Unit 3 - EcologySection 2 - Cycles, Change, and Stability in Ecosystems

Unit 3 - Ecology 1 / 13

Cycling of Matter in Ecosystems

Why do ecosystems appear to be so stable? Most natural environmentsappear to hardly change over time. If organisms are continually beingreborn and dying, along with vegetation, how is it that ecosystemsmaintain a strong sense of stability?

The answer to this question lies in the idea of cycles.

Unit 3 - Ecology 2 / 13

To understand how matter cycles through ecosystems, we must alsounderstand the cycling of organic substances within living things.

Organic substances always contain atoms of carbon and hydrogen, andoften contain oxygen and nitrogen atoms. Proteins, sugars and fats, theimportant chemicals that make up your body, are all organic.

Matter that doesn’t contain a combination of carbon and hydrogen atomsis called inorganic. For example, water (H2O) and ammonia (NH3) areconsidered inorganic.

Unit 3 - Ecology 3 / 13

Food is organic matter. Every time you eat, through the process ofdigestion, the complex organic molecules from your food are broken downinto simpler molecules. Your cells used these simple molecules to buildcomplex molecules, which become part of your own structure.

The materials used in building the bodies of living organisms are limited tothe atoms and molecules that make up the planet. There is no alternativesource of matter. Therefore, to maintain life on Earth, matter must berecycled.

Since all matter is recycled, it is likely that somewhere in your body areatoms that once made up some dinosaur or ancient and extinct plant life.

Unit 3 - Ecology 4 / 13

The Water Cycle (Hydrologic Cycle)

Sun’s heat → evaporation → condensation → precipitation → Sun’s heat→.....

Unit 3 - Ecology 5 / 13

Steps of the Water Cycle:

Heat from the Sun causes water from oceans, lakes, and rivers tomove into a gaseous phase and rise up into the atmosphere. Thisprocess is called evaporation.

As the water vapour strikes cool air in the atmosphere, it will begincondense into tiny droplets of water. These collect upon tiny particlesin the atmosphere to form clouds. This process is calledcondensation.

After enough droplets of water have collected, the clouds will becomeheavier and gravity will pull the droplets to the earth as either rain,snow, sleet, or hail. Most of the water will be returned to oceans,rivers, and lakes; however, wind often moves the clouds above land.This process is called precipitation.

→ The process then repeats...

Unit 3 - Ecology 6 / 13

The Carbon Cycle

Plants absorb CO2 from the atmosphere during photosynthesis → CO2 isthen reintroduced into the atmosphere through plant and animalrespiration, auto and factory emissions, and ocean uptake.

Unit 3 - Ecology 7 / 13

Carbon is the key element for living things. Each year, about 50 to 70billion tonnes of carbon from inorganic carbon dioxide are recycled intomore complex organic substances. This is done through photosynthesis.

6 CO2 + 6 H2O + light −−→ C6H12O6 + 6 O2

Steps of the Carbon Cycle:

Plants absorb CO2 through photosynthesis.

Some of the organic carbon is released back to the environmentthrough cellular respiration as carbon dioxide.

Carbon is also transferred into the soil when plants and animals decay.

As they settle in the soil, fossil fuels emerge. Factory emissionsrelease carbon into the atmosphere.

Ocean uptake (the absorption and release of gases).

Watch: “Crash Course Ecology - Water & Carbon Cycle”

Unit 3 - Ecology 8 / 13

The Nitrogen Cycle

Life depends on the cycling of nitrogen.

Nitrogen atoms are required so that cells can make proteins. Nitrogen isalso required for the synthesis of DNA, the hereditary material found in allliving things.

Unit 3 - Ecology 9 / 13

Steps of the Nitrogen Cycle:

Nitrogen is found abundantly in the atmosphere. Before animals andplants can use nitrogen, it must be fixed (put into a biologicallyuseful compound). This is done through lightning, bacteria, andindustrial fixation.

Once fixated, nitrogen is used for making proteins in plants andanimals.

The plants and animals will pass nitrogen into soil throughdeath/decomposition and excretions.

Now in the soil, nitrogen is often found as ammonia (NH3). Nitrogenfixing bacteria then converts the NH3 into nitrates (NO3).

Nitrates often will be passed into oceans or will eventually surface intop soil. Through a process called denitrification, nitrogen will bereturned to the atmosphere.

Unit 3 - Ecology 10 / 13

The Phosphorus Cycle

Phosphorus is a key element in cell membranes, in molecules that helprelease chemical energy, in the making of DNA, and in the calciumphosphate of bones.

Phosphorus is cycled in two ways: a long-term cycle involving the rockson the Earth’s crust and a short-term cycle involving living organisms.

Unit 3 - Ecology 11 / 13

Steps of Phosphorus’ Short Cycle:

Plants absorb phosphorus from soil.

Animals eat plants.

Decomposers break down plant and animal matter into soil.

Steps of Phosphorus’ Long Cycle:

Phosphate is found in rocks and fossils. Wind and rain erode theserocks.

Rivers carry the gravel and silt (which contain phosphates) to waterbodies. Also, runoff from phosphates in farming end up in largebodies of water.

In the water, the silt and runoff then becomes sediment, and thennew rock.

Geological uplift pushes new rock to the surface.

Unit 3 - Ecology 12 / 13

Assignment: Read pg. 62 - 69

Complete questions #’s 1 - 4, 7a on pg. 65Complete questions #’s 1 - 3, 6, 7, 9 on pg. 69

Unit 3 - Ecology 13 / 13