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Ecosystems: Introduction

Note on Field Science: LabLearner is a lab-based science education program. You have learned many important scientific concepts, facts, and skills over the years. However, as a lab-based system, certain important areas of science have been limited simply for practical reasons. That is a shame because some of the most exciting and important areas of science take place outside, “in the field”. Below are just some of the areas of scientific research and work that will take you right out of the laboratory or library and put you at the rim of a volcano or swimming with sharks!


LabLearner founder Dr. Verner once spoke with his colleague and friend, Dr. Alan Walker, and asked him what was the thing he liked least about working in the field as a paleontologist at an African dig site where he was instrumental in the reconstruction of a nearly complete, 3.5 million-year-old prehuman Homo Erectus skull. Dr. Walker’s reply was short and simple, “Lions!”. This is not an answer you are likely to get from a microbiologist or automotive engineer. Field science is definitely a unique blend of hard science and adventure.

Ecosystems are studied by a hardy and dedicated group of outdoor scientists. The field of biology that deals with ecosystems is termed ecology. However, there are many, many different kinds of ecologists. Some work on the ocean, some in jungles, some in deserts, and some in urban areas where they study the interaction between plants, animals, and man. Ecologists spend a certain percentage of time in the field collecting data and samples. They usually also spend time in the lab analyzing field samples and data, writing scientific papers, giving lectures, and teaching.


Ecosystems: Biotic and Abiotic Factors

An ecosystem is composed of all the organisms that live in a given area, that is the biotic factors, as well as the abiotic factors with which those organisms interact. Abiotic factors are defined as those factors within the ecosystem which are not alive. For example, a pond ecosystem is made up of the animals, plants, and microorganisms living in or around the pond as well as the water, light, dissolved gases, chemicals in the water, and so on. In an ecosystem, the biotic organisms interact not only with the abiotic factors but with each other.

Major Ecosystems of the World

 How is an ecosystem identified? An ecosystem can be identified by its geographic location (i.e., a tropical rainforest), but its boundaries cannot be clearly assigned. For example, a suburban neighborhood could represent an ecosystem. A block within that neighborhood cannot be isolated and referred to as an ecosystem by itself. On the other hand, a subset of the entire neighborhood may be defined as an ecosystem. For example, all the areas of the neighborhood that receive full Sun in the morning every day could be classified as an ecosystem. Organisms may inhabit specific regions of an ecosystem during specific parts of their day. In the full Sun ecosystem for example, some animals and insects may move into this area and inhabit it for the duration of the sunlight, then move elsewhere as the sunlight wanes. Because these organisms move into and out of this specific area of the larger neighborhood ecosystem with regularity, they have created a smaller ecosystem within the larger one that exists during a specified time each day.


Eight Great Ecosystems

Temperate forests can be found in much of Europe and vast stretches of eastern United States. It is characterized by trees that lose their leaves in the fall. Temperate forests receive high levels of precipitation and are therefore relatively humid.

Temperate Forest Ecosystem

Temperate forests can be found in much of Europe and vast stretches of eastern United States. It is characterized by trees that lose their leaves in the fall. Temperate forests receive high levels of precipitation and are therefore relatively humid.

Tropical Rain Forest Ecosystem

High rainfall characterizes tropical rain forests. Located near the Equator, tropical rain forests also have high temperatures year-round. The combination of very high precipitation, sunlight, warm temperatures, and humidity makes vegetation lush and dense with tall trees that do not lose their leaves during the year.

Desert Ecosystem

Annual rainfall in deserts is minimal – zero to less than 25 centimeters per year. Few plants can grow under these conditions and animal life is scarce as well.


Grassland Ecosystem

Grasslands receive plenty of water and support the growth of dense expanses of rolling plains of grass and low shrubs. Few trees populate grasslands. Large herds of herbivore animals can be maintained in the ecosystem, along with much fewer carnivores that prey on them. Natural grasslands of central and western United States were cultivated by eighteenth and nineteenth settlers and now represent some of the most productive agricultural land on Earth.


Taiga Ecosystem

Taiga ecosystems are characterized by an abundance of coniferous trees like spruce and fir. These evergreen trees have needles rather than broad leaves that are not shed in the winter. Thus, taiga ecosystems are green all year.


Tundra Ecosystem

The ground in tundra ecosystems is frozen throughout the year and is referred to as permafrost. Few if any trees service in the tundra and those found there are short and ragged. Migrating mammals and birds populate the tundra at times during the year.


Chaparral Ecosystem

In the United States, the chaparral ecosystem is confined mainly to California. It is characterized by moderate rainfall in the winter and extend dry periods in the summer. The chaparral ecosystem supports small trees (2 – 3 meters) that may lose their leaves during dry periods. Deer, rabbits, and other small mammals populate the chaparral, as do coyotes, lizards, and other preditors. 


Ocean Ecosystem

The ocean is by far the largest ecosystem on Earth. The Earth’s oceans, while given distinct names in different locations (e.g. Pacific Ocean, Atlantic Ocean, Indian Ocean, and so on), is one large body of saltwater that covers approximately 75% of the planet’s surface. By contrast, all of the freshwater in lakes and rivers make up less than 1% of the Earth’s surface. As with other ecosystems, the vast oceans of the world may be further divided into small ecosystems such as beach ecosystems, coral reef ecosystems, open ocean ecosystems, and so on.


Worldwide Distribution of Major Ecosystems

The information thus far may lead you to conclude that ecosystems are discrete, easily identified entities. But this is not so. For example, an ecosystem might be identified by its geographic location (i.e., a tropical rainforest, desert), but its boundaries cannot be clearly assigned. Let’s take a suburban neighborhood for example. The entire neighborhood could represent an ecosystem. On the other hand, a subset of the entire neighborhood may be defined as an ecosystem as well. For example, all the areas of the neighborhood that receive full sunlight in the morning every day could be classified as an ecosystem. Organisms may inhabit specific regions of an ecosystem during specific parts of their day. In the full sunlight ecosystem, for example, some animals and insects may move into this area and inhabit it for the duration of the sunlight, then move elsewhere as the sunlight wanes. Because these organisms move into and out of this specific area of the larger neighborhood ecosystem with regularity, they have created a smaller ecosystem within the larger one that exists during a specified time each day.

The same thing can happen in nature as a whole. Some animals only occupy a given ecosystem at certain times of the year. We are all familiar with bird migrations, for example. A noisy flock of geese fly overhead, and we say that they are “heading south for the winter”. Therefore, at certain times of the year, an ecosystem may host one set of animals that are not present at other times of the year.

One consequence of a lack of discrete boundaries is that ecosystems tend to overlap one another. This allows organisms to move in and out of multiple ecosystems with relative ease. A prime example of this situation would be the world’s major oceans. In reality, they are a single large body of water that humankind has subdivided based on climate and geographical location. This body of water is an extremely large marine ecosystem made up of many smaller, overlapping ecosystems. Whales, other sea mammals, and birds routinely winter in warmer waters but move north to their breeding grounds during the spring. However, geography and climate are not the only things that define an ecosystem. A reef ecosystem differs from the open sea surrounding it, in that certain sea creatures inhabit the reef and never swim or otherwise move away from the reef. Meanwhile, some predator fish such as sharks may visit the reef at a specific time of day but spend the rest of their time in the open ocean. Similar patterns occur in land and estuarine-based ecosystems.

Pacific coast salmon are an example of a species that lives in one ecosystem during part of their lives and another ecosystem at others. Salmon eggs hatch in freshwater rivers that feed into estuaries and then the Pacific Ocean. After hatching, the young salmon spend time in the freshwater rivers and lakes. As they mature, they migrate as smolts (nearly adults) downstream to the ocean, where they feed and grow for a period of a few years. As mature adults, they leave the marine ecosystem and return back to the exact same freshwater rivers and streams where they hatched years earlier and spawn (reproduce). Adult Pacific coast salmon then die very shortly after spawning. Thus, during their short lifetimes, such salmon live in several entirely different ecosystems (freshwater streams, brackish estuaries, and marine ocean).


Producers and Consumers
The organisms within an ecosystem are classified as either producers or consumers. Producers are autotrophic, meaning that they are capable of capturing and storing energy for themselves, they do not consume other organisms. The word autotroph literally means “self-feeder”. Autotrophs can be further classified as either chemoautotrophs or photoautotrophs depending upon the source of energy that they capture. Chemoautotrophs (chemotrophs) capture energy from inorganic chemical sources, usually gases such as hydrogen sulfide. Photoautotrophs (phototrophs) capture light energy. It is the phototrophs that we will mainly focus on as they are responsible for photosynthesis, which is of fundamental importance for most all ecosystems. Plants and most algae (green algae) have chloroplasts and are phototrophs.

Consumers are heterotrophs, meaning that they must obtain energy by consuming other organisms. Heterotrophs can be divided into four major classes: herbivores, carnivores, omnivores, and detritivores (decomposers). Herbivores consume only plants or algae. Omnivores and detritivores (decomposers) consume both plants and animals. Carnivores consume other animals. Detritivores (decomposers) differ from herbivores, carnivores, and omnivores in that they consume dead organic material rather than living organisms.

There are three types of detritivores (decomposers): scavengers, bacteria, and fungi. Scavengers are opportunists. They feed from the remains of organisms killed by predators after carnivores and omnivores have eaten from the carcass. Scavengers come from all parts of the animal kingdom, including insects. Detritivores (decomposers) serve a significant role in nature, especially the bacteria and fungi. Bacteria and fungi complete the cycle of returning nutrients to their inorganic forms for conversion by breaking down the remaining dead tissues of plants and animals through decomposition.


Trophic Levels

Ecosystems have a structure of feeding relationships called trophic levels. A trophic level is a group of organisms that is identified by the organisms’ food source. The supporting trophic level for all other levels in an ecosystem consists of primary producers. Primary producers are responsible for converting non-organic energy sources (that is, not from living or dead organisms) into organic energy sources. Once converted into organic forms, energy moves from one trophic level to the next as organisms are consumed and digested. The figure here illustrates the organization of trophic levels (primary produces, primary consumers, secondary consumers, and detritivores) within an ecosystem and the path that energy takes as it moves from one trophic level to the next.

As energy moves through the system, it is transformed from light (solar) energy to chemical energy and heat. Heat represents energy lost to the ecosystem. Heat is not a true loss as energy cannot be destroyed (Law of Conservation of Energy). However, it is a loss in terms of no longer being in a form that can be used by organisms in the ecosystem. An important characteristic of energy flow through an ecosystem is that the flow is unidirectional. As the figure above illustrates, energy always moves through the different tropic levels in one direction and does not return from one level back. This is significantly different from the flow of nutrients within an ecosystem, as nutrients cycle from inorganic forms to organic forms back to inorganic forms as they pass through the ecosystem.

Not all of the energy from the Sun which reaches the Earth is captured by plants. Of the 1022 joules of energy that reaches the earth on a daily basis, only one percent, or 1020 joules, is captured by plants. The remaining 99 percent is reflected or absorbed by bare soil, rocks, and water. The percentage of energy from the Sun that is captured varies from ecosystem to ecosystem. There are several reasons for this variation. Organisms differ in number and concentration among ecosystems. A rainforest ecosystem will capture and convert significantly greater amounts of light energy than a desert ecosystem due to the differences in plant populations. In addition, light intensity varies due to geographic location and season. Ecosystems located along the equator receive a constant level of light throughout the year. However, light intensity varies with season for ecosystems located North and South of the Equator, with the greatest variation due to season occurring at the North and South Poles.

Light level can also be affected by the presence of dust particles or clouds in the atmosphere, as dust and water vapor decrease the amount of light that can reach the Earth from the Sun. The effect of dust in the atmosphere is most noticeable after catastrophic events such as volcanic eruptions or large wildfires. A series of volcanic eruptions in the early 1800s led to the famous “year without a summer” in Europe and North America in 1816. The dust resulting from the massive eruptions of volcanoes greatly decreased the light energy reaching the Earth. This massive dust cloud caused a summer of freezing weather that prevented crops and other plants from growing.

As with energy capture, energy transfer between trophic levels is inefficient. On average, only 10 percent of the energy present in one level is transferred to the next. This is known as a “10% rule”. The overall effect of decreasing energy levels from the Sun to the detritivores (decomposers) is described by scientists as a pyramid of production and can be illustrated as shown here.

Inefficiency in energy transfer between trophic levels is due to more than just the loss due to heat. Heat is a byproduct of cellular respiration, which is an exothermic process. An exothermic reaction is one in which heat is released. Cellular respiration provides organisms with another form of chemical energy (in the form of high-energy phosphate bonds) as the result of breaking down carbohydrates, lipids, and proteins. The compounds which contain the high-energy phosphate bonds provide fuel for the cell’s maintenance and growth. This energy is consumed by the cell and is not available to transfer to the next level of the ecosystem.


Biomass, Food Chasins, and Food Webs

Ecologists refer to stored energy within a level or ecosystem as biomass. Biomass is the weight of all the organisms within an ecosystem minus the weight of the water they contain. It is measured in units of grams per square meter (g/m2). The close relationship between the amount of biomass in a trophic level and the amount of energy contained by a trophic level means that the efficiency of biomass transfer mirrors that of energy transfer. Thus, biomass can also be represented by a pyramid similar to the energy pyramid discussed above. An African savannah ecosystem biomass pyramid is shown here. In it, you can see the large amount of biomass present at the primary producers tropic level. In the African savannah ecosystem depicted in this pyramid, this would consist mainly of grasses and short shrubs. An antelope represents a primary consumer. Antelopes are herbivores, they do not eat other animals. Consequently, huge herds of antelope can be found in the savannah ecosystem.

In our example, the hyena represents the next trophic level, secondary consumers. Hyenas are carnivorous but also scavengers, that is, they feed on dead animals. While secondary consumers like hyenas may travel in packs, there is simply not enough prey animals or dead animal carcasses to support large herds of these animals. Finally, at the top of the African savannah biomass pyramid are the tertiary consumers (often called apex predators). These are carnivorous animals typically with claws and sharp teeth, useful in killing and devouring their prey. Tertiary consumers are not preyed upon by any higher trophic levels. Not surprisingly, only a relatively few apex predators can be supported by an ecosystem. Their numbers are related directly to the amount of biomass and energy present in the trophic levels below them.

Ecologists can follow the path of energy through an ecosystem either by following a particular food chain or by observing all the different producer-consumer relationships by studying the overall food web. Food chains do not truly exist in nature because organisms rarely eat only one particular food source. Food chains are an artificial means for ecologists to isolate a particular energy path within a food web. The chart above illustrates a typical North American food web. An example of a food chain within that web would be plants, black swallowtail caterpillar, praying mantis, American robin, and red-tailed hawk. This figure also illustrates why organism numbers also follow the pyramid relationship seen in energy and biomass. Each trophic level supports fewer numbers of organisms than it contains. Thus, the ecosystem shown in the food web above might contain a billion separate plants and support only one fox, one red-tailed hawk, and two Eastern king snakes.

In this Core Experience Learning Lab, you will explore how energy and biomass are related through models and experimentation. You will examine how both energy and biomass within an ecosystem decrease as one moves up through the levels of an ecosystem. You will model the inefficiency of energy transfer by examining energy use during cellular respiration. Finally, you will explore the impact of decreasing the amount of plant biomass in an ecosystem on the number of levels and animals per level that can be supported within that ecosystem.


  • Fun Facts
  • Learn the Lingo
  • Get Focused



Biomes and Ecosystems

What is the difference between biomes and ecosystems? Sometimes used synonymously, there is a simple distinction between a biome and an ecosystem. Namely, a biome represents only a subset of the components of an ecosystem. That subset is limited to only the biological organisms in an ecosystem. An ecosystem, on the other, included not only the biotic components of an area but all of the abiotic components as well. Examples of abiotic factors that are often considered components of ecosystems:

  • Minerals
  • Climate
  • Soil
  • Water Sunlight
  • Wind
  • Shade
  • Altitude
  • Slope or grade of land
  • Air quality
  • Pollutants
  • Etc.


The Distribution of Biomass on Earth

Below is a complex but fascinating graphic that appeared in a 2018 issue of The Proceeding of the National Academy of Science USA. Before diving into the graphic, let’s define the scientific units involved. Please click on this figure to enlarge it in a new window:

The unit Gt C refers to the mass of carbon in each of the components shown in the graphic. It represents gigatonnes of carbon. As you know, carbon is a major component of biological organisms. Therefore, it is a good indicator of the amount of biomass in ecosystems. The size of this unit is staggering:

Begin by looking at the box on the left. It represents the entire biotic carbon content of the Earth. That is, approximately 550 gigatonnes of carbon are present in the entire biosphere (all biological organisms on planet Earth). The relative area, indicated by the shapes of different colors, shows the percent contribution of each of the life forms identified. First, notice that plants are by far the major component of biomass of the biosphere. Plants represent 450 of the 550 Gt carbon on Earth, about 80%.

Of the remaining 550 Gt C, almost all is found in microorganisms such protists, bacteria, fungi, and so on. All animals on Earth contribute only about 2 Gt C. That is, animals represent only about 0.36% of the carbon in the entire biosphere. Interestingly, while plants are primarily terrestrial, whereas animals are mainly marine. 

Next, let’s turn to the box on the right. This box is an expanded view of the 2 Gt of carbon present in the biosphere. Arthropods (e.g. insects, spiders, and crustaceans) make up almost half of all animal biomass.  This is followed by fish, mollusks (including snails, clams, squid, octopi, and others), annelids (worms), and cnidarians (including hydra, jellyfish, and sea anemones and coral). Notice that there is more livestock (cattle, poultry, etc.) biomass than human biomass. In fact, humans represent only about 0.01% of the total biomass of the planet. Nonetheless, this is still 30 times more than wild birds and 8 time that of wild mammals. 

Math Note: This type of graphic is similar to the more commonly used pie chart but uses a square rather than a circle. This form of data graphic is called a Voronoi diagram.


The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.


  • Ecosystem: all the organisms in a given area and the abiotic factors with which they interact.
  • Producers: organisms that are autotrophs, which means they make their own food.
  • Consumers: organisms that are heterotrophs, which means they must obtain food by consuming producers or other consumers.
  • Detritivore: an organism that obtains nutrition from detritus. Detritivores are scavengers, bacteria, or fungi.
  • Detritus: dead plant and animal material.
  • Trophic level: a group of organisms in an ecosystem identified by their food source.
  • Biomass: the dry weight of the organisms within an ecosystem. Biomass = live weight – water content of the organism. Also known as dry matter.
  • Law of Conservation of Energy: a law that states that energy is neither created nor destroyed, it simply changes form.
  • Law of Conservation of Matter: a law that states that matter is neither created nor destroyed, it simply changes form.


The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:


Investigation 1:
  • How does energy move through an ecosystem? 
  • What affects the amount of energy in an ecosystem? 


Investigation 2:
  • What affects the efficiency of energy transfer within an ecosystem? 
Investigation 3:
  • What is the relationship between the energy and biomass of producers and the levels that an ecosystem can support? 
  • What affects the amount of energy in an ecosystem?