Chesapeake Bay Trophic Network
In tracing the trophic connections among the populations of even the simplest ecosystems, the emerging picture soon comes to resemble a hopeless jumble, sometimes referred to as a "bird's nest" or "spaghetti" diagram. Yet within such a depiction lie valuable clues to how the ecosystem is functioning. "Ecosystem network analysis" is the rubric applied to a collection of quantitative methods that systematically teases most pertinent information from the full, complicated network description.
Basically, an analysis of an ecosystem trophic network requires that one know "Who eats whom?" and "At what rate?" One usually begins by defining the key component populations that comprise the ecosystem, and then choosing a medium of exchange (e.g., carbon, nitrogen, phosphorus, energy, etc.) The bulk of one's work is devoted to estimating the rates of exchange of this medium among all the compartments. Software packages to assist an investigator in constructing and balancing such networks of exchanges currently are under development by Villy Christensen (ECOPATH2) and Lewi Stone (CARBON).
Ulanowicz is now collaborating with NOAA's Great Lakes Environmental Research Laboratory in the creation of Windows version of the Network Analysis routines. To download the latest version, please go to EcoNetwrk.
In addition, Drs. Cristina Bondavalli and Stefano Allesina of the University of Parma are writing some NETWRK- based software for the Windows environment. It is called the WAND Project.
For those who still like to work in a DOS environment, several analysis packages can still be downloaded from this page:
A new and improved package to analyze a network at several hierarchical levels. The bilateral relationships between any two system components via all trophic pathways are quantified. A description of how much each population feeds at the various trophic levels is provided, and the complex web of relationships is mapped into an equivalent "Lindeman trophic chain", useful for evaluating the efficiencies of the overall system at each level. Thirdly, a number of information- theoretic indices that can be used to describe the organizational status ("health" or "integrity") of the network as a whole are calculated. Finally, all the pathways for recycle within the system are listed, and the system network is decomposed into two webs - one that consists entirely of recycled flow, and the other an acyclic "tree" of straight- through, dissipative flows. NETWRK will run on unbalanced networks, but it is adviseable to use networks wherein each compartment balances to within + or - 5%. To view or download the program documentation, press here. Download the file netwrk.zip.
A flow network is a snapshot of ecosystem kinetics at some specific time or stage. It contains no description of system dynamics. The investigator can use AUTOMOD to assign one of three generic dynamical functions (linear, donor control; Lotka- Volterra; or saturation- type limitation) to all the transfers. The system then can be integrated forward in time to see how it might react to various perturbations in the system parameters. Download the file automod.zip.
A trophic exchange usually has a negative direct effect on the prey population and a positive one on the predator's. These direct impacts are propagated both up and down the trophic web. IMPACTS calculates the overall trophic effects (both direct and indirect) that any given population has on any other. Download the file impacts.zip.
Several utility routines that perform simple operations on datafiles, such as reordering the components, aggregating elements, etc. Download the file misc.zip.
All programs listed here use data stored in standard SCOR format (described in the documentation to NETWRK.) DATBAL is a screen routine that prompts the user for keyboard entry and automatically creates a file in SCOR format. It also can be used to edit existing SCOR data files and to balance automatically any network so that the inputs and outputs associated with each system component are equal. Download the file datbal.zip.
A collection of data on 48 sample flow networks. Data are provided in standard SCOR format, along with a description of the format and a reference for each network. Download the file datall.zip.
All programs were written in primitive FORTRAN and compiled for MS-DOS.
Source codes can be obtained from the author
Other investigators with homepages dealing with ecosystem network analysis include James J. Kay, Robert Christian, Joesph Luczkovich, Dan Baird,...
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