Organisms and Their Environment

What is Ecology?

Ecology: The scientific study of interactions among organisms and their environments

*Ecology reveals relationships among living and nonliving parts of the world!

Ex- what a coyote eats;

-how day length influences the behavior of migrating birds;

-how shrimp help rid ocean fishes of parasites;

-how acid rain threatens some of Earth’s forests

*Ecologists use both descriptive and quantitative research methods to learn about these interactions that take place in the environment.

Aspects of Ecological Study

*As far as we know, life exists only on Earth!

Biosphere: the portion of Earth that supports life

*Extends from high in atmosphere to bottom of ocean.

*Very diverse; supports a wide range of organisms.

*Living things are affected by both the physical/nonliving environment and by other living things.

Abiotic Factors: the nonliving parts of an organism’s environment

Ex- air currents; temp; moisture; light; soil

Biotic Factors: the living organisms that inhabit an environment

Ex- plants; animals; fungi; bacteria

*Remember- All organisms depend on others for food, shelter, reproduction, and/or protection!

Levels of Organization in Ecology

*To help better understand the interactions of the biotic and abiotic parts of the world, ecologists study interactions among organisms at several different levels: (See p.41)

1. individual organism

2. population– a group of organisms of one species that interbreed and live in the same place at the same time

3. community– a collection of interacting populations; No species lives independently of other species!

-A change in one population will cause changes in the other populations.

4. ecosystem– made up of the interactions among the community and the community’s abiotic factors

Organisms in Ecosystems

Habitat– the place where an individual organism lives out its life

-Several species may share a habitat, BUT, the food, shelter, and other essential resources of that habitat are often used in different ways.

*Differences lead to reduced competition!

Niche– the role and position a species has in its environment- how it meets its needs for food and shelter, how it survives, and how it reproduces.

-Being unique is important!  Unique strategies and structures reduce competition w/others!

Ex- polar bears thick coat;

hummingbirds long, skinny beak

*Some species enhance their chances of survival by forming relationships w/other species.

Symbiosis– “living together”- a close and permanent association among organisms of different species

3 Types of Symbiosis:

1. Commensalism– a symbiotic relationship in which one species benefits and the other species is neither benefited or harmed.

Ex- algae growing on turtle shell; bird building a nest in a tree

2. Mutualism– a symbiotic relationship in which both species benefit

Ex- ants and acacia tree; bees and flowers

3. Parasitism– a symbiotic relationship in which one species benefits and the other species is harmed

*Parasites harm, not kill, the host.  If the host dies, the parasite dies unless it can quickly find another host. Ex- dogs and tics; people and mosquitoes

Nutrition and Energy Flow

How Organisms Obtain Energy

*Energy– the ability to do work

-Energy is needed by all organisms for survival.

-The ultimate source of energy is the sun.

Autotrophs (Producers)- organisms that use energy from the sun or energy stored in chemical compounds to make their own nutrients/food

*Plants are the most common terrestrial autotroph.

*Most other organisms depend on autotrophs for nutrients and energy.

Heterotrophs (Consumers)- organisms that cannot make their own food and must feed on other organisms

Types of Heterotrophs:

Herbivores– feed only on plants

Ex.- rabbit, grasshopper, squirrel, bee

Carnivores– kill and feed on other animals

Ex.- lion, bobcat

Omnivores– feed on both plants and animals

Ex.- human, grizzly bear, raccoon, fox

Scavengers– feed on already-dead animals

Ex.- vulture, buzzard, coyote

Decomposers– break down and absorb nutrients from dead plants and animals

Ex.- bacteria, fungi, protozoans

Matter and Energy Flow in Ecosystems

*Nutrients and energy flow through organisms in an ecosystem- from autotrophs to heterotrophs and, eventually, to decomposers.

Food Chain– a simple model that shows how matter and energy move through an ecosystem

-Drawn w/arrows to show the direction in which energy is transferred from one organism to the next

-A portion of the energy is lost at each level as heat, therefore, food chains usually consist of 3, 4, or 5 links only.

Food Web– a model that links all of the possible food chains in an ecosystem together

-Expresses all of the possible feeding relationships

-More realistic than a food chain because most organisms rely on more than one other species for food.

Trophic Level– Each step in a food chain/web is called a trophic level.

-A feeding step in the passage of energy and materials through an ecosystem


Producers/Autotrophs make up the 1st level.

Consumers/Heterotrophs make up all other levels.

Ecological Pyramid– Shows the relative amounts of energy or matter contained at each trophic level

3 types of ecological pyramids:

1. Pyramid of Energy– represents the energy available at each level

-Only about 10% of available energy is passed on to the next level.  Why?

-Most energy is used or lost as heat!

2. Pyramid of Numbers– represents the population size of organisms at each level

-Population as the level

3. Pyramid of Biomass– represents the weight of living material at each level (or amount of potential food available at each level)

-Biomass as the level.

Cycles in Nature

*Ecological pyramids show us that energy is LOST from one trophic level to the next as HEAT generated by the body processes of organisms.

-This energy is constantly renewed by the sun!

*Matter can NOT be renewed;  It is constantly being recycled.

-This is because biological systems do not use up the matter – they transform it.

-All of the elements that make up the bodies or organisms today are the same elements that have been on Earth since life began.

1. The Water Cycle: Water is constantly moving between the atmosphere and Earth due to:

-evaporation (l > g), transpiration (l > g from plants), condensation (g > l), and precipitation (l or s from the sky)

2. The Carbon Cycle: Carbon is the molecule of life- the key ingredient in all living things.

-Carbon is constantly moving between the atmosphere and Earth due to:

photosynthesis (plants use CO2 to make energy rich C molecules); respiration (animals breathe out CO2); eating (we eat plants to get C); decomposition/waste (our bodies give off waste and eventually die, which puts C back into the air and soil); volcanoes; forest fires; burning fossil fuels (all release CO2 into the air and water)

3. The Nitrogen Cycle: All organisms require N to build amino acids and make proteins.

-N is constantly moving between the atmosphere and Earth.

-Air = 78% N, but plants and animals can’t use this N!  Only certain bacteria can use it.

-Plants take in N from soil, where bacteria and lightning has converted it from the air into a more usable form.

-Plants make proteins > animals eat plants to get the N proteins

Urine contains excess N, which returns N to the water and soil.

-Die > decomposers return N to soil > plants reuse this N and bacteria convert this N back into N gas that goes into the air.

4. The Phosphorus Cycle: All organisms require P for growth and development (It is in our DNA!)

-P is not in the atmosphere.

a. Short-Term P Cycle: plants use P in soil; animals eat plants to get P; animals die and decompose and P goes back into soil for plants

b. Long-Term P Cycle: P washed into the sea becomes part of rocks; Rocks erode over time and P dissolves in water and is used by marine life or becomes part of soil again


*Remember, all organisms are affected by both the biotic and abiotic factors in the environment.

-These factors interact and result in conditions that are suitable for life for some organisms and unsuitable for other organisms.

-Ex.- polar bears color and fur

– hummingbirds long beak

Limiting factor: any biotic or abiotic factor that restricts the existence, numbers, reproduction, or distribution of organisms.

-Ex.- not enough water; not enough O2; too hot; too windy – Some species are adapted, but some can NOT survive!

-Often times, the biotic and abiotic factors in a community change.  (Ex- people stop cutting grass; a volcano destroys a forest; an avalanche exposes new rock and buries other rock)

-These changes lead to succession: the natural, orderly changes and species replacements that take place in the communities of an ecosystem

There are 2 types of succession:

1. primary succession: when organisms start to live in a NEW place

Ex.- a new island; new rock made by an erupting volcano

2. secondary succession: when organisms start to live again in a place that had been destroyed by a natural disaster or human actions

Ex.- a flood, fire, or hurricane destroys a community; people stop cutting the grass

-Takes less time them primary succession because soil already exists.

*Overall message of this chapter: We all interact w/everything and everyone around us, and it is these interactions that determine our survival.

author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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