Energy Source - The Sun

All living things require a source of energy in order to survive. Radiant energy from the sun sustains life on Earth. But not all of the Sun’s energy reaches the Earth.

About 30% is reflected back into space. Close to 70% is converted into thermal energy and absorbed by the air, sea and land. A very small percentage (0.023%) of radiant energy is absorbed by plants for photosynthesis. Organisms that can photosynthesize use the light energy of the sun to produce chemical energy in the form of food.

Photosynthesis

Some of the sugars produced are stored by the plant for later use, while others are used to build tissue. Some sugars are combined with other elements to make proteins and fats. What does the plant do when there is no light?

Cellular Respiration

A plant can use the energy that was stored in food (sugars) through a process called cellular respiration. This does not require sunlight, but it does require oxygen. This process occurs continuously. If it were to stop, the plant would quickly die.

Producers (Autotrophs)

So, plants can make their own food through photosynthesis when there is light. Any organism that can make its own food is called a producer. Plants can also use this food to release energy for survival through cellular respiration (in the light or dark).

Consumers (Heterotrophs)

Organisms that must eat other organisms to get their food are called consumers. Consumers must also use respiration to release the food energy to the cell.

Types of Consumers

Recall that organisms that need to eat food are called consumers. Consumers can be further divided into four categories: herbivore, carnivore, omnivore, and scavenger.

Herbivore - eats plants or other producers

Carnivore - eats other animals

Omnivore - eats plants and animals

Scavenger - eats the remains of other animals

Detritivores

• Are heterotrophs that obtain organic nutrients from detritus by internal digestion

• Detritus includes: dead leaves, feathers, hair, dead organisms and feces

• Ex: earthworms, crustaceans, snails, insects

Saprotrophs

• Are heterotrophs that obtain organic nutrients from detritus by external digestion

• Secrete digestive enzymes into dead organic matter, digest externally, and then absorb products

• Also known as decomposers

• Ex: fungi, mushrooms, bacteria

Modes of Nutrition

Nutrient Cycling

Food Chains

The simplest way to show the feeding relationships in an ecosystem is with a food chain. The arrows in a food chain point in the direction that nutrients flow. The level at which an organism feeds is known as its trophic level.

Food Webs

In nature, food chains are part of a much more complex interaction among species. A food web illustrates this series of interconnected food chains.

This is a simple food web; in nature, they are usually more complex. Complex webs are more stable than simple ones. The web may be able to tolerate the loss of one species, but not many.

Ecological Pyramids

One way to visualize relationships that exist between trophic levels is with ecological pyramids. There are three types:

Energy

only 10% is transfer to the next trophic level

Ninety percent is lost to biological processes and as heat. Energy pyramids always have a wide base. This is not always true of the other pyramids.

Energy Loss

Numbers

usually more individuals at lower tropic levels, but not always

On a single tree there can be thousands of sap sucking insects (smaller base).

There are more grasshoppers in a prairie ecosystem than there are animals that eat them (bigger base).

Biomass

usually more biomass at lower tropic levels, but not always

At any point in time, the biomass of algae in a pond is less than the biomass of all the creatures that feed on algae (smaller base).

The mass of all of the grain in a field is greater than the mass of the rodents and birds that feed on the grain (bigger base).