ecosystems by dispersing soil algae and altering soil structure. The snails are not subject to substanding predation by rodents but may occasionally serve as an important food source for migrating birds.     

High-integrity human intervention in ecosystems: Tracking self-organization modes

Humans play major roles in shaping and transforming the ecology of Earth. Unlike natural drivers of ecosystem change, which are erratic and unpredictable, human intervention in ecosystems generally involves planning and management, but often results in detrimental outcomes. Using model studies and aerial-image analysis, we argue that the design of a successful human intervention form calls for the identification of the self-organization modes that drive ecosystem change, and for studying their dynamics. We demonstrate this approach with two examples: grazing management in drought-prone ecosystems, and rehabilitation of degraded vegetation by water harvesting. We show that grazing can increase the resilience to droughts, rather than imposing an additional stress, if managed in a spatially non-uniform manner, and that fragmental restoration along contour bunds is more resilient than the common practice of continuous restoration in vegetation stripes. We conclude by discussing the need for additional studies of self-organization modes and their dynamics.     

Ecosystem Management of Desertified Shrublands in Israel

The objectives of this study were to understand the ecological processes and possible management strategies in desertified shrublands. We hypothesized that biological production and diversity in desertified shrublands in the Negev in Israel are low due to water, soil, and nutrient leakage from the ecosystem. We designed a series of field experiments in order to examine (a) whether source–sink relationships exist between the crusted soil and the shrub patches, (b) whether resources (water, soil, and nutrients) leak from the system, and (c) whether management, which changes the landscape mosaic by introducing new sink patches that reduce leakage of resources, may increase productivity and diversity. The results indicate that the low number of shrub patches, which serve as sinks for resources, leads to water, soil, and nutrient leakage from the ecosystem. This leakage reduces ecosystem production and diversity. We found that artificially created pits, which act as sinks for resources, decrease leakage and increase biomass production and annual plant species diversity. Based on the experimental results, we developed conceptual models for shrubland desertification and ecosystem management. The models are based on a source–sink relationship between two patch types characteristic of shrublands. The models relate landscape productivity to the number of sink patches and suggest that, in cases where there are too few sinks, artificially created sink patches should be added. Management methods were developed to reduce resource leakage in the desertified shrubland of the Negev. Methods included construction of man-made pits in the landscape that add resource-enriched patches to the landscape. These patches are used to create parks consisting of clusters of trees integrated into a matrix of shrubs and herbaceous vegetation. The managed parks are used for recreational purposes and for rangeland. Received 8 July 1997; accepted 7 July 1998.     

Vegetation patterns related to environmental factors in a Negev Desert watershed

Three strip transects, each ca 100 contiguous 0.5×1 m² quadrats, were sampled during the spring bloom of March 1981 across four surface structural units of a Negev Desert research watershed at Sede Boqer, Israel. Presence of all vascular plants was recorded. Data were subjected to detrended correspondence analysis (DCA ordination), and resulting spatial patterns of species distribution and abundance were compared. Large-scale gradients of vegetation were related to differences in soil moisture availability among the four structural units. Where micro-scale vegetation patterns were important, these correlated with rock and crevice microtopography. Species richness was influenced by high numbers of therophytes on the dry upper slope of the watershed and their reduced importance on the lower three units. Relationships between vegetational patterns and known ecosystem properties of the watershed are discussed.