What Is a Pioneer Species?

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. 

To survive under these conditions, pioneer species are usually:

• Hardy enough to withstand harsh environments
• Photosynthetic, due to the lack of soil nutrients
• Able to produce a large volume of seeds with high rates of dispersal
• Wind pollinated, due to the lack of insects 
• Able to survive long dormancy periods
• Early 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

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, including small shrubs and eventually trees, arrive. 

Secondary Succession

Contrary to primary succession, secondary succession occurs after an existing community is disturbed or entirely removed by natural or manmade forces. In this case, the vegetation is removed, but soil remains. This means that pioneer species in secondary succession can start from 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 quicker than in primary succession. Alders, birches, and grasses are common pioneer species in these environments because they thrive in sunny conditions. 

Factors that can impact the development of a community during secondary succession include:

• 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.

Glacial ice

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. 

Lava flow

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.

Flood

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.

Wildfire

Secondary succession also occurred following the Acadia National Park wildfire in 1947, which burned more than 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 seeds 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.

Agriculture

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.