Energy Flow and Chemical Cycling

• Every organism requires energy to carry out life processes such as growing, moving, and reproducing. Photosynthetic producers such as plants convert the light energy from sunlight to the chemical energy of organic compounds.

• Organisms called consumers obtain chemical energy by feeding on the producers or on other consumers.

• organisms called decomposers break down wastes and dead organisms.

• As living things use chemical energy, they release thermal energy in the form of heat to their surroundings.

• Energy enters an ecosystem as light, is converted to chemical energy by producers, and exits the ecosystem as heat.

• Energy is not recycled within an ecosystem, but flows through it and out.

• Producers must continue to receive energy as an input for the ecosystem to survive.

Chemicals such as carbon, oxygen, and nitrogen can be recycled between the living and nonliving parts of ecosystems and the biosphere.

Although energy flows through an ecosystem, while chemicals can be used again and again, the movements of both energy and chemicals are related to patterns of feeding within the ecosystem.

Food Chains• In the desert:

– a grasshopper feeds yellow flowers. – A mouse seizes the grasshopper for its own meal.– These feeding relationships, from flower to

grasshopper to mouse, relate to how energy and chemicals move through the desert ecosystem. Each of these organisms represents a feeding level, or trophic level, in the ecosystem.

• The pathway of food transfer from one trophic level to another is called a food chain.

Producers

• In terrestrial ecosystems, plants are the main producers.

• In aquatic ecosystems, phytoplankton—photosynthetic protists and bacteria—multicellular algae, and aquatic plants are the main producers

Consumers • Organisms in the trophic levels above the

producers are consumers. • They may be categorized according to what

they eat. • A consumer that eats only producers is an

herbivore. • A consumer that eats only other consumers

is a carnivore. • And a consumer that eats both producers

and consumers is an omnivore

Consumers may also be categorized by their position in a particular food chain.

when a consumer feeds directly on producers it is referred to as a primary consumer or first-level consumer.

In terrestrial ecosystems, primary consumers often include:

insects and birds that eat seeds and fruit, as well as grazing mammals such as antelope and deer.

In aquatic ecosystems, primary consumers include:

a variety of zooplankton (mainly protists and microscopic animals such as small shrimps) that feed on phytoplankton.

Secondary consumers(second-level consumers) eat primary consumers.

In aquatic ecosystems, secondary consumers are mainly small fish that eat zooplankton.

Tertiary consumers (TUR shee ehr ee)—third-level consumers—eat secondary consumers.

On land, a tertiary consumer may be a snake eating a mouse.

Decomposer • At each trophic level, organisms produce

waste and eventually die. • These wastes and remains of dead organisms

are called detritus. • Decomposers are consumers that obtain

energy by feeding on and breaking down detritus.

• Animals that eat detritus, often called scavengers, include earthworms, some rodents and insects, crayfish, catfish, and vultures.

•But an ecosystem's main decomposers are bacteria and fungi. These organisms, found in enormous numbers in the soil and in the sediments at the bottom of lakes and oceans, recycle chemicals within the ecosystem

Food Webs• The feeding relationships in an ecosystem

are usually more complicated than the simple food chains you have just read about.

• Since ecosystems contain many different species of animals, plants, and other organisms, consumers have a variety of food sources.

• The pattern of feeding represented by these interconnected and branching food chains is called a food web.

All organisms require energy for growth, reproduction, and, in some species, movement. But there is a limited amount of energy available in an ecosystem—an "energy budget" that is divided among the different trophic levels. This energy budget influences the types and numbers of organisms in the ecosystem

Productivity of Ecosystems

• What determines an ecosystem's energy budget? For most ecosystems, the answer begins with the amount of sunlight that enters the ecosystem. Much of the sunlight that bombards Earth every day bounces back into space or is absorbed by the atmosphere.

• Of the light energy that reaches plants and other producers, only a tiny fraction—about 1 percent—is captured by photosynthesis. The producers convert this light energy to the chemical energy stored in organic compounds. Even such a small percentage of the sun's total energy output is enough to enable Earth's producers to manufacture billions of kilograms of organic material, or biomass, each year.

The rate at which producers in an ecosystem build biomass is called primary productivity. The level of primary productivity in an ecosystem sets the energy budget. In other words, primary productivity determines the maximum amount of energy available to all the higher trophic levels in an ecosystem.

Figure 36-5 contrasts the net primary productivity of several different terrestrial ecosystems. (Net productivity refers to the total amount of organic material produced minus the amount used by the producers themselves to fuel their own life processes.) Notice that the productivity of the tropical rain forest is considerably higher than that of the temperate grassland, which is higher than that of the tundra. The rain forest's climate—warm and humid—and year-round growing season allow for high productivity. In contrast, producers in the typically cold and dry tundra grow more slowly and contribute less biomass than those in the rain forest. Conditions for producers in the grasslands fall in between, and so does their productivity

Ecological Pyramids

• While the producers set an ecosystem's total energy budget, energy is "spent" at each step of the food web. As each consumer feeds, some energy is transferred from the lower trophic level to the higher trophic level. But most of the available energy stored in the prey organism's biomass is lost.

• For example, when a caterpillar eats a leaf, about 50 percent of the energy stored in the leaf passes out of the caterpillar's body in its wastes (feces). The caterpillar uses 35 percent of the leaf's stored energy to support its life processes, such as moving and reproducing. The caterpillar transforms only about 15 percent of the leaf's stored energy into new caterpillar biomass (Figure 36-6).

To depict information about energy, biomass, and numbers of organisms at different trophic levels, ecologists use three types of diagrams: energy pyramids, biomass pyramids, and pyramids of numbers. In each case, the foundation of the pyramid is the producer level. The primary consumers form the next block, and so on.

Energy Pyramids

• An energy pyramid, sometimes called a food pyramid, emphasizes the energy loss from one trophic level to the next (Figure 36-7). In general, an average of only 10 percent of the available energy at a trophic level is converted to biomass in the next higher trophic level. The rest of the energy—about 90 percent—is lost from the ecosystem as heat.

Biomass Pyramids

• biomass pyramid represents the actual biomass (dry mass of all organisms) in each trophic level in an ecosystem. Most biomass pyramids narrow sharply from the producer level at the base to the top-level consumers at the peak (Figure 36-8). There are some exceptions, however. In certain aquatic ecosystems, the zooplankton (primary consumers) consume the phytoplankton (producers) extremely rapidly.

Pyramids of Numbers

• pyramid of numbers depicts the number of individual organisms in each trophic level of an ecosystem. These pyramids are also organized like energy pyramids, with producers found at the foundation and higher trophic levels on each step above them.

• In most cases, the foundation is again the widest section, indicating that there are more individual producers than there are primary consumers, and so on (Figure 36-8). This pyramid emphasizes how few top-level consumers an ecosystem can support. Exceptions to the usual shape of a number pyramid occur when small organisms eat larger ones. For example, a single tree (producer) may be the sole food source for hundreds of insects (primary consumers).

• As a result, the zooplankton have a greater mass at any given time than the phytoplankton. The phytoplankton grow and reproduce at such a rapid rate that they can support a consumer population that has a greater biomass. A biomass pyramid for this ecosystem would appear top-heavy.