Stability of the earth's ecological systems is undergoing a crisis unknown for some 65 million years. Ecosystems are rapidly breaking down and changing, exposing habitats, species and resources to highly stressful conditions. Best estimates indicate that the present rates of species extinction are 1,000 to 10,000 times greater than the average over geological time.
Because human activity is today's major cause of ecosystem change and loss, researchers are attempting to understand the complex interactions that act to preserve ecosystem stability. With improved knowledge, ways may become available to control ecosystem breakdown and the downswing in landscape and species diversity - or biodiversity. Losses of species and habitats are responsible for desertification, exacerbation of the poverty of local populations and the disappearance of plants, animals and microorganisms whose genetic resources could be essential for ecosystem services and the development of better crops or pharmaceuticals.
The structure and dynamics of ecosystems - with their interacting networks of animals, plants, bacteria and resources - are extremely complex. While interacting mechanisms among species and food-web interconnections have been studied extensively, a major determinant of ecosystem health - landscape or habitat richness - has been largely ignored. When global changes induce a downswing in habitat variety, the varied ecological niches providing sites for species growth begin to disappear, leading to local, regional, or even outright extinction of species.
Our field studies indicate that landscape, resource and species richness are determined by a few animal or plant species, whose powerful effects are the major determinants of ecosystem structure and dynamics. These species, which we call "ecosystem engineers," would include dam-building beavers in North American forests or various shrubs growing in the Negev desert. The dams create water reservoirs that allow for colonization of new species. The shrubs create soil-mounds increasing water infiltration and accumulation of nutrients used by other species. Without recognizing that the engineers are integral to system function, properties, such as ecosystem stability, bio-productivity, biodiversity and resilience to change cannot be properly modeled or understood. Ecosystem engineers should also be taken into account in conservation, restoration and management practices. Fortunately, the number of keystone ecosystem-engineer species is small, and they are susceptible to theoretical, experimental and observational investigations.
In this study, we will probe the roles of plants as ecosystem engineers in drylands, areas where water is the major resource barrier. Our long-term field-research in the Negev provides evidence for the existence of several vegetative ecosystem engineers and demonstrates their impact on resource flow and species richness. These studies found that cyanobacteria and shrubs modulate the landscape, the former, by forming soil crusts, which generate runoff, and the latter, by creating soil mounds, which increase water infiltration. These processes maintain the variety and abundance of herbaceous species growing in the ecosystem. In the northern Negev, where mean annual rainfall is about 200 mm, we have estimated that the engineers enable as many as 300 species to populate the area rather than only 25, in their absence.
Along with field studies, theoretical progress in understanding landscape modulation by ecosystem engineers has also been achieved. Our recent mathematical models for vegetation growth in water-limited systems suggest that vegetative ecosystem engineers, such as trees and shrubs, may self-organize into a variety of spatial patterns. These range from bare soil at low rainfall, via spot, stripe and gap patterns as precipitation increases, to uniform vegetation at high rainfall. These patterns dictate soil-water distributions that are vital for the survival of other species. However, a comprehensive theory elucidating the relationship of such landscape modulation and its attendant resource distribution to biodiversity has not yet been proposed.
With the aid of the James S. McDonnell Foundation, we expect to develop a comprehensive theory that ties the concept of ecosystem engineers to the appearance of vegetation patterns and elucidates the relationships among landscape modulation, resource distribution and biodiversity. The theory will be developed for the case model of waterlimited systems and will be backed up by experiments and field studies along a 500-km rainfall gradient in Israel ranging from 100 to 900 nun mean annual precipitation. In the future, the principles we develop could be applied to other terrestrial ecological systems such as forests, which are nutrient or light controlled.
Our studies will develop novel mathematical models of water-vegetation dynamics along with algorithms for the estimation of species richness. These models will be used to simulate ecosystem-engineer patterns along the rainfall gradient at various topographies and under different disturbance regimes, such as grazing, vegetation clear-cutting, fires and drought. We will identify mechanisms leading to high pattern diversity, including spatial chaos, and investigate the resource niches they create for other species. The algorithm will be used to correlate resource niches to species richness, and to relate global changes affecting ecosystem engineer patterns, to species loss events.
In order to assess the reliability of our models and algorithms, it is essential that their predictions can be confirmed on natural and experimental ecosystems. Israel is the ideal place to carry this out as we have six long-term ecological research sites, running from the central Negev desert in the south to the rainy north; they represent extreme desert, dry shrub-land and Mediterranean woodland ecosystems. In addition, we have the same ecosystem engineer, white asphodel (Asphodelus ramosus), occupying the entire range, enabling the verification of our theoretical approach over an extremely broad band of resource levels.
This is the first modeling effort to elucidate the role of ecosystem engineers in maintaining biodiversity, and the first attempt to experimentally relate ecosystem-engineer resource modulation to species richness. This investigation will help identify how global changes, resulting from climate and land use, will affect the engineers' operation and therefore biodiversity. It will also be important for conservation ecology and for restoration ecology if a key engineering species becomes extinct due to human-induced disturbances. The unified theory to be developed is expected to uncover previously unknown processes operating in the organization and control of life supporting systems and to suggest new ways of ecosystem management.