http://www.uen.org/themepark/cycles/nature.shtml

Nature’s cycles have to do with how the earth renews itself. The living things within an ecosystem interact with each other and also with their non-living environment to form an ecological unit that is largely self-contained. Sometimes this renewal process is gradual and gentle. Sometimes it is violent and destructive. Nevertheless, ecosystems contain within themselves the resources to regenerate themselves

Water Cycles

All of the water that is on the earth has always been here. Earth never gets water added to it–nor does water disappear from the earth. Water is constantly recycled in a process known as the hydrologic or water cycle.

Fresh water is more scarce than you might think. 97% of all the water on the earth is in the oceans, and so only 3% is fresh water. About 2.4% of the water on earth is permanently frozen in glaciers and at the polar ice caps. About 1/2 of 1% of the water on earth is groundwater. Only about 1/100 of 1% of the water on earth is in the rivers and lakes. Water is essential to life on earth, so it is important that we protect our water resources.

Nature has a way of keeping the amount of water on the earth relatively constant. A large amount of water evaporates from the surfaces of oceans, rivers, and lakes every day. It forms water vapor that rises into the air until it cools, condenses, and forms water droplets. Millions of these droplets come together to form clouds. When clouds get heavy enough, gravity tugs on the droplets, and the clouds release their water as rain or snow. This precipitation falls into streams and rivers, which flow back to the oceans, seas, and lakes, where the water cycle can begin again.

Energy

The sun is the source of all life on our earth. Every form of energy, except for atomic energy, can be traced back to the sun. Happily, the earth is at the best possible distance from the sun for the sun’s heat to provide this energy for life. Energy from sunlight is used by plants to make food from air, water, and the minerals in the soil. This energy is stored by plants who are the primary producers in ecosystems.

Energy sources such as the fossil fuels of coal, petroleum, and natural gas are really just ancient stockpiles of the sun’s energy stored in plants and the animals that ate those plants that are thousands or millions of years old. These fuels came from plants that used sunlight when they lived long ago. When these plants died, they fell to the ground where their remains piled up over thousands or millions of years. As this pile grew large, the remains at the very bottom became pressed together. Over time, these remains changed. Some became a gas–natural gas. Some became a liquid–petroleum. Some became a solid or a rock–coal. We use these forms of energy to power vehicles, heat homes, and run industries. Fossil fuels are considered nonrenewable sources of energy because they cannot be replaced once they are used up.

Chemical Cycles

Balance is essential to the earth. Chemical cycles keep the amount of elements on the earth in a perfect balance.

Oxygen Cycle

The amount of oxygen in and around the earth is fixed. But this oxygen is fed again and again through the world’s living systems in a never-ending circle called the oxygen cycle. Our needs are just part of this cycle. The cycle involves a continual exchange of gases between the air and animals and plants. In a process called respiration, animals and plants take oxygen from air and give back carbon dioxide. In a process called photosynthesis, plants take carbon dioxide from air and water and give back oxygen. Respiration and photosynthesis are effectively opposite processes. Respiration is an oxidation reaction, which takes oxygen from the air or from water. Photosynthesis is a reduction reaction. It adds oxygen gas to the air. Enormous quantities of oxygen are taken in by plants and animals every day, and huge quantities of oxygen are returned to the air by plants. These amount exactly balance so that overall the amount of oxygen in the air stays the same.

Carbon Cycle

Carbon is essential for the chemical processes that support life. It plays such an important role in life that sometimes we say that life is “carbon-based”. But there is only a limited amount of carbon on the earth. So carbon is constantly cycling around the earth, turning up in a lot of different forms and places. The reactions that move carbon around make up a giant web called the carbon cycle.

Plants get carbon by taking carbon dioxide from the air. They use the carbon dioxide and the energy from sunlight to make food. Animals get their carbon by eating those plants or by eating animals that have eaten those plants. When organisms breathe, they take oxygen from the air. During respiration, the oxygen reacts with food to provide energy. Respiration produces carbon dioxide which is released to the air.

Volcanic eruptions are a source of carbon. When a volcano erupts, it releases huge amounts of carbon dioxide. But remember–the earth needs it elements to stay in balance. So the effect of volcanoes is balanced by weathering which is a chemical reaction between rainwater and rocks that absorbs carbon dioxide from the air to create rock carbonate minerals. Left to themselves, these natural processes are in perfect balance. But human activities can disturb the cycle and increase the amount of carbon dioxide in the atmosphere. This could cause problems in the future because carbon dioxide is vital for controlling the world’s climate.

Nitrogen Cycle

Nitrogen atoms are constantly moving in a giant circle from the air, through the soil, into the bodies of plants and animals, and eventually back to the air. This whole process is called the nitrogen cycle. All living things need nitrogen to develop and grow. Even though the earth’s atmosphere is made up of 78% nitrogen, plants and animals cannot use it in this form because the nitrogen atoms are too firmly bound together in molecules. So plants must draw their nitrogen from nitrogen compounds dissolved in the soil, and animals get their nitrogen by eating plants or by eating other animals that eat plants.

The nitrogen gets into the soil in a couple of different ways. A small quantity of the nitrogen found the soil by way of lightning. Lightning changes atmospheric nitrogen into nitrogen dioxide which is soluble in water. The nitrogen oxides dissolve in rainwater to form nitric acid which is absorbed by soil. The rest of the nitrogen in soil comes from bacteria. Bacteria are the only living things capable of getting nitrogen directly from the air. This is called “fixing”. The process is started by certain kinds of bacteria in the soil that can extract nitrogen from the air. Then other bacteria convert the nitrogen into nitrogen compounds called nitrates, This process is called nitrification. Plants absorb the nitrates and turn them into more complex nitrogen compounds. Bacteria also help return nitrogen to the air. Bacteria in the soil decompose animal waste and the remains of dead animals and plants and produce ammonia. Nitrifying bacteria turn the ammonia into nitrates. Other bacteria, called denitrifying bacteria, convert some of the nitrates back into nitrogen gas, which is released into the air. All these different steps form a massive cycle. The effect is that, over time, bacteria in the soil return almost the same amount of nitrogen to the air as other bacteria take from the air. This keeps the nitrogen content of the earth and its atmosphere in a perfect balance.

Unfortunately, humans are interfering with the natural balance when they overuse artificially produced nitrates as agricultural fertilizers. Before these nitrates can be converted into atmospheric nitrogen, they are often carried off from the soil by rain or irrigation. These dissolved nitrates are carried to streams and rivers and even seep down to groundwater. In some parts of the world, water for humans and animals contains such high concentrations of nitrates that it is unsafe for consumption. These excessive amount of nitrates, when they reach rivers and lakes, cause too much algae to grow. This over-abundance of algae uses up too much of the oxygen in the water. When oxygen levels fall, other forms of life in the water die off.

Geophyscial Cycles

The surface of the earth does not hold still. Continents and islands have been coming together, splitting apart, folding, sinking, rising, and rotating for millions of years. They are propelled by forces deep within the earth.

In recent years, the ability of satellites to send back photographs of the entire surface of the earth has greatly enabled geologists and other scientists to learn more about the geophysical cycles that form our planet.

Rock Cycles

Even rocks have a cycle. Rocks are continually circulating in the mantle just below the crust of the earth. They are sometimes thrust up into the crust due to convection currents. Imagine really thick jam slowly cooking in a big pot on a stove. The jam is thick, and when it reaches a high temperature, convection currents circulate through it. Occasionally big bubbles of steam erupt from the jam and splash onto the top of the stove. This is how rocks get thrust up onto the top of the crust from the boiling mantle below. Rocks can also reach the surface when they are spit out by volcanoes.

Once on the surface of the earth, rocks cool down. Over time, they are broken up or worn down by weather, and the fragments are carried back to the ocean by way of wind, rain, and the flow of rivers and streams. All of these small pieces of rock collect as sediment at the bottom of seas and oceans. The sediment slowly solidifies into rock and is sometimes drawn back down in to the mantle at subduction zones or reaches the surface again as sea levels change or plates collide.

Life Cycle of Plants

There are over 375,000 different species of plants on the earth. They range from tiny, single-celled algae to huge sequoia trees. Life on earth would not be possible without plants because they are the only living things that are capable of converting sunlight into energy. That energy fuels the other processes of life on earth.

Because plants make their own food, they are able to live almost everywhere on earth in a wide range of habitats. Many plants have developed special adaptations to help them survive.

Life Cycles of Animals

The life cycle of an organism refers to the sequence of developmental stages that it passes through on its way to adulthood. Mammals, reptiles, amphibians, birds, fish, insects and other invertebrates–they each have their own unique way of reproducing life. There is an amazing variety of life cycles within the animal world. Surprisingly, only about 3% of all animal species give birth to live young as part of their life cycle. Most animals lay eggs.

Animals need to eat, to grow, to be safe, and to reproduce. This is all part of the life cycle. Their bodies are adapted in a wonderful range of ways to solve these problems of survival.