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Rarity: A Complicated Issue



Crepis nana

Dwarf Hawk's Beard (Crepis nana). Burnt Cape. Only ocurrence on the Island of
Newfoundland. Rare in northern Labrador. Photo: John E. Maunder. [CLICK image to
enlarge.]



Almost 300 of the approximately 1300 types of vascular plant found on the Island of Newfoundland and in adjacent Labrador are considered to be RARE.

Over 100 of these are found in our "limestone regions" - Burnt Cape, alone, is home to over 30 of them.

But, what is actually meant by "RARE"?

It turns out that there are at least three BASIC types of "species rarity":

    1. small population size and narrow distribution - "rare in all respects, but occuring only within a small area"
    2. large population size but narrow distribution - "locally common, but occuring only within a small area"
    3. small population size and wide distribution - "consistently rare over a broad area"

    There is also "local rarity". A species may be very common over most of its range, but rare in outlying areas. Small disjunct populations are a special example of this type of rarity.

    Plus, there is "political rarity". A species may be common within one political jurisdiction, but rare in an adjoining political jurisdiction, simply because the range of that species peters out just beyond that political border.

    And, of course, some species just SEEM to be rare because they are inconspicuous, hard to distinguish, or simply hard to find.

Why are some species rare?
    Usually because of limited habitat; although, sometimes, also, because of regular targeted attacks on individuals.


    Some species are just fussy, requiring very specific living conditions that may be rare and/or scattered.

    Some species are isolated in small geographical or ecological units, from which there is little or no opportunity to spread.

    Some species may have formerly been more common and widespread, but have become rare as the result of human-mediated habitat loss or degradation - including climate change. (Of course, for some species, the reverse may also be true.)

    Some species may be specifically exploited by humans as a desirable resource, to the point of significant population reduction or even extinction, or targeted as pests or vermin.

    Some species may be attacked by pests that have become more common because of human-mediated ecological change, or physical introduction.




[Page last updated: January 4, 2012]





















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