Biology I

Chapter 2, Section 2

Nutrition and Energy Flow

Ecologists trace the flow of energy through communities to discover nutritional relationships between organisms.

Producers – Autotrophs

The ultimate source of energy of life is the sun. An organism that uses light energy or energy stored in chemical compounds to make energy-rich compounds is an autotroph. Plants are the most familiar of autotrophs but some organisms such as green algae also make their own nutrients. Other organisms in the biosphere depend on autotrophs for nutrients and energy. These dependent organisms are called consumers.

Consumers – Heterotrophs

An organism that cannot make its own food and feeds on other organism is called a heterotroph. Heterotrophs display a variety of feeding relationships.

1. Feeds only on plants is a herbivore (rabbits, grasshoppers, squirrels, bees) 2. Eat other heterotrophs (lions kill and eat only other animals carnivores). 3. Scavengers do not kill for food but scavenge and eat animals that have already died (vultures). What would our ecosystems be like without scavengers? 4. Omnivores: Humans are an example of a third type of heterotroph. Most people eat a variety of foods that include both animal and plant materials. 5. Decomposers (such as bacteria and fungi) break down the complex compounds of dead and decaying plants and animals into simpler molecules that can be more easily absorbed.

A food chain is a simple model that scientists use to show how matter and energy moves through the ecosystem. In a food chain, nutrients and energy move from autotrophs to heterotrophs and eventually to decompose.

First order – heterotrophs - herbivores such as the deer, cardinal turtle and fish obtain food from photosynthetic organisms.

Second order – heterotrophs – some carnivores feed on first-order heterotrophs. Owls feed on fishes or mice, worms or small insects.

Third order - heterotrophs – Carnivores are animals that feed on second-order heterotrophs (bears attack other animals such as the deer and bears also rely on a large diet of berries and so are termed omnivores.

Decomposers – at every level in a food chain, bacteria and fungi break down living matter and help release nutrients.

Tropic levels represent links in the chain. Tropic level is the passage of energy and materials. First level of heterotroph is an organism that feeds on plants. A second order heterotroph is an organism that feeds on a first order heterotroph. A food chain represents only one possible route for the transfer of matter and energy through an ecosystem. Food web shows all the possible feeding relationships at each tropic level in a community. A food web is a more realistic model than a food chain because most organisms depend on more than one other species for food.