Introduction

 The Amazon Rainforest is an incredibly complex ecosystem.  The unique conditions of an almost constant temperature and a high, regular precipitation rate are what allow the incredible amount of biodiversity to occur there.  Because of its size and high rate of productivity, the Amazon Rainforest ecosystem plays a substantial role in many of the biogeochemical cycles of the world.

Definition of an Ecosystem -

An ecosystem is defined as an open thermodynamic system composed by the living community or communities and their abiotic surroundings in which movements of matter (nutrients) and energy take place.  Living matter and organisms cannot exist without the following abiotic factors:
• Atmosphere (air)
• Hidrosphere (water)
• Litosphere (soil and rocks)
All the components of an ecosystem maintain the open exchange of matter and energy on which the system is based.  There are several attributes inherent to ecosystem that are based in this exchange:
• Primary productivity - Energy fixation rate by primary producers, of which 99.99% are photosynthetic organisms, although chimiosynthetic organisms are considered primary producers as well.
• Secondary productivity - Energy and matter fixation rate by consumers.
• Decomposition rate - Velocity at which organic matter is degraded into chemically and physically simpler elements.
• Use of energy efficiency:
o Ratio of the rate of the matter and energy that is fixed in a trophic level of the ecosystem to that of a previous level.
o Inefficiency: Food that is excreted and/or not used, that is the matter and energy used in metabolism.
o Efficiency: Ingested and assimilated food, as well as matter and energy used in the production of new tissues.
• Standing biomass: All the matter of the ecosystem (dry weight of the organisms.)
• Velocity and pattern of circulation of nutrients: Velocity and places by which the nutrients (matter) travel.
• Velocity and pattern of circulation of energy:  Velocity and places by which each particle of energy that enters the ecosystem travels.
Ecosystems share some attributes with communities as well:
• Trophic structure - Trophic and connecting levels in regard to nutrient and energy circulation.
• Key species - These species are crucial because their activities determine the pattern of circulation of matter and energy and help maintain the ecosystem's equilibrium.

Energy
 
 

As we can see from the red arrows, the movement of energy is open and in only one direction. This means that the energy goes through each trophic level one time. As it goes from one level to another, it is lost with metabolism and in the form of heat. This means that, for example, the energy ingested by producers does not pass completely to consumers. After the energy is lost as heat, it cannot be used for work anymore.
 
 

Matter

Matter, on the other hand, has a closed cycle and can follow several paths. Matter can't be lost from the ecosystem, because all nutrients are degraded and restored to the cycle.
Matter is distributed in air (parts of plants above the ground), soil (chemical elements, organic matter, and dead matter), and roots (parts of plants under the ground). This distribution varies from ecosystem to ecosystem. (BIOLOGY)

The Amazon Rainforest Ecosystem

The two factors which enable a rainforest to exist are an almost constant temperature and a high, regular precipitation rate.  These two characteristics are the basis for all functions which occur.
The actual forest begins with a thick, nearly impenetrable wall of trees, vines, and shrubs. Once inside this wall, the forest is actually very open, with relatively little underbrush. The forest is comprised of several canopy levels, each with its unique group of flora and fauna.  The physical structure of the forest is very efficient, capturing most of the sunlight before it reaches the ground story.  This is the reason for relatively little vegetation on the floor of the forest.
Most of the nutrients in the forest is locked up in the vegetation.  Unlike what one would assume, the soil is not very rich or thick.  It is estimated that there is only about 1" of leaf litter and 1"-2" of topsoil.  However, the conditions in the rainforest (high moisture and warm temperature) allow for a very quick decomposition rate, so nutrients are recycled very quickly.  One major problem is that when trees are logged, nutrients are taken away from the system with no way for them to be returned. (Newman)
Growth Cycle

 The growth cycle in the Amazon begins with an opening in the canopy known as a gap.  The gap can be caused by trees naturally dying or by being removed by deforestation.  This is followed by the building phase (a state of re-growth,) the mature phase, and then back, in some cases, to the degenerative phase.  When gaps are too large, such as what can happen with deforestation, the climax (present) species gives way to other pioneer species.  Once this happens, it is very difficult for the forest to return back to its initial composition and diversity. (Whitmore)

Nutrient Cycles

 The most important biogeochemical cycles in the Amazon Rainforest are the Carbon, Nitrogen, Oxygen, and Hydrogen cycles.

It is important to look at these cycles because even minor disruptions in flow, with either inputs or outputs, can greatly alter the transfer of the element, and therefore the ecosystem.  This can cause either an accumulation of a certain element in a form that might be harmful (such as C in the atmosphere) or a lack of necessary elements (such as nutrients being removed from the cycle by logging.)  (BIOLOGY)
 One important tool to help understand nutrient cycles and to predict possible future problems is computer modeling.

Characterization of Modeling Techniques

 Two programs that we used to help understand the nutrient cycles and the ecosystem are VenSim and ArcView GIS.

VenSim is a program that models system dynamics.  The user sets different levels and rates of flow for some element, and the model calculates how the system behaves over time.  This is a very powerful tool for looking at the nutrient cycles.
One reason VenSim is such a useful program for modeling is its extensive data analysis capabilities.  Once data sets are created, they can be viewed as graphs, tables, and causes strips.  Also, several different data sets can be viewed simultaneously, allowing for instant comparisons.

ArcView Gis is a mapping program that with the use of ArcView Spatial Analyst can be used to do a Population Viability Analysis.  Population Viability Analysis is a technique for determining the probability that a species will become extinct within any given time period.  PVA has the unique ability to take into account a great deal of subtle information about a species, including the age-specific birth and death rates, the ratio of males to females at birth, the negative effects of inbreeding on small populations, and even randomly occurring calamities, such as floods and climatological events.  Since many aspects of PVA models are randomly determined, a model is normally run many times in order to establish the statistical properties of the results.  Thus, a common output of these analyses is a histogram of Time to Extinction, from which it is possible to determine the probability that the species will have gone extinct at a given time from present.