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.

A model study of terraced riverbeds as novel ecosystems.

Riverbed terracing has been introduced in ancient times to retain water and soil, to reduce hydrological connectivity and erosion and to increase primary and secondary productivity of agro-ecological systems. These presently abandoned human-made landscapes have become novel ecosystems and a potential source of ecosystem services to humans in drylands. We use a mathematical-modeling approach to study factors that regulate terraced riverbeds and affect community and ecosystem attributes such as productivity, functional diversity and resilience to droughts. We introduce a model that captures the relationships between rainfall pattern, runoff coupling between adjacent terraces, and vegetation growth, taking into account competition for water and light. We found that a large number of weak rainfall events results in lower total biomass and functional diversity across the terraced riverbed compared with a few strong rainfall events. We further analyzed the filtering of species traits from pools of functional groups that make different tradeoffs between investment in above-ground biomass to capture canopy resources and investment in below-ground biomass to capture soil resources. Pools characterized by concave tradeoffs give rise to higher functional diversity, lower biomass production and lower resilience to droughts, as compared with convex pools. New empirical studies are needed to test these model predictions.

Multiscale mapping of species diversity under changed land use using imaging spectroscopy.

Land use changes are one of the most important factors causing environmental transformations and species diversity alterations. The aim of the current study was to develop a geoinformatics-based framework to quantify alpha and beta diversity indices in two sites in Israel with different land uses, i.e., an agricultural system of fruit orchards, an afforestation system of planted groves, and an unmanaged system of groves. The framework comprises four scaling steps: (1) classification of a tree species distribution (SD) map using imaging spectroscopy (IS) at a pixel size of 1 m; (2) estimation of local species richness by calculating the alpha diversity index for 30-m grid cells; (3) calculation of beta diversity for different land use categories and sub-categories at different sizes; and (4) calculation of the beta diversity difference between the two sites. The SD was classified based on a hyperspectral image with 448 bands within the 380-2500 nm spectral range and a spatial resolution of 1 m. Twenty-three tree species were classified with high overall accuracy values of 82.57% and 86.93% for the two sites. Significantly high values of the alpha index characterize the unmanaged land use, and the lowest values were calculated for the agricultural land use. In addition, high values of alpha indices were found at the borders between the polygons related to the "edge-effect" phenomenon, whereas low alpha indices were found in areas with high invasion species rates. The beta index value, calculated for 58 polygons, was significantly lower in the agricultural land use. The suggested framework of this study succeeded in quantifying land use effects on tree species distribution, evenness, and richness. IS and spatial statistics techniques offer an opportunity to study woody plant species variation with a multiscale approach that is useful for managing land use, especially under increasing environmental changes.

Emerged or imposed: a theory on the role of physical templates and self-organisation for vegetation patchiness.

In this article, we develop a unifying framework for the understanding of spatial vegetation patterns in heterogeneous landscapes. While much recent research has focused on self-organised vegetation the prevailing view is still that biological patchiness is mostly due to top-down control by the physical landscape template, disturbances or predators. We suggest that vegetation patchiness in real landscapes is controlled both by the physical template and by self-organisation simultaneously, and introduce a conceptual model for the relative roles of the two mechanisms. The model considers four factors that control whether vegetation patchiness is emerged or imposed: soil patch size, plant size, resource input and resource availability. The last three factors determine the plant-patch size, and the plant-to-soil patch size ratio determines the impact of self-organisation, which becomes important when this ratio is sufficiently small. A field study and numerical simulations of a mathematical model support the conceptual model and give further insight by providing examples of self-organised and template-controlled vegetation patterns co-occurring in the same landscape. We conclude that real landscapes are generally mixtures of template-induced and self-organised patchiness. Patchiness variability increases due to source-sink resource relations, and decreases for species of larger patch sizes.

Mechanisms of vegetation-ring formation in water-limited systems.

A common patch form in dryland landscapes is the vegetation ring. Vegetation patch formation has recently been attributed to self-organization processes that act to increase the availability of water to vegetation patches under conditions of water scarcity. The view of ring formation as a water-limited process, however, has remained largely unexplored. Using laboratory experiments and model studies we identify two distinct mechanisms of ring formation. The first mechanism pertains to conditions of high infiltration contrast between vegetated and bare soil, under which overland water flow is intercepted at the patch periphery. The decreasing amount of water that the patch core receives as the patch expands, leads to central dieback and ring formation. The second mechanism pertains to plants with large lateral root zones, and involves central dieback and ring formation due to increasing water uptake by the newly recruited individuals at the patch periphery. In general the two mechanisms act in concert, but the relative importance of each mechanism depends on environmental conditions. We found that strong seasonal rainfall variability favors ring formation by the overland-flow mechanism, while a uniform rainfall regime favors ring formation by the water-uptake mechanism. Our results explain the formation of rings by fast-growing species with confined root zones in a dry-Mediterranean climate, such as Poa bulbosa. They also explain the formation of rings by slowly growing species with highly extended root zones, such as Larrea tridentata (Creosotebush).

Harvester ant response to spatial and temporal heterogeneity in seed availability: pattern in the process of granivory.

The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied in an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availability for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape patch types (shrub patches and inter-shrub patches) in 1997. Shrub patches were shown to have higher plant and seed-rain density than inter-shrub patches. Patch use and seed selection by M. arenarius foragers were monitored through the spring, summer and autumn of 1997. After a pulse of seed production in the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summer, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two patch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increase in the use of shrub patches by individual foragers. In the autumn, a pulse of seed production by the shrub species Atractylis serratuloides and Noaea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in the proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behaviour showed linked changes in seed selection and patch use resulting in important differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productivity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephemeral plants in the landscape.

Porcupine disturbances and vegetation pattern along a resource gradient in a desert.

In the Negev Desert, Israel, the Indian crested porcupine, Hystrix indica, digs similar sized, discrete, elongated pits (257±21.3 cm3; n=144) while foraging for below-ground plant storage organs. In these digs, soil moisture content is higher than in the surrounding soil matrix. The digs disturb population and community structure due to porcupine consumption or damage of 18 species of plants, and repopulation by 55 plant species. Over the past 14 years we have studied dig dynamics on a rocky hillslope with three distinct habitats as regards soil moisture content. Midslope soil moisture is the highest, decreasing towards upper and lower slope. We have counted a total of 6,609 digs in the area: 2141 on the upper, 3211 in the middle and 1257 on the lower part of the slope. The number of digs at midslope is significantly higher than on the other parts of the slope (ANOVA; P<0.0001). There is a significant (P<0.05) correlation between the mean number of porcupine digs and the cumulative rainfall amount for the 2 years prior to dig formation. To study plant repopulation in digs, all plants in 144 digs along the slope and from equal sized plots in the undistarbed soil matrix were collected. In all, 288 samples with 20 584 plants were collected, 2042 from the matrix and 18,542 from digs. Of the 55 species, we focused on the abundance patterns of Filago desertorum, Picris cyanocarpa and Bromus rubens, which made up 69.5% of all the individuals in the digs and 68.3% in the matrix. Our results showed that all three species increased in abundance in the digs as compared to the matrix. F. desertorum density increased by a factor of 2.9, P. cyanocarpa by 9.5 and B. rubens by 12.0. There were species-specific responses in abundance to the location of the digs along the moisture gradient. The only species whose abundance responded to the moisture gradient was F. desertorum. P. cyanocarpa demonstrated peak abundance in the location with the poorest moisture regime, while B. rubens showed peak abundance at the intermediate part of the moisture gradient. We suggest a scheme for integrating the increase in density and the species-specific responses to the digs along a water gradient based on R.H. Whittaker's view of individual species abundances along an environmental gradient.

Relationship among settling, demography and habitat selection: an approach and a case study.

We explore the demographic consequences of site selection by animals on their abundance among habitats. We found that pre and post settling survivorship are important links between the behavioral decisions where to settle and the distribution of a population among habitats. This was demonstrated for 10 generations of the desert isopod, Hemilepistus reaumuri, in three habitats in the Negev Desert, Israel. The populations exhibit low survivorship before settling (≈12%) and high survivorship (≈55%) after settling. According to our model this implies high site selection. Theoretical considerations and the case study led us to suggest the following relationship among settling, demography and habitat selection: 1) Individuals search for suitable settling sites to inhabit and reproduce. 2) Their decision where and when to settle is a cost benefit decision. They weigh the benefit of searching for a high quality site against mortality due to increased searching time. 3) The individual's decision to settle determines the pre and post settling survivorship pattern. 4) Survivorship pattern dictates density pattern in time and space. 5) Density pattern in a given habitat determines its quality for the individual. 6) Settling selection among habitats and the number of safe sites controls the distribution of densities among habitats.

The relationship between sit and wait foraging strategy and dispersal in the desert scorpion, Scorpio maurus palmatus.

We examine the role of the sit and wait foraging strategy and of the stability and predictability of the biotic environment in shaping population biology and life history traits of the desert scorpion, Scorpio maurus palmatus. The study was carried out in a loessial plain in the Negev desert, Israel. We used a grid system (5x80 m) within the Hammada scoparia plant association. The area was divided into 1,600 squares (50x50 cm) and the presence (within ±5 cm) and width of each scorpion burrow was recorded for eight years (1973-1980). It was found that: (1) Dispersal is a common phenomenon for all S. maurus palmatus age groups and takes place only after a heavy rainfall when soil moisture is high. (2) Dispersion patterns change from clumped to random as a function of age. (3) Dispersal is age specific and density dependent. (4) During the dry seasons a high percentage of the population goes into the resting stage.In order to integrate the above findings, a graphical model was constructed with the main assumption that the S. maurus palmatus prey availability relationship is a decreasing function between the scorpion burrow and its food resources. Data and model analysis led to the conclusion that the desert scorpion, S. maurus palmatus, is confronted with a relatively predictable physical environmeent and unpredictable biological environment. Two alternative strategies are available for S. maurus palmatus to overcome prey unpredictability; dispersal during the rainy season and inactivity during the dry seasons.We suggest that many other burrowing desert animals may face the problem of unpredictability of biological resources. Thus it seems that the S. maurus palmatus biological relationship model can be applied, with modifications, to a wide spectrum of desert burrowing animals.

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

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.

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.