9.5: Succession
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Ecological Succession
Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state. Thus, disturbance can initiate successional change. Ecological succession is the process by which natural communities replace (or “succeed”) one another over time. Successional changes are often orderly and predictable. Succession may be initiated either by formation of new, unoccupied habitat, such as from a lava flow or a severe landslide, or by some form of disturbance of a community, such as from a fire, logging, or hurricane.
For example, when an old farm field in the midwestern U.S. is abandoned and left alone for many years, it gradually becomes a meadow, then a few bushes grow, and eventually, trees completely fill in the field, producing a forest. Each plant community creates conditions that subsequently allow different plant communities to thrive. For example, early colonizers like grasses might add nutrients to the soil, whereas later ones like shrubs and trees might create cover and shade. Succession stops temporarily when a “climax” community forms; such communities remain in relative equilibrium until a disturbance restarts the succession process.
Understanding how succession happens in a variety of ecosystems—and what kinds of disturbances and time spans lead to the formation of different plant and animal communities—is important for scientists who want to understand ecosystem dynamics and effectively protect or restore natural communities. For example, many natural communities in North America have adapted to periodic disturbances from wildfires: This can help maintain prairie or savanna communities that depend on open habitat and nutrient cycling that might occur as a result of fire.
Primary Succession and Pioneer Species
Primary succession occurs when new substrate is formed or rock is exposed: for example, following the eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new land is continually being formed. On the Big Island, approximately 32 acres of land are added each year. First, weathering and other natural forces break down the substrate enough for the establishment of pioneer species such as hearty plants and lichens with few soil requirements (Figure \(\PageIndex{1}\)). These species help to further break down the mineral-rich lava into the soil where other, less hardy species will grow and eventually replace the pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing organic material and contribute to soil formation. Over time the area will reach an equilibrium state, with a set of organisms quite different from the pioneer species.
The predictable change in a community following a disturbance that completely removes and/or destroys the soil such as glacial retreat, volcanic eruption, or newly uplifted land. Primary succession describes how organisms populate an area for the first time.
Because primary succession begins without intact soil and the presence of organisms it generally occurs over much longer time scales than secondary succession.
The early stages of primary succession are dominated by species with small propagules (seed and spores), which can be dispersed long distances. The early colonizers—often algae, fungi, and lichens—stabilize the substrate and create soils by breaking down rocks into smaller particles. Nitrogen supplies are limited in new soils, and nitrogen-fixing species tend to play an important role early in primary succession. Thus early, successional species may alter the substrate in such a way that facilitates colonization by other species. Over time organic matter gradually accumulates, favoring the growth of herbaceous plants like grass, ferns and herbs. These plants further improve the habitat by creating more organic matter when they die, and providing habitats for insects and other small animals. This leads to the occurrence of larger vascular plants like shrubs, or trees. More animals are then attracted to the area and a climax community is reached.
Secondary succession
A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire (Figure \(\PageIndex{3}\)). Wildfires will burn most vegetation and kill those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe fires, the area will soon be ready for new life to take hold.
The predictable change in a community following a disturbances that does not completely destroy the soil such as a forest fire, hurricane, flood, or farming. The disturbance significantly alters the area, but it is not rendered completely lifeless. Because the soil retains nutrients and seeds colonization by pioneer species occurs quickly.
Before the fire, the vegetation was dominated by tall trees with access to the major plant energy resource: sunlight. Their height gave them access to sunlight while also shading the ground and other low-lying species. After the fire, though, these trees are no longer dominant. Thus, the first plants to grow back are usually annual plants followed within a few years by quickly growing and spreading grasses and other pioneer species. Due to, at least in part, changes in the environment brought on by the growth of the grasses and other species, over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which will remain stable until the next disturbance.
Climatic factors may be very important, but on a much longer time-scales than any other. Changes in temperature and rainfall patterns will promote changes in communities. As the climate warmed at the end of each ice age, great successional changes took place. The tundra vegetation and bare glacial till deposits underwent succession to mixed deciduous forest. The greenhouse effect resulting in increased temperatures is likely to bring profound community changes in the next century. Geological and climatic catastrophes such as volcanic eruptions, earthquakes, avalanches, meteors, floods, fires, and high winds also bring successional changes.
In general, communities in early succession will be dominated by fast-growing, well-dispersed species (opportunist, fugitive, or r-selected life histories). As succession proceeds, these species will tend to be replaced by more competitive (K-selected) species. Trends in ecosystem and community properties in succession have been suggested, but few appear to be general. For example, species diversity almost necessarily increases during early succession as new species arrive, but may decline in later succession as competition eliminates opportunistic species and leads to dominance by locally superior competitors.
Dynamics in secondary succession are strongly influenced by pre-disturbance conditions, including soil development, seed banks, remaining organic matter, and residual living organisms. Because of residual fertility and pre-existing organisms, community change in early stages of secondary succession can be relatively rapid. Secondary succession is much more commonly observed and studied than primary succession. Particularly common types of secondary succession include responses to natural disturbances such as fire, flood, and severe winds, and to human-caused disturbances such as logging and agriculture.
Unlike in primary succession, the species that dominate secondary succession, are usually present from the start of the process. In some systems, the successional pathways are fairly consistent, and thus, are easy to predict. In others, there are many possible pathways, potentially leading to alternate stable states.
In ecology, the theory of alternative stable states predicts that ecosystems can exist under multiple "states" (sets of unique biotic and abiotic conditions). These alternative states are non-transitory and therefore considered stable over ecologically-relevant timescales. However, ecosystems may transition from one stable state to another, (sometimes termed a phase shift or regime shift), when disturbed. Due to ecological feedbacks, ecosystems display resistance to state shifts and therefore tend to remain in one state unless disturbances are large enough.
Climax Communities?
Ecological succession is a foundational concept in ecology, which as a field examines the structure and dynamics of biological communities. Today, the concept of ecological succession continues to be studied from new angles as humans modify the global environment more than ever before. As new nuances have been added to the original theory, insights have emerged that are valuable to humans interested in managing natural resources.
For example, recent studies show that even in “climax” communities, changes in what resources are available may shift the balance of the species composition over time, even without a formal disturbance. While other natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Thus, modern ecologists have largely abandoned the idea of stable end-stage climax community in favor of non-equilibrium ideas of ecosystem dynamics. Other work has examined the impact of biodiversity loss, invasive species, climate change and other anthropogenic factors in altering the way ecosystems respond to change.
As native species go extinct or become rare, new species enter ecosystems, and climate baselines shift, the communities that once dominated an ecosystem may be less likely to eventually return after a disturbance. However, studying succession can also provide valuable insights for ecologists and wildlife managers interested in restoring those natural systems: through careful management such as controlled burning or invasive species control, people can help ecological communities stay strong.
Watch the video to see how successional change is good for communities. As you watch write down 2 examples of how disturbances may lead to higher biodiversity.
Attribution:
This page is a modified derivative of:
- Ecological Succession by Tara Jo Holmberg via General Ecology; license CC BY-SA 4.0
- Primary succession via Wikipedia; license CC BY-SA 4.0