Living systems operate by expending energy.  Organisms need energy for growth, reproduction, and their own metabolic processes.  In short, if there is no energy, there are no life functions! Yet, no organism can create energy—organisms can only use energy from other sources.  You probably know that you get your energy from the plants and animals you eat.  But where does the energy in your food come from? For most life on Earth, sunlight is the ultimate energy source.  Over the last few decades, however, researchers have discovered that there are other energy sources for life.  For some organisms, chemical energy stored in inorganic chemical compounds serves as the ultimate energy source for life processes.

Only algae, certain bacteria, and plants like the one in Figure 3–4 can capture energy from sunlight or chemicals and convert it into forms that living cells can use.  These organisms are called autotrophs.

Autotrophs use solar or chemical energy to produce “food” by assembling inorganic compounds into complex organic molecules.
 

But autotrophs do more than feed themselves.  Autotrophs store energy in forms that make it available to other organisms that eat them.  That’s why autotrophs are also called primary producers. Primary producers The first producers of energy-rich compounds that are later used by other organisms.

Primary producers are, therefore, essential to the flow of energy through the biosphere.

Energy From the Sun

The best-known and most common primary producers harness solar energy through the process of photosynthesis.

Photosynthesis captures light energy and uses it to power chemical reactions that convert carbon dioxide and water into oxygen and energy-rich carbohydrates such as sugars and starches.

This process adds oxygen to the atmosphere and removes carbon dioxide.  Without photosynthetic producers, the air would not contain enough oxygen for you to breathe!  Plants are the main photosynthetic producers on land.  Algae fill that role in freshwater ecosystems and in the sunlit upper layers of the ocean.  Photosynthetic bacteria, most commonly cyanobacteria, are important primary producers in ecosystems such as tidal flats and salt marshes.

Life Without Light

About 30 years ago, biologists discovered thriving ecosystems around volcanic vents in total darkness on the deep ocean floor.  There was no light for photosynthesis, so who or what were the primary producers? Research revealed that these deep-sea ecosystems depended on primary producers that harness chemical energy from inorganic molecules such as hydrogen sulfide.  These organisms carry out a process called chemosynthesis.

Chemosynthesis
A biological process in which chemical energy is used to produce carbohydrates

Chemosynthetic organisms are not only found in the deepest, darkest ocean, however.  Several types of chemosynthetic producers have since been discovered in more parts of the biosphere than anyone expected.  Some chemosynthetic bacteria live in harsh environments, such as deep-sea volcanic vents or hot springs.  Others live in tidal marshes along the coast.

What are primary producers?
 

REVIEW & DO NOW Answer the following questions:
What are autotrophs? Give an example of an autotroph. What are primary producers? Give an example of a primary producer. Do all primary producers get their energy from the sun?	What is photosynthesis? Give an example of a photosynthetic organism. What is chemosynthesis? Give an example of a chemosynthetic organism.

Consumers
How do consumers obtain energy and nutrients?

Animals, fungi, and many bacteria cannot directly harness energy from the environment as primary producers do. 

Heterotrophs
organisms which must acquire energy from other organisms by ingesting them in one way or another

Heterotrophs are also called consumers.

Consumers
Organisms that rely on other organisms for energy and nutrients

Types of Consumers

Consumers are classified by the ways in which they acquire energy and nutrients.  As you will see, the definition of food can vary quite a lot among consumers.
 

Carnivores Carnivores kill and eat other animals.  Carnivores include snakes, dogs, otters, and this lion.  Catching and killing prey can be difficult and requires energy, but meat is generally rich in nutrients and energy and is easy to digest.		Herbivores Herbivores like this long-necked giraffe obtain energy and nutrients by eating plant leaves, roots, seeds, or fruits.  Common herbivores include cows, caterpillars, macaws, and deer.
Omnivores Omnivores are animals whose diets naturally include a variety of different foods that usually include both plants and animals.  Humans, coati, pigs, and this brown bear are omnivores.		Scavengers Scavengers are animals that consume the carcasses of other animals that have been killed by predators or have died of other causes.  This African vulture is a scavenger.
Decomposers Decomposers, such as bacteria and fungi (like this mushroom), “feed” by chemically breaking down organic matter.  The decay caused by decomposers is part of the process that produces detritus—small pieces of dead and decaying plant and animal remains.		Detritivores Detritivores like this giant earthworm feed on detritus particles, often chewing or grinding them into even smaller pieces.  Many types of mites, snails, shrimp, and crabs are detritivores.  Detritivores commonly digest decomposers that live on, and in, detritus particles.

Beyond Consumer Categories

Categorizing consumers is important, but these simple categories often don’t express the real complexity of nature.  Take herbivores, for instance.  Seeds and fruits are usually rich in energy and nutrients, and they are often easy to digest.  Leaves are generally poor in nutrients and are usually very difficult to digest.  For that reason, herbivores that eat different plant parts often differ greatly in the ways they obtain and digest their food.  In fact, only a handful of birds eat leaves, because the kind of digestive system needed to handle leaves efficiently is heavy and difficult to fly around with!
 

Moreover, organisms in nature often do not stay inside the tidy categories ecologists place them in.  For example, some animals often described as carnivores, such as hyenas, will scavenge if they get a chance.  Many aquatic animals eat a mixture of algae, bits of animal carcasses, and detritus particles—including the feces of other animals!  So, these categories make a nice place to start talking about ecosystems, but it is important to expand on this topic by

discussing the way that energy and nutrients move through ecosystems.

How do consumers obtain energy and nutrients?
 

REVIEW & DO NOW Answer the following questions:
What is a heterotroph? Give an example of a heterotroph. What is a consumer? Give an example of a consumer. Are all heterotrophs consumers? What are carnivores? Give an example of a carnivore. What are herbivores? Give an example of a herbivore.	What is an omnivore? Give an example of an omnivore. What is a scavenger? Give an example of a scavenger. What is a decomposer? Give an example of a decomposer. What is a detritovore? Give an example of a detritovore.