You are watching: Which of these examples describes a pioneer species that is starting primary succession?
Penstemon flowers grow in barren land in Mount St. Helens, which erupted in 1980.Steve Satushek / Getty Images
A pioneer species is one that’s typically the first to colonize a barren ecosystem. These hardy plant and microbial species are also the first to return to environments that have been disrupted by events like wildfires and deforestation. Once they arrive, pioneer species begin the recovery of the ecosystem by making it more hospitable for later species. This is typically accomplished through soil stabilization, nutrient enrichment, reduction of light availability and wind exposure, and temperature moderation.
Hardy enough to withstand harsh environmentsPhotosynthetic, due to the lack of soil nutrientsAble to produce a large volume of seeds with high rates of dispersalWind pollinated, due to the lack of insects Able to survive long dormancy periodsEarly to mature and dependent on asexual reproduction
With the increase in wildfire frequency in the Western United States — and deforested areas expanding worldwide — it’s more important than ever to understand what pioneer species are and their role in ecosystem recovery and growth.
Pioneer Species and Ecological Succession
Ecological succession describes the changes in species structure that an ecosystem undergoes over time. This is a gradual process that can occur in a previously barren environment (as in the case of primary succession), or in an area that has been cleared due to a serious disturbance (as with secondary succession). Pioneer species play an integral role in these processes by preparing the new or recently disturbed ecosystem for more complex communities.
Primary succession occurs in areas with no existing plants, animals, insects, seeds, or soil — usually where there was not a prior community. However, this type of succession can technically occur even where a former community has been disturbed or removed — but there cannot be any existing organic matter to qualify as primary succession.
Fungi and lichen are the most common pioneer species in primary succession because they have the ability to break down minerals to form soil and subsequently develop organic matter. Once pioneer species colonize the area and start to build soil, other species — like grasses — begin to move in. The complexity of the new community increases as more new species arrive, including small shrubs and eventually trees.
In contrast to primary succession, secondary succession occurs after an existing community is disturbed — or entirely removed — by natural or man-made forces. In this case, the vegetation is removed but soil remains. This means that pioneer species in secondary succession can start from either roots and seeds in the residual soil. Alternatively, seeds can be carried by wind or by animals visiting from neighboring communities. Grasses, alders, birches, and pine trees are examples of plants that begin secondary succession.
Behavior of the community following a disturbance depends on a number of factors, but mostly on the nature of the pre-disturbance ecosystem. That said, because secondary succession begins with some remnants of the original community, change typically occurs much more quickly than in primary succession. Alders, birches, and grasses are common pioneer species in these environments because they thrive in sunny conditions.
Soil condition. The overall quality of the soil that remains after a disturbance can have a substantial impact on secondary succession. This may include everything from soil pH to the density and makeup of the soil. Residual organic matter. Likewise, the amount of organic matter remaining in the soil after the disturbance affects the speed of succession and the types of pioneer species. The more organic matter in the soil, the faster secondary succession is likely to occur. Existing seed banks. Depending on how the community was disturbed, seeds may remain in the soil. This is also impacted by how close the area is to outside sources of seeds — and can lead to a higher abundance of certain pioneer species.Residual living organisms. If roots and other underground plant structures survive the disturbance, secondary succession will occur more quickly and in a way that more closely reflects the original ecosystem.
Examples of Pioneer Species
Lichens, fungi, bacteria, fireweed, grasses, alder, and willow are examples of pioneer species. Here are some common circumstances where pioneer species have aided in succession:
Primary succession is studied less frequently and in less detail than secondary succession. However, one of the most basic examples of primary succession occurred in Yellowstone after the Pinedale Glacial Maximum when the area was covered in glacial ice. After the ice removed the soil and vegetation from the environment — and after the glacial period came to an end — the area was recolonized by pioneer species that broke down the bedrock and formed soil for other plants to colonize.
Following the Mount Saint Helens eruptions in 1980, the surrounding areas were left barren and ash-covered with very little surviving plants and animals. Even so, some underground animals survived, as did some underground root systems of plants like willow and black cottonwood. In the early aftermath of this destruction, these surviving root systems, as well as alder and fir, were able to colonize the raw landslide debris and lava flows.
In 1995, flooding of the Moorman’s and Rapidan rivers in Shenandoah National Park caused widespread destruction of plant and animal life — much of which was replaced with gravel and boulders. Since then, plant and wildlife communities have started to rebuild through secondary succession.
Secondary succession also occurred following the Acadia National Park wildfire in 1947, which burned over 10,000 acres of the park. After the fire, some of the previously wooded areas were logged for timber salvage and cleanup — with some logs left behind to promote regrowth of the forest ecosystems. Through secondary succession, the forests regrew with the help of existing root systems, stump sprouts, and seed carried on the wind.
Trees like birch and aspen that had not previously grown in the area took advantage of the newly sunny conditions and flourished early on. Once these deciduous trees formed a canopy, the spruce and fir that had originally thrived in the region were able to return, resulting in the mix of deciduous and evergreen trees that’s present today.
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Agriculture — especially slash and burn type agriculture — can have devastating impacts on the natural environment. During fallow periods immediately following agricultural use, secondary succession occurs when remaining seeds, root systems, weeds, and other pioneer species start to recolonize the land. This process is similar to what occurs in the wake of logging and other deforestation.