Ecological succession is the process by which an ecosystem or community experiences change. A climax community is found when this ecosystem stops experiencing sporadic change and instead reaches stability. This means that, under the current climate conditions, little to no changes are occurring. Most diagrams depict this as a forest, though the Sahara Desert has remained unchanged for almost five thousand years.
If conditions change, the climax community will change and succession will begin again. Think what would happen if the desert suddenly had more consistent rainfall or the rainforests experienced long term drought. Those conditions then become "unideal" or "unstable" for its residents and it must change (experience succession) again until it reaches another climax community.
Primary succession occurs after a disaster when only bedrock is in the area. Think about if a parking lot was left alone to reclaim the environment and grow vegetation; the process for plants to overtake cement or rock is a slow and tedious one. Since soil and nutrients in the soil are required to foster plant growth, pioneer species like lichens are important. These are the first complex organisms involved in succession, and with the help of erosion and weathering, these species will break down rock and create soil from the remains. Since this process is so long and arduous, it will take hundreds (or thousands!) of years for other plants (like grasses) to take root and start growing. As individual organisms die and decompose, this promotes nutrient increase and soil development, which helps succession move faster. More and more complex plants, like shrubs and trees, will start to compete for resources, and forest will eventually form when hardwood trees win the interspecies "competition." This creates, in many years, a pipeline from rock to pioneer species to much larger, more complex, and ecologically more tolerant plant species.
There is little difference between primary and secondary succession after they start. Secondary succession is only different in that the soil is not wiped out; it remains, and bare bedrock is not exposed. Often, a natural disaster is more tame (such as a wildfire or a flood) and wipes away the living organisms, leaving only soil. Now the pioneer species will be grasses and ‘weeds’ like dandelions. The rest of the process remains the same, and will eventually foster competition after the ecosystem's entire rebuild.
Most diagrams of succession, primary or secondary, only show the vegetation (flora) that is changing (replaced) over time. As mentioned before, the plants present become larger or more complex the further along succession processes are. You must also realize that the animals (fauna) will also be changed (replaced). Buffalo are at home in the open grasslands, for example, but would have a hard time navigating the redwood forest.
Keystone species is depended upon by other species in the ecosystem such that if it were removed, the ecosystem would undergo dramatic changes. These keystone species are disproportionate in size, meaning that they have relatively low population density for how extremely important their effect on their ecosystem is. The wolves of Yellowstone National Park were reintroduced in 1995__ after nearly a century of absence. The video link explains some of the changes that the park experienced because of the wolves (including the shape of the rivers). The changes to the park showed that the wolves are an extremely valuable species. Keystone species are often undervalued and, as a result of their importance combined with their numbers, are much more prone to going extinct.
An indicator species is one that reflects the health of its ecosystem. If an indicator species experiences death or much lower population density, this can indicate that an ecosystem's health is poor or failing. For example, amphibians like frogs experience life both on land and in water, so their health could indicate whether or not both parts of an ecosystem are healthy and functioning properly.