Ecological succession is the change of communities over time, where one community replaces an existing community. An example is primary succession on bare rock. Eventually, weathering will begin to break down the rock and moss and lichens will grow on the rock causing it to further break down. Soil begins to form. This soil allows for grasses to grow. They will eventually add humus and produce more soil, allowing for shrubs to grow, Once enough soil has formed, eventually a forest community will develop. As each plant community changes, so do the animals that live in each community. The flow of energy between trophic levels is demonstrated by a food chain or web. Producers contain the greatest amount of energy absorbed directly from the sun. Herbivores consume producers and gain energy, although some is lost as heat. Carnivores consume herbivores, gain energy but also lose some as heat. Decomposers gain energy from the breakdown of dead organic matter. Limiting factors are any necessary resources an organism needs for its survival. It can be food, space, light, etc. Depending on how available these factors are, limits the size a population can be. Limiting factors are related to carrying capacity, which is the amount of organisms an area can support indefinitely. For example, if an area can support 30.000 deer but there are 50,000 deer, there will be a population explosion, but once carrying capacity has been surpassed, increased competition for food and space(limiting factors)will lead to many deer dying of starvation. A much studied example is the Kaibab deer, where there were no wolves as predators and deer increased until they exceeded the carrying capacity of the area, leading to a large amount of the population to die of starvation.