Nature Word of the Week: Accidental

This week's Nature Word is: Accidental

We all know the meaning of accidental in every day speech, so we probably have a good idea of what it would mean when used to describe species occurrence.  But let's take a closer look at this and other words used to talk about how often a particular species is seen in a given area.  (These words are most commonly used in reference to bird species, but they could also apply to any highly-mobile group of animals.)

Image credit: University of Columbia, SECAC, NASA, via Wikipedia

Merriam-Webster lists three definitions for accidental, but the one that fits best, when describing species occurrence is this: 

occurring unexpectedly or by chance

The National Audubon Society Field Guide to North American Birds (Western Region, second edition, August 2000) defines accidental as:

A species that has appeared in a given area only a very few times and whose normal range is in another area.

Accidental species then, probably took a wrong turn at Albuquerque.


At the opposite end of the occurrence scale, we have the term permanent resident.  This, of course, is self-explanatory - it describes an animal species that lives in the area all year round.  You could also have a winter or summer resident if the species is migratory (moving between a winter range and a summer range). 

Somewhere in the middle of the scale, we have a couple of different terms:

• Casual - A species that has appeared in a given area somewhat more frequently than an accidental, but whose normal range is in another area.  (The National Audubon Society Field Guide to North American Birds Western Region, second edition, August 2000)
•  Erratic - Occurring in numbers some years, but very scarce or even absent in others.  (US Fish and Wildlife Service)

You could also use erratic in conjunction with migration, to describe an erratic migrant - a species that moves through an area in some years but not others.

Related to species occurrence, we have a whole list of terms which describe species distribution or range (Definitions below from The National Audubon Society Field Guide to North American Birds Western Region, second edition, August 2000):

•  Circumpolar - Of or inhabiting the Arctic (or Antarctic) regions in both the Eastern and Western hemispheres.
• Cosmopolitan - Worldwide in distribution, or at least occurring in all continents except Antarctica.
• Equatorial - Of or inhabiting the equatorial regions.
• Local - Occurring in relatively small, restricted areas within the range, rather than commonly and widespread throughout the range.  (Species whose occurrence is local usually have highly specialized habitat requirements.)
• Pelagic - Of or inhabiting the open ocean.
• Riparian - Of or inhabiting the banks of rivers or streams.
• Subalpine - Of or inhabiting/pertaining to the stunted forest or other vegetation immediately below the treeless, barren alpine zone on high mountains.
• Woodland, coastal, etc.,