Cycles in Our Ecosystems 
 
Anything that has mass and occupies space is matter. This matter must be used over and over again in our closed ecosystem, we call this recycling. We will discuss a few of the very important cycles life depends on. 
 
The Water Cycle 
 
The movement of water among the oceans and our atmosphere, and the movement of fresh water (lakes, rivers, and streams) and our atmosphere make up the water cycle. There are several parts we will consider of the water cycle. 
 
Precipitation 
 
Water moves from the atmosphere to the land or bodies of water as precipitation (rain, snow, sleet, and hail). About 91 % of the water falls into our oceans, the rest falls on lakes, rivers, ponds, streams and the land as fresh water. Remember: fresh water does not necessarily mean it is clean water, it simply means it is low in salt content. Think of ocean water as salt-water. 
 
Evaporation 
 
When water goes from the bodies of water on the Earth back into the atmosphere, evaporation has occurred. This cycle is driven by the energy from the sun. When water vapor cools as it undergoes the process of condensation. Condensation is the process of water going from the gas phase into the liquid phase. In order for water to condensate, the moisture must have some form of particulate to attach to. This particulate can be in the form of dust, smoke, or other forms of pollution as well as solid surfaces. When condensation occurs in the atmosphere and falls back to Earth we again have precipitation. This is why it is called the water cycle. 
 
Ground Water 
 
As the water falls to the Earth, some of it falls onto the land. Some of the water seeps into the ground and enters underground caves, rocks with small pores and is often stored there. This water in under ground is called ground water. Ground water may stay in the Earth for hundreds to thousands of years. It can also slowly flow through the passages underground. Some ground water may drain into rivers, streams, form springs, or enter into the ocean. 
 
Water and Life 
 
All organisms require water to survive. Humans are about 70% water. RECALL: water transports wastes from our tissues and cells. Water also regulates our body temperature when we sweat and the sweat evaporates (this cools our body). Plants also need water in order to survive (they are living organisms). When water is returned from organisms back into the environment or atmosphere, the process is called transpiration. If there was not any water on Earth, there would not be any life. 
 
The Carbon Cycle 
 
Carbon is in all living organisms. The movement of carbon from the environment into living things and back into the environment again is called the carbon cycle. 
 
Photosynthesis (This should be a review) 
 
Photosynthesis is the process by which carbon cycles from the environment into living organisms. During photosynthesis, plants use carbon dioxide from the air to make sugars. Most animals get the carbon they need by eating the plants (when they eat the plants they are taking carbon into their body to make molecules that will contain carbon. 
 
Respiration (This should be a review). 
 
Carbon returns to the environment during respiration. RECALL: Respiration occurs in both plants and animals. When respiration occurs, sugar molecules are broken down to release energy. Carbon dioxide and water are released as by-products. Remember: we exhale carbon dioxide and water. 
 
Decomposition 
 
The breakdown of dead materials into carbon dioxide and water is called decomposition. Recall: Bacteria and some fungi decompose dead materials. When these organisms decompose the dead material, they are returning the carbon to the environment. 
 
Combustion 
 
The carbon in coal, oils and natural gas returns to the atmosphere when we burn the fuels. They release carbon dioxide into the environment when they are burned. The process of burning fuel is called combustion. We use combustion to heat our houses, run our vehicles, and make electricity. 
 
The Nitrogen Cycle 
 
The movement of nitrogen from the environment to living things and back is called the nitrogen cycle. About 78% of the Earth’s atmosphere is nitrogen gas. Most organisms cannot use the atmospheric nitrogen (nitrogen in our atmosphere), but there are specific types of bacteria that convert atmospheric nitrogen into a form of nitrogen plants can take in and use. These bacteria are carrying out nitrogen fixation. After the plants get the nitrogen into their tissues, other organisms consume the plants and take the nitrogen into their body to be used. 
 
Bacteria in the soil perform the final step of the nitrogen cycle. These bacteria are of a different species than those that carry out nitrogen fixation. These bacteria break down dead organisms and animal wastes. This process produces nitrogen gas, which is returned to the atmosphere. 
 
 
SECTION 2 
 
Ecological Succession 
 
Succession is defined as the gradual development of a community over time. There are two types of succession. The first type we will discuss is primary succession and the second type is called secondary succession. 
 
 
Primary Succession 
 
Primary succession occurs when a community develops on an area where living organisms did not live before. A bare rock can be an example of a location where living organisms had not grown in the past. We will use this as our example of primary succession. 
 
If we observed a bare rock over many years, we would have noticed that the first life forms to begin growing on the rock are lichens. The acids produced by the lichens begin to weather or break down the rock. These first organisms are called pioneer species. The lichens that die over time and small fragments of rock that begins to break away from the larger rock become soil. The next organism that becomes established are mosses, which may be dropped from an animals paw, hoof, or other part of their body or the moss may have spores that are blown onto the area by the wind. Once some of the mosses and lichen begin to die off, the soil become deeper and small species of grasses and flowers can begin to grow. 
 
The grasses and flowers may get to the area by wind blowing them onto the site or animals that have eaten the seeds of the grasses and flowers may deposit them on the site. As time passes and more plant material accumulates from plants, mosses and lichens dieing, the soil continues to get deeper. This deeper soil provides a “root bed” for larger plants like small trees and shrubs. 
 
After many more years of soil formation, larger trees can become established onto the site and the area ends up being a forest. Remember that new plants arrive on an area by the wind blowing the seeds or spores and animals can carry the seeds onto the site. Many seeds that are eaten by birds are spread to new areas after the seed has passed through the digestive system of the bird. Many seeds actually cannot germinate unless they pass through the digestive system of an organism. 
 
Secondary Succession 
 
This is very similar to primary succession, but the area has once had an existing community on it. The community or ecosystem may have been destroyed by fire, other natural disasters or by the interference of humans. A good example would be an area that once had a forest on it, but the forest was removed to make farmland and the farmer stops farming. Secondary succession begins the first year the farmer stops farming. During the first year, many weeds like crabgrass and broom sedge (a form of grass) grows on the field. During the second year, new plants begin to grow from the seeds that have blown into the field or have been carried into the field by an animal. In five to fifteen years (depending on the location) small trees like pines begin to grow. After many years of the pines growing, hardwood trees begin to grow under the pines and as the pines die and fall to the ground, the hardwoods receive more sunlight in which to grow. After 80 to 100 years you will end up with a mature hardwood forest (oaks, hickories, some maples, sweetgum, and other hardwoods) and a few pines on the drier ridges in the area. 
 
 
Does succession end? Well, the answer is yes. What type of ecosystem is the end result? It depends on the climate (how much precipitation and the temperatures the area has over time). In the southeast United States, we end up with a mature hardwood forest. In the northern United States and southern Canada, you would find the Coniferous Forest (Pines, Spruces and Fir Trees). If you had an area with relatively high temperatures on average and low precipitation, you would end up with a desert. If the area had relatively low temperatures and low precipitation, you may have Tundra.