Succession is the gradual change in plant and animal communities in an area following disturbance or the creation of new substrate. Primary succession occurs on newly exposed geological substrates not significantly modified by organisms. Secondary succession occurs in areas where disturbance destroys a community without destroying the soil. Succession generally ends with a climax community whose populations remain stable until disrupted by disturbance.Community changes during succession include increases in species diversity and changes in species composition. Primary forest succession around Glacier Bay may require about 1,500 years, while secondary forest succession on the Piedmont Plateau takes about 150 years. Meanwhile, succession in the intertidal zone requires 1 to 3 years and succession within a desert stream occurs in less than 2 months. Despite the great differences in the time required, all these successional sequences show increased species diversity over time.Ecosystem changes during succession include increases in biomass, primary production, respiration, and nutrient retention. Succession at Glacier Bay produces changes in several ecosystem properties, including increased soil depth, organic content, moisture, and nitrogen. Over the same successional sequence, several soil properties show decreases, including soil bulk density, pH, and phosphorus concentration. During ecosystem development on lava flows in Hawaii, organic matter and nitrogen content of soils increased over the first 150,000 years and then declined by 1.4 and 4.1 million years. Weatherable mineral phosphorus in soils was largely depleted on lava flows 20,000 years. The percentage of soil phosphorus in refractory form made up the majority of phosphorus on lava flows 20,000 years old or older. Nitrogen losses from these ecosystems increased over time, while phosphorus losses decreased. Succession at the Hubbard Brook Experimental Forest increased nutrient retention by the forest ecosystem. Several ecosystem properties change predictably during succession in Sycamore Creek, Arizona, including biomass, primary production, respiration, and nitrogen retention.Mechanisms that drive ecological succession include facilitation, tolerance, and inhibition. Most studies of succession support the facilitation model, the inhibition model, or some combination of the two. Both facilitation and inhibition occur during intertidal succession. Facilitation and inhibition also occur during secondary and primary forest succession.Community stability may be due to lack of disturbance or community resistance or resilience in the face of disturbance. Ecologists generally define stability as the persistence of a community or ecosystem in the face of disturbance. Resistance is the ability of a community or ecosystem to maintain structure and/or function in the face of potential disturbance. The ability to bounce back after disturbance is called resilience. A resilient community or ecosystem may be completely disrupted by disturbance but quickly return to its former state. Studies of the Park Grass Experiment suggest that our perception of stability is affected by the scale of measurement. Studies in Sycamore Creek indicate that resilience is sometimes influenced by resource availability and that resistance may result from landscape-level phenomena.
Repeat photography can be used to detect long-term ecological change. Most successional sequences and most community and ecosystem responses to climatic change take place over very long periods of time. Repeat photography has become a valuable tool to help ecologists study these long-term changes.
