Pioneer species are hardy species which are the first to colonize previously disrupted or damaged ecosystems, beginning a chain of ecological succession that ultimately leads to a more biodiverse steady-state ecosystem. Some lichens grow on rocks without soil, so may be among the first of life forms, and break down the rock into soil for other plants. Since some uncolonized 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. Pioneer species will die creating plant litter, and break down as "leaf mold" after some time, making new soil for secondary succession (see below), and nutrients for small fish and aquatic plants in adjacent bodies of water.
Examples of the plants and organism that colonize such areas are:
- Barren sand - Lyme grass (Leymus arenarius), Sea couch grass (Agropyron pungens), Marram grass (Ammophila breviligulata)
- Salt water - green algae, Marine eel grass (Zostera spp.), Pickleweed (Salicornia virginica), and Cordgrass (hybrid Spartina × townsendii) and (Spartina anglica).
- Clear water - algae, mosses, Freshwater eel grass (Vallisneria americana).
- Lava fields - green algae
- Construction sites - <founder species types needed>
- Bare clay - <founder species types needed>
- Mountains - Lichens
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. 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 effect 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.
Secondary succession and pioneer species
Pioneer species can also be found in secondary succession, such as an established ecosystem being reduced by an event such as: a forest fire, deforestation, or clearing; quickly colonizing open spaces which previously supported vegetation.
Common examples of the plants in such areas include:
- Birch - Betula spp.
- Raspberry - Rubus spp.
- Heaths - Ericaceae spp.
- Graminoids, forbs, and wildflowers - native, introduced, and invasive species: such as fire dependent seed, cone, and resprouter chaparral genera.
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