Moshe Shachak

Organisms as ecosystem engineers

Interactions between organisms are a major determinant of the distribution and abundance of species. Ecology textbooks (e.g., Ricklefs 1984, Krebs 1985, Begon et al. 1990) summarise these important interactions as intra- and interspecific competition for abiotic and biotic resources, predation, parasitism and mutualism. Conspicuously lacking from the list of key processes in most text books is the role that many organisms play in the creation, modification and maintenance of habitats. These activities do not involve direct trophic interactions between species, but they are nevertheless important and common. The ecological literature is rich in examples of habitat modification by organisms, some of which have been extensively studied (e.g. Thayer 1979, Naiman et al. 1988).

POSITIVE AND NEGATIVE EFFECTS OF ORGANISMS AS PHYSICAL ECOSYSTEM ENGINEERS

Physical ecosystem engineers are organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance, or creation of habitats. Ecological effects of engineers on many other species occur in virtually all ecosystems because the physical state changes directly create nonfood resources such as living space, directly control abiotic resources, and indirectly modulate abiotic forces that, in turn, affect resource use by other organisms. Trophic interactions and resource competition do not constitute engineering. Engineering can have significant or trivial effects on other species, may involve the physical structure of an organism (like a tree) or structures made by an organism (like a beaver dam), and can, but does not invariably, have feedback effects on the engineer. We argue that engineering has both negative and positive effects on species richness and abundances at small scales, but the net effects are probably positive at larger scales encompassing engineered and nonengineered environments in ecological and evolutionary space and time. Models of the population dynamics of engineers suggest that the engineer/habitat equilibrium is often, but not always, locally stable and may show long-term cycles, with potential ramifications for community and ecosystem stability. As yet, data adequate to parameterize such a model do not exist for any engineer species. Because engineers control flows of energy and materials but do not have to participate in these flows, energy, mass, and stoichiometry do not appear to be useful in predicting which engineers will have big effects. Empirical observations suggest some potential generalizations about which species will be important engineers in which ecosystems. We point out some of the obvious, and not so obvious, ways in which engineering and trophic relations interact, and we call for greater research on physical ecosystem engineers, their impacts, and their interface with trophic relations.

Infiltration through three contrasting biological soil crusts in patterned landscapes in the Negev, Israel

Abstract We examined the role of soil crusts in infiltration processes in three contrasting environments in the Northern, Central, and Central-Western Negev, Israel. The removal of a thin cyanobacterial-dominant crust from a sandy dune at Nizzana in the Central-Western Negev and of a well-developed lichen-dominant and a cyanobacterial-dominant crust from a loess-covered hillslope at Sayeret Shaked in the Northern Negev resulted in three to fivefold increases in sorptivity and steady-state infiltration under both ponding and tension. The removal of a depositional crust colonised by cyanobacteria from a loess floodplain at Sede Zin in the Central Negev resulted in an increased infiltration under tension, but had no significant effect under ponding. We attribute the lack of effect under ponding to exposure of surface silts to water, which resulted in the clogging of matrix pores and surface sealing. The removal of the crusts in all three landscapes influences resource flows, particularly the redistribution of runoff water, which is essential for the maintenance of desert soil surface patterning. It would also have marked effects on germination, establishment and survival of vascular plants and soil biota, leading ultimately to desertification.

Vegetation patterns along a rainfall gradient

A continuum model for vegetation patterns in water limited systems is presented. The model involves two variables, the vegetation biomass density and the soil water density, and takes into account positive feedback relations between the two. The model predicts transitions from bare-soil at low precipitation to homogeneous vegetation at high precipitation through intermediate states of spot, stripe and gap patterns. It also predicts the appearance of ring-like shapes as transient forms toward asymptotic stripes. All these patterns have been identified in observations made on two types of perennial grasses in the Northern Negev. Another prediction of the model is the existence of wide precipitation ranges where different stable states coexist, e.g. a bare soil state and a spot pattern, a spot pattern and a stripe pattern, and so on. This result suggests the interpretation of desertification followed by recovery as an hysteresis loop and sheds light on the irreversibility of desertification. 2003 Elsevier Ltd. All rights reserved.

Ecosystem Management of Desertified Shrublands in Israel

ABSTRACT 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.

Litter as a regulator of N and C dynamics in macrophytic patches in Negev desert soils

Abstract In desert ecosystems, nutrient cycling activity is concentrated in brief periods of intense biological activity following wetting events. Release and uptake of N from litter and microbial biomass may be important regulators of N availability to plants and N loss to denitrification and NH 3 volatilization. Litter and microbial biomass dynamics may also be important to the maintenance of shrub-dominated patches of high fertility in desert ecosystems. We have measured soil C and N cycling processes (respiration, NH 4 + and NO 3 − dynamics, denitrification and microbial biomass C and N dynamics) in rewetted Negev desert soil treated with different size classes of desert plant litter in 30-day laboratory incubations. The results suggest that litter plays a strong role in conserving N following wetting events in Negev soils. Amounts of soil NH 4 + and NO 3 − and microbial biomass N were reduced in litter-amended treatments, suggesting that significant quantities of N were sequestered in litter, especially the largest size classes of litter. Denitrification was a significant sink for N, and was stimulated by the presence of litter, but was less important than immobilization of N in litter. Immobilization and release of N by litter may be especially important in the N cycle in desert ecosystems, moderating seasonal patterns of N availability and regulating patch interactions that facilitate the development of “islands of fertility” in these ecosystems.

Nitrogen fixation in macro- and microphytic patches in the Negev desert

Abstract Nitrogen fixation is thought to be a major input to the N budget of deserts, and it is generally considered to be carried out by cyanobacteria in the soil microphytic crust, not by free-living heterotrophic bacteria. We have compared N fixation in Negev desert microphytic soil crusts and macrophytic patch soils. We evaluated four different types of crusts in two sites, which vary in their rainfall amount from cyanobacterial-dominated crusts in the dry (

Woody Species as Landscape Modulators and Their Effect on Biodiversity Patterns

ABSTRACT Ecological research on organism-environment interactions has developed asymmetrically. Modulation of organisms by the environment has received much attention, while theoretical studies on the environmental impact of organisms have until recently been limited. We propose a theoretical framework for studying the environmental impacts of woody plants in order to understand their effects on biodiversity. We adopt pattern formation theory to discuss how woody plants organize ecological systems on the patch and landscape levels through patch formation, and how organism patchiness creates resource patchiness that affects biodiversity. We suggest an integrative model that links organisms as landscape modulators through resource distribution and species filtering from larger to smaller spatial scales. Our “biodiversity cycling hypothesis” states that in organism-modulated landscapes, disturbance enables the coexistence of different developmental stages of vegetation patches, thereby increasing biodiversity. This hypothesis emphasizes that species and landscape diversity vary with the development, renewal, maturation, and decay of biotically induced patches.

The effect of microphytes on the spectral reflectance of vegetation in semiarid regions

The normalized difference vegetation index (NDVI), which is derived from satellite sensor images, is widely used as a measure of vegetation and ecosystem dynamics, change in land use, desertification, and climatic change processes on a regional or global scale. Surprisingly, in semiarid regions, relatively high values of NDVI were measured in landscapes where little, if any, photosynthetic activity of higher plants exists. We tested the hypothesis that the high NDVI values may be caused by the photosynthetic activity of microphytes (lower plants), consisting of mosses, lichens, algae, and cyanobacteria, which cover most of the rock and soil surfaces in semiarid regions. We found that the spectral reflectance curves of lower plants can be similar to those of the higher ones and their derived NDVI values can be as high as 0.30 units. We conclude that, in semiarid environments, the reflectance of lower plant communities may lead to misinterpretation of the vegetation dynamics and overestimation of ecosystem productivity.

Microphytic crusts, shrub patches and water harvesting in the Negev Desert: the Shikim system

Human-made contour banks are a central component of theShikim water harvesting system in Israel’s Negev Desert.Efficient water capture depends on the presence of a stable microphytic crustwhich directs surplus surface runoff into the banks where it is stored. We usedsimulated rainfall to examine the impact of soil surface disturbance on runoffand sediment transport, and the effect of this on the efficiency of resourcecapture within the Shikim system. Two disturbance regimes:1) removal of the microphytic crust only, and 2) removal of the crust and shrubpatches by cultivation, were compared with an undisturbed control. In theundisturbed state, 32% of rainfall was redistributed as runoff. This runoffpenetrated approximately 27% deeper under the shrub patches compared with themicrophytic crust. When the microphytic crust was destroyed by simulatedtrampling, the runoff coefficient declined to 13%, and there was no significantdifference in water penetration between shrub and crust patches. Completedestruction of the shrub hummocks and crust by cultivation resulted in adeclinein the runoff coefficient to 6%. The result of sustained disturbance in thesepatchy Negev shrublands is a breakdown in spatial heterogeneity, a loss ofecosystem function, a reduction in ecosystem goods and services such as plantdiversity and production, and ultimately a reduction in pastoral productivity.These results reinforce the view that microphytic crusts are critical for theefficient operation of the Shikim water harvesting system.Given that practices such as cultivation and trampling which disturbmicrophyticcrusts result in enhanced infiltration, crusts should be left intact tomaximisethe water harvesting efficiency in these desert landscapes.