Robert E. Ulanowicz

The Seasonal Dynamics of The Chesapeake Bay Ecosystem

The full suite of carbon exchanges among the 36 most important components of the Chesapeake Bay mesohaline ecosystem is estimated to examine the seasonal trends in energy flow and the trophic dynamics of the ecosystem. The networks provide information on the rates of energy transfer between the trophic components in a system wherein autochthonous production is dominated by phytoplankton production. A key seasonal feature of the system is that the summer grazing of primary producers by zooplankton is greatly reduced due to top—down control of zooplankton by ctenophores and sea nettles. Some of the ungrazed phytoplankton is left to fuel the activities of the pelagic microbial community, and the remainder falls to the bottom where it augments the deposit—feeding assemblage of polychaetes, amphipods, and blue crabs. There is a dominant seasonal cycle in the activities of all subcommunities, which is greatest in the summer and least in the cold season. However, the overall topology of the ecosystem does not ap...

Compartments revealed in food-web structure

Compartments in food webs are subgroups of taxa in which many strong interactions occur within the subgroups and few weak interactions occur between the subgroups. Theoretically, compartments increase the stability in networks, such as food webs. Compartments have been difficult to detect in empirical food webs because of incompatible approaches or insufficient methodological rigour. Here we show that a method for detecting compartments from the social networking science identified significant compartments in three of five complex, empirical food webs. Detection of compartments was influenced by food web resolution, such as interactions with weights. Because the method identifies compartmental boundaries in which interactions are concentrated, it is compatible with the definition of compartments. The method is rigorous because it maximizes an explicit function, identifies the number of non-overlapping compartments, assigns membership to compartments, and tests the statistical significance of the results. A graphical presentation reveals systemic relationships and taxa-specific positions as structured by compartments. From this graphic, we explore two scenarios of disturbance to develop a hypothesis for testing how compartmentalized interactions increase stability in food webs.

Quantitative methods for ecological network analysis

The analysis of networks of ecological trophic transfers is a useful complement to simulation modeling in the quest for understanding whole-ecosystem dynamics. Trophic networks can be studied in quantitative and systematic fashion at several levels. Indirect relationships between any two individual taxa in an ecosystem, which often differ in either nature or magnitude from their direct influences, can be assayed using techniques from linear algebra. The same mathematics can also be employed to ascertain where along the trophic continuum any individual taxon is operating, or to map the web of connections into a virtual linear chain that summarizes trophodynamic performance by the system. Backtracking algorithms with pruning have been written which identify pathways for the recycle of materials and energy within the system. The pattern of such cycling often reveals modes of control or types of functions exhibited by various groups of taxa. The performance of the system as a whole at processing material and energy can be quantified using information theory. In particular, the complexity of process interactions can be parsed into separate terms that distinguish organized, efficient performance from the capacity for further development and recovery from disturbance. Finally, the sensitivities of the information-theoretic system indices appear to identify the dynamical bottlenecks in ecosystem functioning.

An hypothesis on the development of natural communities

Abstract A knowledge of the flow structure within a natural community is assumed to be sufficient to describe the behavior of far-from-equilibrium, self-organizing systems. This postulate permits the definition of a non-conservative, macroscopic variable quantifying the ascendency of a natural community. Self-organizing, dissipative systems are hypothesized to develop over time so as to optimize their ascendency. These assumptions appear to be supported by observed trends in ecosystem development. The theory possibly provides a caricature of development phenomena common to ecosystems, organisms, economic communities, evolution, and a host of other self-organizing phenomena.

Life and the production of entropy

It appears that living communities serve to augment the rate of entropy production over what it would be in the absence of biota. This hypothesis might be tested by comparing the spectra of electromagnetic fluxes incident to and emanating from the surface of the Earth. An added measure of the value of stored energy to ecosystems is derived by using the economic theory of discounting.

The Trophic Consequences of Oyster Stock Rehabilitation in Chesapeake Bay

There is mounting speculation that overharvesting of oyster stocks (Crassostrea virginica) in Chesapeake Bay may be a factor contributing to the decline in water quality and shifts in the dominance of species inhabiting the estuary. The trophic consequences of increasing the oyster population may be addressed using a simple quasi-equilibrium, mass action model of the exchanges transpiring in the Chesapeake mesohaline ecosystem. According to output from the model, increasing oyster abundance would decrease phytoplankton productivity as well as stocks of pelagic microbes, ctenophores, medusae, and particulate organic carbon. Recently acquired field data on phytoplankton productivity, bacterioplankton, and labile organic carbon in the vicinity of rafted oyster aquaculture support model predictions. The model also indicates that more oysters should increase benthic primary production, fish stocks, and mesozooplankton densities. Hence, augmenting the oyster community by restoring beds or introducing raft culture represents a potentially significant adjunct to the goal of mitigating eutrophication through curtailment of nutrient inputs. *** DIRECT SUPPORT *** A01BY059 00005

A Detailed Guide to Network Analysis

This chapter will introduce a number of the more frequently used network analysis techniques, many of which trace their origins from Leontief’s (1936) economic input-output analysis and Shannon’s information theory (1948). We begin by presenting a standard format for recording all of the flow data related to the ecosystem in a network representation. Having constructed the representation, the direct and indirect relationships that exist among the different components can then be examined in depth using a suite of network analyses that have been developed through the years by a number of researchers. These analyses consist of calculating a number of measures which synthesise some part of the information about flows of energy or materials through an ecosystem. These measures and analytical techniques cover from the microscopic level, the level of a component, right up to the macroscopic level, the level of the whole ecosystem. The measures are presented in a gradation from micro to macroscopic. The set of measures described herein provides a very rich description of an ecosystem; a description which looks at the system from many different perspectives. Clearly anyone applying these measures will have need for only those which reflect the perspectives of interest to the researcher.

Identifying the Structure of Cycling in Ecosystems

Given a steady-state network of flows within an ecosystem, it becomes possible to systematically and automatically enumerate all distinct simple cycles of flows. Cycles may be grouped according to shared smallest arcs. The original network may then be decomposed into a web of purely cycled flow and a residual acyclic graph consisting of once-through pathways. The aggregation of elementary cycles according to shared vulnerable arcs seems particularly effective in locating those transfers which stress is most likely to disturb.

A package for the analysis of ecosystem flow networks

In a single software package NETWRK affords an ecosystem researcher or manager with several new avenues for extracting heretofore unavailable information about the flows of material or energy in the ecosystem. Clues about how an ecosystem may be reacting to a perturbation are often manifested first as changes in the relationships among the constituent taxa. Systematic analyses that quantify these configurational changes are only now beginning to appear as ecological software. NETWRK consolidates four analytical techniques: (1) The study of indirect trophic effects allows the user to infer how a given taxonomic group affects or is affected by other compartments with which it may not be directly connected. (2) The myriad of trophic interactions is simplified into a linear chain of trophic transfers (sensu Lindeman), revealing how efficiently the system is processing the medium in question. (3) The full structure of pathways of recycle is elaborated, possibly depicting domains of control within the system. (4) The overall trophic status of the system is assessed by several indices deriving from thermodynamics and information theory. A before-after comparison of these system indices allows the user to render quantitative judgements about the extent to which the community in question has been impacted. This nine year old program has been tested extensively and is available for use in both MS-DOS and Macintosh environments in either FORTRAN or Pascal. Copies are available from the authors.

Scale and Biodiversity Policy: A Hierarchical Approach

There exists a broad consensus supporting the protection of biological diversity; but the exact meaning of this consensus for policy is not clear. In the United States, for example, the Endangered Species Act emphasizes protection of species. But this emphasis has led to the question: Since approximately 99% of all species that have existed on earth are now extinct, how can it be so urgent that we reduce anthropogenic species extinctions? The standard answer to this question — that extinction itself is not bad, but rather that the accelerated rate and broadened scale of extinctions is unacceptable — likewise raises more questions than answers. One might ask, what would be an “acceptable” rate of extinctions? If species are not sacrosact, what then is the proper target of protection?