28.8:
Ecological Succession
Ecological succession is a gradual change in community structure over time. This change is often initiated by environmental disturbances. Ecologists describe how a community develops following an environmental disturbance as either primary succession or secondary succession.
Primary succession occurs when organisms colonize bare rock introduced by retreating glaciers or volcanic eruptions. In both cases, the rock lacks both soil and life forms. Pioneer species are hardy organisms that can thrive in the harsh conditions created by such environmental disturbances.
Lichens, symbiotic organisms comprised of a fungus and algae or cyanobacteria are common pioneer species. Lichens absorb nutrients from the rock and generate biomass through photosynthesis. Over time, lichens accelerate the weathering of rocks by releasing acids and penetrating the rock surface.
Dead biomass from lichens contributes to soil formation. The soil is then sequentially colonized by different types of plants. Mosses often appear first, then grasses, followed by shrubs, shade intolerant trees, such as pines, and shade tolerant trees such as oaks.
Eventually, the species composition of the new community will become relatively stable. Such a climax community will change little until a disturbance occurs. Overall, it may take hundreds of years for primary succession to produce a climax community.
Secondary succession occurs when organisms re-colonize a habitat that was cleared by a disturbance such as a flood or fire that left the soil in place. Following such a disturbance, pioneer species like annual plants, herbaceous perennials, and grasses, typically reinhabit the soil first, followed by shrubs and trees. The climax community that emerges in secondary succession may be very different than the previous one. Secondary succession establishes a climax community more rapidly, typically in about 150 years for oak and hickory forests.
28.8:
Ecological Succession
Ecological succession is influenced by the processes of facilitation, inhibition, and toleration. Facilitation occurs when early successional species create more favorable ecological conditions for subsequent species, such as enhanced nutrient, water, or light availability. In contrast, inhibition happens when early successional species create unfavorable ecological conditions for potential successive species, such as limiting resource availability. In some cases, later successional species only have the chance to thrive if a disturbance negatively impacts the early inhibitory species. Finally, toleration occurs when the ecological conditions created by early successional species neither aid nor impede the emergence of later succession species.
For example, ecologists have extensively studied primary succession resulting from glacier retreats at Glacier Bay in Alaska. Over a period of 1,500 years, pioneer species such as liverworts paved the way for creeping shrubs, which in turn set the stage for larger shrubs and trees like alder. Eventually, a climax community emerged that was dominated by spruce trees. Facilitation and inhibition influenced this succession pattern. Dryas shrubs and alders improved the nitrogen content of the soil, facilitating the establishment of spruce seedlings. However, competition and leaf litter produced by these early successional species also hindered later species’ germination and seedling survival.
Understanding ecological succession is important because humans significantly impact ecological communities. Agriculture, clear-cutting, and overgrazing by livestock disturb terrestrial ecosystems, causing species diversity to decline. Although ecosystems can naturally recover from such disturbances via ecological succession, severe damage (e.g., soil nutrient loss or toxic chemicals) may prolong or prevent recovery. To address this issue, restoration ecologists apply the principles of ecological succession to accelerate the time to climax community—thus, repairing the damaged ecosystem.
Suggested Reading
Chang, Cynthia C., and Benjamin L. Turner. "Ecological succession in a changing world." Journal of Ecology 107, no. 2 (2019): 503-509. [Source]
Emery, Sarah. 2010. “Succession: A Closer Look.” Nature Education Knowledge 3 (10): 45. [Source]
Chen, Jie, Hans-Peter Blume, and Lothar Beyer. “Weathering of Rocks Induced by Lichen Colonization — a Review.” Catena 39, no. 2 (March 1, 2000): 121–46. [Source]