The program SPECIES illustrates the way ecosystems develop diversity of species and ecosystem productivity. The species that become part of an ecosystem's self-organizational process are seeded from other ecosystems by dispersal. Birds often fly carrying seeds; other species may be transported by wind or water.

Islands and other isolated locations may not receive as many species by dispersal and are slow to develop as high a diversity as can be sustained. Over long periods of time, thousands to millions of years, micro-evolution may occur with species developing. In the famous example of the Galapagos Islands studied by Darwin, some of the species were found transported from the mainland and others were evolved within the islands, apparently by being isolated temporarily on separate islands long enough to develop different characteristics.

When there are only a few species available to self-organization, the community production may be less than its full potential, because some of the species may not be as well adapted to perform the various functions as those in situations where more species are available.

The model in Figure III-16 shows the interplay of dispersal, local speciation, and production. As the diagram shows, the more species there are, the more of the environmental resource is efficiently used to form products, although production is ultimately limited by the inflow of renewable resource (sun, wind, rain, nutrients). The diagram shows the production partitioned into three pathways. The first is the respiration and consumption that is in proportion to the production (K2*R2). In the second pathway, the produce is utilized in proportion to the square of the number of species (K3*N*N), thus representing the additional energy that goes into competition or into various niches separation mechanisms that prevent that competition. The third pathway (K4*R2) is the produce remaining, which goes into generating new species at a very slow rate (microevolution). The energy for speciation is the energy to support extra individuals with variations that are necessary to the process of isolation and selection.

The diversity N (expressed is species per 1000 individuals counted) is a balance between the immigration and seeding from outside sources (S) and the extinction. Immigration is diminished as the inverse square of the distance of the source (the mainland). Two pathways of extinction are included: one in proportion to the number of species (K6*N); the other in proportion to the square of the species (K5*N*N), the latter representing the effect of competition, consumption, and other interactions among species in causing extinction.

In the simulation, species increase causing production to increase. Both level off as the system becomes limited by resources-including the resource of species availability. Each unit of time represents 100 years and the horizontal scale represents 32,000 years. After the program has plotted one graph, the distance from the mainland, initially 100 miles is increased by 100 miles and run again. The result is a family of curves each for a different distance. As shown in Figure III-16b, increasing distance causes lower diversities and lower productivities.

Notice lines in the program to see how to make a simulation run successive curves. Time and initial conditions are reset; the change is made in the variable under study (in this case distance D); and then the program is sent back to its cycle of iteration.

Other Examples

The model may be applied to other situations where seeding of information is limited by distance so that the rate of developing information anew is occurring at the same rate as dispersal. Other biological examples are species developing on isolated mountaintops where species adapted that can provide seedings are far away.

This model represents the principles of developing and maintaining information, since the seeding of species is really the seeding of genetic information. The model can also be used for information transfer and maintenance in human systems. Cultural information affecting human productivity was transmitted by travelers to remote islands slowly before the days of cheap transportation. There was a limit to the information and technology that could be sustained on a small area.

"What if" Experiments

1. What is the effect of increasing the basic energy sources as if the Galapagos Islands began to receive higher rainfall? Increase J.
   


2. What would be the effect of an increase in losses of living components from pollution so that greater respiration is required for maintenance? Double the value of K2 and K6.
   


3. What is the effect of starting with a larger diversity? Set N to 100.
   


4. What is the effect of a higher concentration of species at the source of seeding. Double the value of S.

COMPUTER MINIMODELS AND SIMULATION EXERCISES FOR SCIENCE AND SOCIAL STUDIES

Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Copyright 1994

Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Enrique Ortega
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007