Pioneer species are resilient species that are the first to colonize barren environments, or to repopulate disrupted biodiverse steady-state ecosystems as part of ecological succession. A number of kinds of events can create good conditions for pioneers, including disruption by natural disasters, such as wildfire, flood, mudslide, lava flow or a climate-related extinction event[1] or by anthropogenic habitat destruction, such as through land clearance for agriculture or construction or industrial damage. Pioneer species play an important role in creating soil in primary succession, and stabilizing soil and nutrients in secondary succession.
For humans, because pioneer species quickly occupy disrupted spaces they are sometimes treated as weeds or nuisance wildlife, such as the common dandelion or stinging nettle.[2] Even though humans have mixed relationships with these plants, these species tend to help improve the ecosystem because they can break up compacted soils and accumulate nutrients that help with a transition back to a more mature ecosystem.[3] [4] In human managed ecological restoration or agroforestry, trees and herbaceous pioneers can be used to restore soil qualities and provide shelter for slower growing or more demanding plants.[5] [6] Some systems use introduced species to restore the ecosystem, or for environmental remediation.[7] The durable and characteristics of pioneer species can also make them to become potential invasive species, or interfere with biodiversity by quickly occupying an area.[8]
Some lichens grow on rocks without soil, so may be among the first of life forms, and break down the rocks into soil for plants.[9] Since some uninhabited land may have thin, poor quality soils with few nutrients, pioneer species are often hardy plants with adaptations such as long roots, root nodes containing nitrogen-fixing bacteria, and leaves that employ transpiration. Note that they are often photosynthetic plants, as no other source of energy (such as other species) except light energy is often available in the early stages of succession, thus making it less likely for a pioneer species to be non-photosynthetic.
The plants that are often pioneer species also tend to be wind-pollinated rather than insect-pollinated, as insects are unlikely to be present in the usually barren conditions in which pioneer species grow; however, pioneer species tend to reproduce asexually altogether, as the extreme or barren conditions present make it more favourable to reproduce asexually in order to increase reproductive success rather than invest energy into sexual reproduction. Pioneer species will eventually die, create plant litter, and break down as "leaf mold" after some time, making new soil for secondary succession (see below), and releasing nutrients for small fish and aquatic plants in adjacent bodies of water.[10]
Some examples of pioneering plant species:
The pioneering fauna will colonize an area only after flora and fungi have inhabited the area. Soil fauna, ranging from microscopic protists to larger invertebrates, have a role in soil formation and nutrient cycling. Bacteria and fungi are the most important groups in the breakdown of organic detritus left by primary producing plants such as skeletal soil, moss and algae. Soil invertebrates enhance fungal activity by breaking down detritus. As soil develops, earthworms and ants alter soil characteristics. Worm burrows aerate soil and ant hills alter sediment particle size dispersal, altering soil character profoundly.
Though vertebrates in general would not be considered pioneer species, there are exceptions. Natterjack toads are specialists in open, sparsely vegetated habitats which may be at an early seral stage.[13] Wide-ranging generalists visit early succession stage habitats, but are not obligate species of those habitats because they use a mosaic of different habitats.
Vertebrates can affect early seral stages. Herbivores may alter plant growth. Fossorial mammals could alter soil and plant community development. In a profound example, a seabird colony transfers considerable nitrogen into infertile soils, thereby altering plant growth. A keystone species may facilitate the introduction of pioneer species by creating new niches. For example, beavers may flood an area, allowing new species to immigrate.[14]
The concept of ecologic succession also applies to underwater habitats. If a space becomes newly available in a reef surrounding, haplosclerid and calcareous sponges are the first animals to initially occur in this environment in greater numbers than other species. These types of sponges grow faster and have a shorter life-span than the species which follow them in this habitat.[15]
The term pioneer species is also used to refer to the first species, usually plants, to return to an area after disturbance as part of the process of secondary succession. Disturbances may include floods, tornadoes, forest fires, deforestation, or clearing by other means.[16]
Pioneer species tend to be fast-growing, shade-intolerant, and tend to reproduce large numbers of offspring quickly. The seeds of pioneer species can sometimes remain viable for years or decades in the soil seed bank and often are triggered to sprout by disturbance.[17] Mycorrhizal fungi have a powerful influence on the growth of pioneer species.[18]
Some examples of the plants in such areas include: