This is a model of ecological succession leading to a climax ecosystem. On a bare field a few grasses and herbs grow producing a small amount of biomass (B in Figure III-15). In later seasons taller grasses and shrubs replace these and then later tree seedlings and larger trees. The growth of this biomass depends on renewable outside energy sources (I) but also on the diversity (D), the number of different kinds of plants and animals. As the quantity of biomass increases, it provides niches for seeding from outside to increase the diversity. The greater the diversity the more productivity. This increases biomass. The numbers in the program of this model were from a tropical rain forest in Puerto Rico.
In this model diversity is the number of species per 1000 individual organisms. The number of species (D) is a balance between new species evolved and seeded and those lost by local extinction. The more biomass and the more diversity there are in a system the more productivity is required to maintain them. In the model the extra energy needed to support the diversity is indicated by the quadratic drains from the biomass storage (K5*D*D).
For example, in the tropical rain forest there are many species of frogs. It has been necessary for them to use extra energy to develop different colors, habitats, croaks, and mating behaviors to enable them to occupy different niches and survive together. However, diversity increases division of labor and improves efficiency. Depreciation and respiration of biomass are indicated by the pathway K6*B.
Often complex ecosystems are used by humans who harvest a yield of wood or wildlife. The yield from the system is K4*B. In the tropical forest example, this was the cutting and export of selected trees without clear-cutting. The varieties of plants and animals depend on the climate, I, which includes sun, rain, wind, tide, temperature and seasonal changes.
The simulation graph in Figure III-15 shows growth of the productivity, biomass and diversity. Gross production (PG), which is proportional to the outside energy sources (I), increases with biomass and diversity and levels quickly. Net production (DB) is the growth of biomass minus its respiration and use for diversity. At first most of gross production is net gain going into biomass. When gross production is no longer increasing, net production decreases as more of the production goes for maintenance. Biomass and diversity both level to a steady state.
Examples of Succession and Climax Systems
Ecosystems with similar species of plants and animals are found in similar climate zones. The major types of ecosystems with their typical organisms are called biomes. The climax species in the grasslands biome are grasses; in coral reefs they are coral; in the tundra special grasses and herbs develop which can live in a short growing season and permafrost soil. Different species occupy the same niche in different areas. As examples we have the large herbivores in the tundra, the caribou live in the tundra in North America and the reindeer in the same niche of the tundra in Northern Europe.
Human civilizations also go through succession and climax. They start with a few people in an area; the population grows as the economic assets grow. Finally, at least for a time, the culture may come to a steady state climax with high diversity. Businesses may go through stages of growth (succession) and then, if they survive, they come to an equilibrium (climax).
"What if" Experimental Problems
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
Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007