
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 are the first of life forms, and break down the rock into soil for other...
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http://en.wikipedia.org/wiki/Pioneer_species

Pioneer tree species are those which colonise an area of cleared land before any other species. Species such as Birch and Scots Pine, which are tolerant of adverse soil and climate conditions, may act as a 'nurse crop'. That is, they may initially colonise an area to eventually produce suitable conditions for tree species of later successional stag...
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http://www.encyclo.co.uk/local/21605

Plant species that dominate a community in the early stages of succession.
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http://www.physicalgeography.net/physgeoglos/p.html

the first species to colonise a barren area, beginning a new successional sequence.
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http://www.seafriends.org.nz/books/glossary.htm

a species that is an early occupant of newly created or disturbed areas. A member of the early stage communities in ecological succession.
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https://forums.skadi.net/threads/11491-Biogeography-A-Glossary
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