Biological communities, like the organisms that comprise them, can and do change over time. Ecological time focuses on community events that occur over decades or centuries. Geological time focuses on events lasting thousands of years or more.
Community succession is the sequential replacement of species by immigration of new species and local extinction of older ones following a disturbance that creates unoccupied habitats for colonization. The initial rapid colonizer species are the pioneer community. Eventually a climax community of more or less stable but slower growing species eventually develops.
During succession productivity declines and diversity increases. These trends tend to increase the biomass (total weight of living tissue) in a community. Succession occurs because each community stage prepares the environment for the stage following it.
Primary succession begins with bare rock and takes a very long time to occur. Weathering by wind and rain plus the actions of pioneer species such as lichens and mosses begin the buildup of soil. Herbaceous plants, including the grasses, grow on deeper soil and shade out shorter pioneer species. Pine trees or deciduous trees eventually take root and in most biomes will form a climax community of plants that are stabile in the environment. The young produced by climax species can live in that environment, unlike the young produced by successional species.
Secondary succession occurs when an environment has been disturbed, such as by fire, geological activity, or human intervention (farming or deforestation in most cases). This form of succession often begins in an abandoned field with soil layers already in place. Compared to primary succession, which must take long periods of time to build or accumulate soil, secondary succession occurs rapidly. The herbaceous pioneering plants give way to pines, which in turn may give way to a hardwood deciduous forest (in the classical old field succession models developed in the eastern deciduous forest biome).
Early researchers assumed climax communities were determined for each environment. Today we recognize the outcome of competition among whatever species are present as establishing the climax community.
Climax communities tend to be more stable than successional communities. Early stages of succession show most growth and are most productive. Pioneer communities lack diversity, make poor use of inputs, and lose heat and nutrients. As succession proceeds, species variety increases and nutrients are recycled more. Climax communities make fuller use of inputs and maintain themselves, thus, they are more stable. Human activity (such as clearing a climax forest community to establish a farm field consisting of a cultivated pioneering species, say corn or wheat) replaces climax communities with simpler communities.
Communities are composed of species that evolve, so the community must also evolve. Comparing marine communities of 500 million years ago with modern communities shows modern communities composed of quite different organisms. Modern communities also tend to be more complex, although this may be a reflection of the nature of the fossil record as well as differences between biological and fossil species.
The basic effect of human activity on communities is community simplification, an overall reduction of species diversity. Agriculture is a purposeful human intervention in which we create a monoculture of a single favored (crop) species such as corn. Most of the agricultural species are derived from pioneering communities.
Inadvertent human intervention can simplify communities and produce stressed communities that have fewer species as well as a superabundance of some species. Disturbances favor early successional (pioneer) species that can grow and reproduce rapidly.