Eugene P. Odum

Detritus Food Webs in Conventional and No-tillage Agroecosystems

onservation tillage-crop planting systems that leave 30% or more of crop residues on the soil surface instead of plowing them under-is becoming widely adopted in US agriculture. The total area under conservation tillage is estimated to be between 24 and 36 million hectares, or about one-third of the nations cropland, which represents an increase of about 1,25% during the past decade (Christensen and Magleby 1983). The primary reasons for this increase are that conservation

Perturbation Theory and the Subsidy-Stress Gradient

Mans impact on his environment often involves a gradient of subsidy and stress effects, which vary along nutrient, climatic, developmental (successional), and other important ecological gradients. Since performance curves that simulate subsidy-stress responses are unimodal, the zone of optimality can be determined on the basis of relatively few experiments where specific input factors are of concern. We have emphasized the importance of assessing the response of the perturbed system at different levels of organization. In practice, at least two levels, the ecosystem and the population (species) level, are necessary for a complete impact evaluation. We have stressed the need for judicious use of terms and the desirability of sticking as closely as possible to basic dictionary definitions if one expects to communicate with large numbers of people. Most of all, we have tried to show how a systems approach, based on simple flow diagram models, can clarify the intended meaning of terms and the cause-and-effect relations, as well as identify the basic processes involved.

A Hierarchical approach to evaluating the significance of soil biodiversity to biogeochemical cycling

The significance of biodiversity to biogeochemical cycling is viewed most directly through the specific biogeochemical transformations that organisms perform. Although functional diversity in soils can be great, it is exceeded to a high degree by the richness of soil species. It is generally inferred from this richness that soil systems have a high level of functional redundancy. As such, indices of species richness probably contribute little to understanding the functioning of soil ecosystems. Another approach stresses the value of identifying “keystone” organisms, that is those that play an exceptionally important role in determining the structure and function of ecosystems. Both views tend to ignore the importance of biodiversity in maintaining the numerous and complex interactions among organisms in soils and their contributions to biogeochemical cycling. We describe some of those interactions and their importance to ecosystem function.

Nature's pulsing paradigm

While the steady state is often seen as the final result of development in nature, a more realistic concept may be that nature pulses regularly to make a pulsing steady stata—a new paradigm gaining acceptance in ecology and many other fields. In this paper we compare tidal salt marshes, tidal freshwater marshes, and seasonally flooded fresh-water wetlands as examples of pulsed ecosystems. Despite marked differences in species composition, biodiversity, and community structure, these wetland types are functionally similar because of the common denominator of water flow pulses. Often a period of high production alternates with a period of rapid consumption in these fluctuating water-level systems, a biotic pulsing to which many life histories, such as that of the wood stork, are adapted. Pulsing of medium frequency and amplitude often provides an energy subsidy for the community thus enhancing its productivity. The energy of large-scale pulses such as storms are usually dissipated in natural ecosystems with little harm to the biotic network; however, when seawalls, dikes, or stabilized sand dunes are constructed to confront these strong pulses, the whole ecosystem (and associated human structures) may be severly damaged when the barriers fail because too much of the storm energy is concentrated on them. The relationship between biologically mediated internal pulsing, such as plant-herbivore or predator-prey cycles, and physical external pulsing is discussed not only in wetlands but in other ecosystem types as well. An intriguing hypothesis is that ecosystem performance and species survival are enhanced when external and internal pulses are coupled. We suggest that if pulsing is general, then what is sustainable in ecosystems, is a repeating oscillation that is often poised on the edge of chaos.

THE STATUS OF THREE ECOSYSTEM-LEVEL HYPOTHESES REGARDING SALT MARSH ESTUARIES: TIDAL SUBSIDY, OUTWELLING, AND DETRITUS-BASED FOOD CHAINS

Abstract During the first 10 years of salt marsh research at Sapelo Island (1952–1962), three general hypotheses emerged as follows: (1) tides provide an energy subsidy that enhances productivity, (2) organic matter is exported from productive estuaries to offshore waters (outwelling), and (3) detritus rather than grazing food chains predominate in the salt marsh ecosystem. These hypotheses, which we judge to be “emergent properties” of the salt marsh estuary as a whole, have now been challenged and tested in many places up and down the coast. The tidal subsidy hypothesis has been verified sufficiently to stand as a general principle. Outwelling seems to be strictly a local question depending on relative productivity of inshore and offshore waters and the magnitude of water flow in and out of the estuary; some estuaries export while some import, and the material exported (or imported) may involve nutrients, organic matter, or organisms. Although dominance of detritus-based food chains has been verified for most shallow water estuaries (in contrast to dominance of the grazing food chain in open water marine habitats), recent work has indicated that detritus complexes are like autotroph-heterotroph microcosms with algae, protozoa, fungi and bacteria providing major energy sources for detritus consumers which in turn are the chief food for fish and higher trophic levels in general.

The emergence of ecology as a new integrative discipline.

It is self-evident that science should not only be reductionist in the sense of seeking to understand phenomena by detailed study of smaller and smaller components, but also synthetic and holistic in the sense of seeking to understand large components as functional wholes. A human being, for example, is not only a hierarchal system composed of organs, cells, enzyme systems, and genes as subsystems, but is also a component of supraindividual hierarchal systems such as populations, cultural systems, and ecosystems. Science and technology during the past half century have been so preoccupied with reductionism that supraindividual systems have suffered benign neglect. We are abysmally ignorant of the ecosystems of which we are dependent-parts. As a result, today we have only half a science of man. It is perhaps this situation, as much as any other, that contributes to the current public dissatisfaction with the scientist who has become so specialized that he is unable to respond to the larger-scale problems that now require attention. There is a rich literature on hierarchal theory and philosophy which deserves to be read by todays specialists (1). As expressed by Novikoff (2), there is both continuity and

The Cybernetic Nature of Ecosystems

Cybernetic systems are systems with feedback (Wiener 1948). They are a special class of cause-and-effect (input-output) systems in which input is determined, at least in part, by output. The portion of output that is returned to input is the feedback, and this may become the basis for feedback control. Very small feedbacks may exert very large effects. Figure la illustrates a basic input-output system. Energy, matter, or information coming from the environment causes the system to respond; this reaction is transmitted as energy, matter, or information output back to the environment. Figure lb shows a feedback system consisting of two component subsystems. This system is determinate because its behavior is governed only by past causes. Its feedback structure may passively or emergently make its behavior stable, regular, or otherwise predictable, and may enable it to damp disturbance. Figure Ic illustrates a feedback control system in which the feedback subsystem is a controller through which information about desired output can be introduced. Actual output information is fed back to the controller, and the deviation of actual from desired becomes the basis for corrective action. This system is teleological because its behavior is guided by future or desired goals. The behavior is stable, regular, and purposeful since the feedback organization has been designed to actively achieve such characteristics. Feedback control systems may be manmade or natural, and may have living or nonliving components. Controllercontrolled system pairs that are common in experience include thermostatfurnace, guidance system-missile, driver-car, eye-hand, brain-body, etc. Both determinate and teleologic feedback systems are cybernetic because they contain feedback.

Homeostasis of the Nonfat Components of Migrating Birds

Obesity in migratory birds appears to differ from obesity in man in that gains and losses in body weight do not involve changes in the tissue structure of the body; fat is added to and used from preexisting tissue spaces without appreciable change in the water content or the nonfat dry weight of the body as a whole. Evidence is presented which supports the hypothesis that the nonfat body is essentially homeostatic during migration despite very large scale changes in total body weight.

Soil biology, soil ecology, and global change

SummaryThis overview paper addresses aspects of scaling in space and time, and scaling in relation to micro-and macrohabitats. Ecological processes in soils are examined for possible generalizations about processes and organisms, across a wide range of different habitats. Problems of scaling in space and time that have an important impact on processes associated with global change are outlined.

Tidal Marshes as Outwelling/Pulsing Systems

Now that we are beginning to understand that the balance of nature is a pulsing one and not a steady-state as is the case at the organism level (i.e., homeorhesis rather than homeostasis) estuaries become important sites for research because they pulse so strongly. The external tidal pulses interface in a complex manner with internal biological and life history pulses. I review the concept that productivity of near shore ocean waters can be enhanced not only by upwelling of nutrients from deeper waters but also by outwelling of nutrients and organic matter from fertile estuaries. I conclude 1) that the extent of the latter depends on the level of production within the estuary, the tidal amplitude and the geomorphology of the estuarine landscape and 2) the outwelling from tidal marshes where it occurs is often intermittent and largest during rain storms and storm tides.