Ecological succession is the natural process of change within an ecological community. Primary succession occurs within an open landscape, where no biological community exists. Primary succession is begun by the establishment of pioneer species, which slowly create soil. As the soil forms and improves, different species of plants begin to colonize the area, eventually crowding or shading the pioneer species out. Over time, these species too are replaced by others, and with the change of plant species, the kinds of animals that can live in the area changes also.
Secondary succession occurs when an area with an established biological community is disturbed. This can be natural, like a wildfire or hurricane, or it can be man-made, such as lumber harvesting or land clearing operations. In either case, as long as the soil remains, plants will usually begin to reestablish themselves, and succession will soon be underway once again.
When humans deforest an area, there are a couple of issues that can become problematical. One is erosion. Plant roots hold soil in place, and is the plants are removed from an area, wind and water can take the soil away. This makes it hard for plants to grow back, and the lost soil can pollute waterways or become an air pollution issue as well. Urban development often involves creating large areas of paved surfaces, which force rain and snowmelt to run off instead of soaking into the ground. This intensifies erosion and makes it harder for plants to grow well.
When an area is suddenly deforested, the animals that lived there are displaced too. Even if the plants grow back eventually, the animals may not be able to establish new populations in that area; urbanization interrupts natural migration pathways, creating "islands" which animals cannot get to.
Pollution impacts species differently. While some native species may be able to sustain populations in polluted areas, others will succumb, and the natural balance of the biological community will be upset. This paves the way for invasive species, which can compete with the native species and completely upset the natural succession process.