Scripts for "Ecosystems"
How Does Energy Flow Through Ecosystems?

An ecosystem consists of all organisms living within a community plus the physical features of the environment. The organisms are called the biotic factors of the community and the physical environmental contains the abiotic factors. As with communities, the size of a given ecosystem is defined by the researcher. An ecosystem can be as large as the Saharan desert or as small as the life within the aquarium shown here.

Production efficiency
Production efficiency is defined as the percentage of energy assimilated by an organism that is converted into biomass. Biomass is all of the matter that forms the organs and tissues within body of an organism. In the example shown here, energy is stored within the leaves eaten by a caterpillar. The caterpillar acquires 1000 joules of energy during a certain period from these leaves. However, the caterpillar cannot digest all of the tough plant matter, and 500 joules of energy is lost in the feces. During the same time period, another 320 joules of energy is lost as heat or utilized in metabolism. This leaves 180 joules for production of new tissues as the caterpillar grows. Dividing 180 by 1000 gives a value of 0.18, meaning that only 18% of the energy taken in by the caterpillar is converted into biomass. Thus production efficiency is 18%.Production efficiency of insects is usually between 10 and 40% which is relatively high. Microorganisms also have high production efficiencies due, in part, to their rapid rate of reproduction. On the other hand, vertebrate animals tend to have much lower production efficiencies because they devote more energy to metabolism than to producing more biomass. For example, production efficiency in this squirrel is only 1.6%.Production efficiency also varies among vertebrates, depending in part on the extent to which they utilize energy to main a constant body temperature. Ectothermic vertebrates, such as fish and reptiles, have production efficiencies around 10%, whereas the value is in the range of 1-2% for the endothermic mammals and birds. One consequence of this for terrestrial ecosystems is that a sparsely vegetated region, such as a desert, can support healthy populations of snakes and lizards, whereas mammals might starve.

Energy is lost
The loss of energy with each transfer up the food chain can be represented by a “pyramid of production” as shown here. Each block of the pyramid represents a trophic level and the number followed by “J” is a measurement of the amount of energy stored in each level. “J” stands for Joule, a unit of energy that equals 0.24 calories. Note that of the total energy in sunlight surrounding primary producers, only 1% is stored in their collective bodies. At each transfer of energy to a higher level, 90% of this energy is lost. So at the level of a tertiary consumer, like a snake, only 0.1% of the energy originally stored in plants is available.This pyramid adds figures from a real ecosystem in Michigan. The ecosystem contained about 6 million plants, which contained enough energy to support about 700,000 primary consumers (mainly small animals such as insects). These small animals could support 350,000 secondary consumers, but there were only 3 tertiary consumers in the ecosystem. These 3 animals were the “top carnivores” in the system—such as snakes, owls or foxes. So the loss of energy at each transfer results in fewer organisms at each higher trophic level.Humans eat plants and animals and are thus both primary and secondary consumers. The pyramid of production shows us that there is much more energy available to humans from plants than from animals that feed on the same energy base.

Productivity pyramids
In a few rare cases, even a pyramid of biomass can be inverted. In this aquatic food chain, the primary producers, phytoplankton, contain a high amount of energy and also reproduce much faster than the zooplankton herbivores which in turn reproduce faster than fish. Thus, although the biomass of phytoplankton is relatively low at any point in time, over a given time period a large biomass has been produced and eaten.
If the same food chain is used to construct a pyramid of production, the typical shape is restored. Since productivity pyramids are based on the rate of energy production, they are never inverted. The laws of thermodynamics have not been violated and the highest amounts of available energy are always found at the lowest trophic level.

What Factors Influence Primary Production In Ecosystems?

Photosynthetic Organisms
All organisms that make organic compounds by photosynthesis are primary producers. This includes the photosynthetic prokaryotes and single-celled protists as well as the multicellular algae and plants. Together, these organisms account for most of earth's biomass. While prokaryotes that make organic compounds by chemosynthesis are also primary producers, they are few in number, so contribute little to the total primary production on earth.

Most Limiting Nutrient
A lack of nutrients in usable form can limit plant growth. This is why farmers add fertilizer to their fields to increase crop production. Nutrient limitation is illustrated in this graph of a salt marsh in Canada where both phosphorus and nitrogen were present in minimal amount. When phosphorus alone was added to the marsh, primary production did not change. When nitrogen alone was added, production doubled showing that nitrogen was the limiting factor in plant growth in the marsh. However adding both nitrogen and phosphorus resulted in an even greater increase in primary production. Thus, once sufficient nitrogen was present, plant growth was limited by lack of phosphorus. These results illustrate an important principle which states that species biomass is limited by the scarcest factor. In the case of the marsh, when both nitrogen and phosphorus became abundant, productivity increased but then became limited by another nutrient.

Availability of Light
In aquatic ecosystems, light availability (rather than water) is a major limiting factor. This is because light rapidly dissipates as it penetrates into water. In lakes and coastal waters, more than half of the solar radiation has been absorbed one meter below the surface, and light rarely penetrates beyond 50 meters (164 feet). Even in the clear water of the open ocean, light cannot penetrate more than 200 meters. Thus most of the marine environment is too dark to support photosynthesis. Primary production by cyanobacteria and algae is thus limited to the upper 50-200 meters of water in all aquatic systems.

Low Nutrient Levels
The most important nutrients affecting primary production in aquatic ecosystems are nitrogen and phosphorus which are usually present in very low concentrations. For example, the nitrogen concentration of soil is about 0.5% as compared to 0. 00005% in sea water. Much of the nitrogen in marine environments is on the ocean floor where many decomposers live. In some areas of the ocean, upwelling currents carry nitrogen-rich sediment from the ocean floor to the surface. Primary production is high in these regions, and consumers such as fish are also plentiful. Some of the largest regions of upwelling occur in the Antarctic and along the coasts of Peru and California. This chart compares the rate of primary production in 7 aquatic habitats. Note that the highest productivity is found in algal beds and coral reefs which are located in warm, shallow water in which nutrients can be recycled.

Why Are Biogeochemical Cycles Important?

When considering an ecosystem, the abiotic factors fall into two categories: resources, such as nutrients and water, and other physical factors such as atmospheric conditions and temperature. We will first consider nutrients. You have no doubt seen a periodic table of the elements in your high school chemistry class. Of the 92 stable elements, only 30 or so are vital to life and thus considered nutrients. Some nutrients, such as iron and iodine, are only needed in tiny amounts, while others are required in large quantities. Most important among the latter are carbon, nitrogen, oxygen, and phosphorus. They are shown in blue on the table.
Carbon is the element that forms the backbone of organic molecules. These molecules in turn form the structure of all cells and tissues. Nitrogen and phosphorus are needed in smaller amounts, but are a crucial part of key molecules such as DNA and proteins. Oxygen is the gas that we breath and is crucial for cellular respiration. It also combines with nitrogen and phosphorus to form biologically active compounds.
And of course there is water, which is composed of the elements oxygen and hydrogen. Hydrogen also combines with other elements and is found, for example, in ammonia, methane, and all organic molecules.

Produced by Humans
Throughout most of human history, nitrogen has been fixed mainly by bacteria. Nitrogen is essential for plant growth, and for centuries, agriculture was limited by a lack of sufficient nitrogen in the soil. You probably remember the story of Indians helping the pilgrims grow corn by burying dead fish in the soil—an early attempt to add nitrogen. As the importance of nitrogen to crops became better understood, manure was used and legume crops were periodically grown, since they contained nitrogen-fixing bacteria within their roots.
Early in the 20th century, a major boost to agricultural occurred when an industrial procedure for fixing atmospheric nitrogen was developed. Now, liquid ammonia can be converted into ammonium and nitrate compounds on a large scale and used as fertilizer.
Since the 1960’s, nitrogen-based fertilizers have been used extensively to grow more crops on the same amount of land. The practice has become necessary in order to produce sufficient food for the growing human population. So much human-made ammonium and nitrate is added to the ecosystem, that 24% of all terrestrial nitrogen fixation is now of human origin.

Fresh Water
Of all the water on earth, only 2.5% is fresh water. Most of this fresh water is locked up in glaciers and in the polar ice caps. The main source of usable fresh water is ground water, but it is only 0.5% of the earth’s water. Note that the total fresh water supply in lakes and rivers is relatively quite small.

Human Use
Fresh water is becoming a scarce resource. Humans now use 54% of all the accessible fresh water on earth. This leaves only 46% to support all other forms of life. The availability of uncontaminated water is also a problem. More that a billion people around the globe lack access to a consistent supply of clean water. The problem is greatest on the continents of Asia and Africa.