Day 2: Community Ecology 

Matter consists of Elements and Compounds and matter is anything that has mass and takes up space. It can exist in three physical states-solid, liquid, and gas- and two chemical forms- elements and compounds. Organic compounds are the chemicals of life. Plastics, table surga, vitamins, aspirin, penicillin, and most of the chemicals in your body are called organic compounds, which contain at least two carbon atoms combined with atoms of one or more other elements. The exception is methane (CH4), with only one carbon atom. Matter comes to life through cells, genes, and chromosomes. Matter can change by undergoing a physical change or a chemical change (chemical reaction). We can change elements and compounds from one physical or chemical form to another. We cannot, however, create or destroy any of the atoms involved in the physical or chemical change. All we can do is rearrange atoms, ions, or molecules into different spatial patterns (physical changes) or chemical combinations (chemical changes). This finding, based on many thousands of measurements, describes an unbreakable scientific law known as the law of conservation of matter. 

(see textbook pages 40-46)

Tropical rain forests are found near the earth's equator and contain an amazing variety of life. These lush forests are warm year round and have high humidity because it rains almost daily. Rain forests cover only 2% of the earth's land but contain up to half of the world's known terrestrial plant and animal species. These properties make rain forests natural laboratories in which to study ecosystems- communities of organisms that interact with one another and with the physical enviornment of matter and energy in which they live. 

To date, at least half of the earth's rain forests have been destroyed or degraded by humans cutting down trees, growing crops, grazing cattle, and building settlements. The destruction and degradation of these centers of biodiversity is increasing. Ecologists warn that without protections, most of these forests will be gone or severely degraded by the end of this century. 

Why should we care that tropical rain forests are disappearing? Scientists give three reasons. First, clearing these forests reduces the earth's vital biodiversity by destroying the habitats for many of the earth's species. Second, destroying these forests contributes to atomospheric warming and speeds up climate change, which you will learn later this year. How does this occur? Eliminating large areas of trees faster than they can grow back decreases the ability of the forests to remove some of the human-generated emissions of carbon dioxide, a gas that contributes to atmospheric warming and climate change. 

Third, large-scale loss of tropical rain forests can change regional weather and climate patterns. Sometimes such changes can prevent the regrowth of rain forests in cleared or degraded areas. 

When the ecological tipping point is reached, tropical rain forests in such areas, become less diverse tropical grasslands. 

AP Big Ideas 

The laws of thermodynamics and law of conservation of matter to ecosystems. Energy flows in one direction; from the sun through living organisms in a food web. These energy conversions underlie all ecological processes. YOU SHOULD be able to: 

1. map a food web, placing the arrows in the correct direction of energy flow

2. apply the 10% rule

3. understand and apply the idea of primary productivity

4. follow nutrients (carbon, nitrogen, & phosphorus and water through their cycles 

Earth's Life Support System Has Four Major Components

These four main systems interact with one another. They are the atmosphere (air), the hydrosphere (water), the geosphere (rock, soil, and sediment), and the biosphere (living things). 

The atmosphere is a spherical mass of air surrounding the earth's surface. Its innermost layer, the troposphere, extends about 17 kilometers (11 miles) above sea level at the equator and about 7 kilometers (4 miles) above the earth's North and South Poles. The troposphere contains the air we breathe. It is 78% nitrogen (N2) and 21% oxygen (02). The remaining 1% of air is mostly water vapor, carbon dioxide, and methane. 

The next layer of the atmosphere is the stratosphere. It reaches 17 to 50 kilometers (11-31 miles) above the earth's surface. The layer of the stratosphere closest to the earth's surface contains enough ozone (O3), gas to filter out about 95% of the sun's harmful ultraviolet (UV) radiation. This global sunscreen allows life to exist on the surface of the planet. 

The hydrosphere includes all of the water on or near the earth's surface. It is found as water vapor in the atmosphere, as liquid water on the surface and underground, and as ice-polar ice, icebergs, glaciers, and ice in frozen soil-layers called permafrost. Salty oceans that cover about 71% of the earth's surface contain 97% of the planet's water and support almost half of the world's species. About 2.5% of the earth's water is freshwater and three-fourths of that is ice. 

The geosphere contains the earth's rocks, minerals, and soil. It consists of an intensely hot core, a thick mantle of very hot rock, and a thin outer crust of rock and soil. The crust's upper portion contains soil chemicals or nutrients that organisms need to live, grow, and reproduce. It also contains nonrenewable fossil fuels-coal, oil, and natural gas-minerals that we extract and use. 

The biosphere consists of the parts of the atmosphere, hydrosphere, and geosphere where life is found. If you compare the earth with an apple, the biosphere would be as thick as the apple's skin. 

Three Factors Sustain the Earth's Life

Life depends on three interconnected factors on Earth: 

1. One-way flow of high-quality energy from the sun. The sun's energy supports plant growth, which provides energy for plants and animals, in keeping with the solar energy principle of sustainability. As solar energy interacts with carbon dioxide (CO2), water vapor, and several other gases in the troposphere, it warms the troposphere-a process known as the greenhouse effect. Without this natural process, the earth would be too cold to support most of the forms of life we find here today. 

2. Cycling of nutrients through parts of the biosphere. Nutrients are chemicals that organisms need to survive. Because the earth does not get significant inputs of matter from space, its fixed supply of nutrients must be recycled to support life. This is in keeping with the chemical cycling principle of sustainability. 

3. Gravity allows the planet to hold on to its atmosphere and enables the movement and cycling of chemicals through air, water, soil, and organisms. 

Key Points: 

1. Some organisms produce the nutrients they need, others get the nutrients they need by consuming other organisms, and some recycle nutrients back to producers by decomposing the wastes and remains of other organisms. 

2. Soil is a renewable resource that provides nutrients that support terrestrial plants and helps purify water and control the earth's climate. 

Ecosystems Have Several Important Components

Scientists classify matter into levels of organization ranging from atoms to galaxies. Ecologists study five levels of matter-the biosphere, ecosystems, communities, populations, and organisms- which are shown below:

The biosphere and its ecosystems are made up of living (biotic) and nonliving (abiotic) components. Nonliving components include water, air, nutrients, rocks, heat, and solar energy. Living components include plants, animals, and microbes. 

Ecologists assign each organism in an ecosystem to a feeding level, or trophic level, based on its source of nutrients. Organisms are classified as producers and consumers. Producers (also called autotrophs) are organisms, such as green plants, that make the nutrients they need from compounds and energy obtained from their environment. In the process known as photosynthesis, plants capture solar energy that falls on their leaves. They use it to combine carbon dioxide and water and form carbohydrates such as glucose (C6H12O6), to store chemical energy that plants need and emit oxygen (O2) gas into the atmosphere. This oxygen keeps us and most other animal species alive. The following chemical reaction summarizes all the overall process: 

Carbon dioxide + water + solar energy ---> glucose + oxygen 

About 2.8 billion years ago, producer organisms called cyanobacteria, most of them floating on the surface of the ocean, started carrying out photosynthesis, which added oxygen to the atmosphere. After several hundred million years, oxygen levels reached about 21%-high enough to keep oxygen-breathing animals alive. 

Today, most producers on land are trees and other green plants. In freshwater and ocean ecosystems, algae and aquatic plants growing near shorelines are the major producers. In the open water of the oceans, floating and drifting microscopic organisms known as phytoplankton are the dominant producers. 

Some producer bacteria live in dark and extremely hot water around fissures on the ocean floor. Their source of energy is heat from the earth's interior, or geothermal energy. They are an exception to the solar energy principle of sustainability. 

The other organisms in an ecosystem are consumers (also called heterotrophs) that cannot produce the nutrients they need. They get their nutrients by feeding on other organisms (producers or other consumers) or on their wastes and remains. 

There are several types of consumers. Primary consumer, or herbivores (plant eaters), are animals that eat mostly green plants. Examples are caterpillars, giraffes, and zooplankton (tiny sea animals that feed on phytoplankton). Carnivores (meat eaters) are animals that feed on the flesh of other animals. Some carnivores, including spiders, lions, and most small fishes, are secondary consumers that feed on the flesh of herbivores. Other carnivores such as tigers, hawks, and killer whales (orcas) are tertiary (or higher-level) consumers that feed on the flesh of herbivores and other carnivores. Omnivores such as pigs, rats, and humans eat both plants and animals. 

Soil Is the Foundation of Life on Land

Terrestrial life depends on soil, one of the most important components of the earth's natural capital. The minerals that make up your muscles, bones, and most other parts of your body come almost entirely from soil. Soil also supplies most of the nutrients needed for plant growth and purifies water. Through aerobic respiration, organisms living in soil remove some of the carbon dioxide in the atmosphere and store it as organic carbon compounds, thereby helping to control the earth's climate. 

Soil is much more than the dirt that we wash off our hands and clothes. Soil is a complex mixture of rock pieces and particles, mineral nutrients, decaying organic matter, water, air, and living organisms that support animal life. Life on land depends on roughly 15 centimeters (6 inches) of topsoil-the earth's living skin. 

Soil is a renewable resource but it is renewed very slowly and becomes a nonrenewable resource if we deplete it faster than nature can replenish it. The formation of just 2.5 centimeters (1 inch) of topsoil can take hundreds to thousands of years. Removing plant cover from soil exposes its topsoil to erosion by water and wind. This explains why protecting and renewing topsoil is key to sustainability. 

Checkpoint: 

1. Can you identify the abiotic and biotic components of the earth's life-support system? 

2. Since plants are the link between the sun and primary consumers, explain why plants are critical to sustaining all other organims. 

3. Can you identify several reasons why microbes are critical to continued life on earth? 

As energy flows through ecosystems in food chains and food webs, there is a decrease in the high-quality chemcial energy available to organisms at each successive feeding level. 

Chemical energy, stored as nutrients in the bodies and wastes of organisms, flows through ecosystems from one trophic (feeding) level to another. A sequences of organisms with each one serving as a source of nutrients or energy for the next level of organisms is called a food chain. 

Every use and transfer of energy by organisms from one feeding level to another involves a loss of some high quality energy to the environmetn as low-quality energy in the form of heat, in accordance with the second law of thermodynamics. A graphic display of the energy loss of each trophic level is called a pyramid of energy flow. This can illustrate energy loss for a food chain, assuming a 90% energy loss for each level of the chain. 

The large loss in chemical energy between successive trophic levels explains why food chains and webs rarely have more than four or five trophic levels. 

Some Ecosystems Produce Plant Matter Faster Than Others Do 

Gross primary productivity (GPP) is the rate at which an ecosystem's producers (such as plants and phytoplankton) convert solar energy into chemical energy stored in compounds found in their tissues. To stay alive, grow, and reproduce, producers must use some of their stored c hemical energy for their own respiration. Net primary productivity (NPP) is the rate at which producers use photosynthesis to produce and store chemical energy minus the rate at which they use some of this stored chemical energy through aerobic respiration. NPP measures how fast producers can make the chemical energy that is stored in their tissues and that is potentially available to other organisms (consumers) in an ecosystem. 

Gross primary productivity is similar to the rate at which you make money, or the number of dollars you earn per year. Net primary productivity is similar to the amount of money earned per year that you can spend after subtracting your expenses such as the costs of transportation, clothes, food, and supplies. 

Terrestrial ecosystems and aquatic life zones differ in their NPP. Despite its low NPP, the open ocean produces more of the earth's biomass per year than any other ecosystem or life zone. This happens because oceans cover 71% of the earth's surface and contain huge numbers of phytoplankton and other producers. 

Tropical rain forests have a high net primary productivity because they have a large number and variety of producer trees and other plants. When these fores are cleared (Core Case Study) or burned to make way for crops or for grazing cattle, they suffer a sharp drop in net primary productivity. They also lose much of their diverse array of plant and animal species. 

Only the plant matter represented by NPP is available as nutrients for consumers. Thus, the planet's NPP ultimately limits the number of consumers (including humans) that can survive on the earth. This is an important lesson from nature.