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15.3: Community Dynamics

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    Community Dynamics

    Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. 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.

    Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly exposed or newly formed land is colonized by living things; in secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain.

    Primary Succession and Pioneer Species

    Primary succession occurs when new land 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 is added each year. First, weathering and other natural forces break down the substrate enough for the establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species (Figure \(\PageIndex{1}\)). These species help to further break down the mineral rich lava into 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.

    Photo shows a succulent plant growing in bare earth.
    Figure \(\PageIndex{1}\): During primary succession in lava on Maui, Hawaii, succulent plants are the pioneer species. (credit: Forest and Kim Starr)

    Secondary succession

    A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire (Figure \(\PageIndex{2}\)). 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.

    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.

    The three illustrations show secondary succession of an oak and hickory forest. The first illustration shows a plot of land covered with pioneer species, including grasses and perennials. The second illustration shows the same plot of land later covered with intermediate species, including shrubs, pines, oak, and hickory. The third illustration shows the plot of land covered with a climax community of mature oak and hickory. This community remains stable until the next disturbance.
    Figure \(\PageIndex{2}\): Secondary succession is shown in an oak and hickory forest after a forest fire.

    Disturbance

    Disturbance is an event of destruction of some part of a community, an occurrence that is followed by a sometimes pronged period of ecological recovery called succession. All communities are dynamic, changing over time in their species composition and functional attributes (such as productivity, decomposition, and nutrient cycling). However, the rate of change depends on the stability of environmental conditions, which is greatest in communities that are close to the end-point of a succession. In contrast, the most dynamic communities are associated with the younger stages of succession. Disturbances can occur on two spatial scales.

    • Stand-replacing disturbances are caused by wildfire, a disease epidemic, clear-cutting, and other cataclysmic events (Figure \(\PageIndex{3}\)). This kind of disturbance is extensive and results in the immediate replacement of a community with a different one, followed by a period of successional recovery. Over time, succession may regenerate a community similar to what existed before the disruption, or a different one may result. The younger stages of a sere (successional sequence) are especially dynamic in terms of community change. 
    • Microdisturbances are local disruptions that affect small areas within an otherwise intact community. A microdisturbance may, for instance, be associated with the death of an individual large tree, which results in a gap in the canopy, below which community change is relatively dynamic as species compete to take advantage of the additional sunlight. Similarly, the death of an individual coral head represents a microdisturbance within a tropical reef community. Although ecological changes are dynamic within a gap created by a recent microdisturbance, at the stand level the community is relatively stable. Gap-phase community dynamics occur in all ecosystems but are especially important during later stages of succession, such as in older-growth forests.

     

    Figure \(\PageIndex{3}\): Ecosystems are occasionally subjected to catastrophic disturbances, such as these forest fires in 2002 in the boreal forest of northern Quebec. The individual fires are marked with a red dot, and their smoke plumes are blowing to the south. The large white mass at the bottom right is cloud cover. Source: NASA photo ID 751339; https://www.dvidshub.net/image/751339/fires-quebec-canada-send-smoke-us-natural-hazards#.VOS_vXUtHIU 

    Contributors and Attributions

    Modified by Kyle Whittinghill from the following sources

    This page titled 15.3: Community Dynamics is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Melissa Ha and Rachel Schleiger (ASCCC Open Educational Resources Initiative) .