Did You Know?
The fireweed plant, which is found in abundance in North America, derives its name from the fact that it can easily establish itself in areas cleared by fire.
Biological communities come into existence and eventually get replaced in an orderly manner, in what is known as ecological succession. It's a gradual process, by which the ecosystem changes and develops over a period of time. It can occur in a barren habitat with absolutely little or no soil (primary succession), or in a habitat where soil is present, but natural vegetation was destroyed (secondary succession).
In either case, ground conditions are far from favorable, and therefore, only a select species can crop up and establish themselves. These species, which start life in the otherwise defunct ecosystem, are called the pioneer species.
What are Pioneer Species?
Going by the definition, pioneer species are hardy species which establish themselves in a disrupted ecosystem and trigger the process of ecological succession. These include species like algae, lichens, and mosses, which grow in poor, nutrient-deficient soil, and put up with extreme conditions with immense ease.
Most pioneer plants are annuals, i.e., species with a life-cycle of a year, which eventually give way to the emergence of perennials. When pioneer species grow, they fix nitrogen into the soil. Basically, they absorb atmospheric nitrogen through their leaves, and transport it to their roots, where soil microbes help them covert it to a biological form, which can be used by pioneer species themselves as well as other plants and microorganisms.
The organic mulch layer that forms as a result of leaf drop or when pioneer species die, improves the quality of soil, thus resulting in favorable conditions for the growth of new species. It's also worth noting that pioneer species create favorable climate for less hardy species, thus giving them enough time to establish. Once new species start growing, it attracts a range of animals to the site, eventually turning it into a flourishing ecosystem.
Pioneer species are known for their rapid growth, which, along with their opportunistic nature, helps them quickly capitalize on any opening available. They have a shorter lifespan as compared to climax species, which are known to survive for over hundred years.
Pioneer species in primary succession are smaller than those in secondary succession. These include various species of lichens, mosses, etc. As for species that are involved in secondary succession, they are known for their tendency to disperse a lot of seeds, which helps them spread very quickly.
In the aftermath of a volcanic eruption, the entire area, with all plants and animals, is covered with lava. There is no soil in the area to support the growth of new plants. The prevailing abiotic factors provide a perfect opening for pioneer species like lichens and mosses, which are adapted to live in harsh conditions. So, lichens colonize the hardened lava, and eventually break apart the lava rock to form soil. This soil, along with the humus which is formed when lichens decompose, provides favorable conditions for the growth of new species.
Similarly, in the event of a forest fire or deforestation, pioneering species are quick to establish themselves in the affected area. One of the best examples of a pioneer species in secondary succession is fireweed, which is known to grow in acidic soil that is typical to burnt sites in the aftermath of forest fires.
Similarly, mountain avens is a pioneer species which is found in abundance in the rough terrain of North America.
While the term pioneer fauna is used by quite a few sources, it's technically incorrect, considering that animals come into the picture only after pioneer species―pioneer plants to be precise―have established themselves.