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Ranges

We have many means for describing the spatial distribution of organisms.  Perhaps the most common is the concept of range.  A range is simply the geographic area that a creature occupies.  There are, however, several specific variants of range the fall into two broad classes--contiguous or disjunct.

Contiguous Ranges

Think of a contiguous range as being a single area that a kind of creature occupies, and that hypothetically any individual could travel to any location within the range without ever leaving it.  Consider the badger (the North American one Taxidea taxus, not the UWSP chemistry professor, the perennial Big Ten football team, or any of the variety of the overseas organisms going falsely by the term "badger").  On the map below it is clear that there is only a single [relatively large] area in North America where the [real] badger exists, and that an individual badger born in Alberta conceivably could travel to Mexico without leaving the range.  This is the idea of a simple contiguous range.

Cosmopolitan ranges occur for organisms that exist almost everywhere.  Of the approximately 2,500,000 species that currently exist, only a few thousand at most are cosmopolitan.  These tend to fall into three classes:  higher-order carnivores, large marine creatures, and certain microscopic organisms.  Cosmopolitan species must be highly tolerant and adaptable to a wide array of environmental conditions.  An example is the common bat, which can exist almost anywhere so long as its general insect food supply is consistently available.

The prefix "pan" means worldwide, but restricted to whatever follows.  The example below is of a group of non-migratory birds that require yearround supplies of fruit, and also have tolerance requirements for high temperature and continuous foliage cover availability.  We stretch the meaning of "contiguous" here, as there are wide tropical gaps to the trogon range over oceans, and an hiatus in New Guinea/Oceania east of Wallace's Line where antecedent species fill the trogon niche.

The Alcid birds illustrate another variant of limited worldwide range, these being residents of a high latitude northern hemisphere marine range.  Why do these not occur in similar environmental conditions in the southern hemisphere?  Presumably due to competitive exclusion by the penguins (a label that referred to any swimming marine fish-eater in Shakespeare's time and writing).

Disjunct Ranges

Now things get interesting for biogeographers seeking explanation of observable spatial distributions.  The redwood tree group below is an example; how can we explain the wide separation of the rather small ranges for four very similar members of the redwood family?

The ranges of two shrubs on the next map are distinctive for having wide separation on opposite sides of the Equator, or "divided by the tropics".  The usual explanation for this type of distribution is that something has transported the species across the intervening area.  The transport agent might be migratory organisms, ocean currents, winds, or other natural mechanisms.

Until recently six kinds of bison existed, two in ranges very remote from the other four.  Likewise, the two kinds of tapir have equatorial ranges on opposite sides of the Earth.  In both cases the separate populations are taxonomic relatives.  The explanation for these vicariant separations is that bison and tapirs formerly existed in much larger contiguous ranges that have since contracted into smaller relict areas.  The fossil record outside the modern ranges supports this; ancestral bison remains occur throughout Siberia and Alaska, and tapir fossils occur in North America and eastern China.

Incidentally, vicariance is also the explanation for the redwood ranges.  Redwood fossils occur throughout the world where no modern redwoods exist. 

The last form of disjunction is not truly a single range, but rather the separate ranges of taxonomically distinct organisms that have very similar form, behavior, and niche.  Kangaroo rats (a complete misnomer), gerbils, jeraboas, and hopping mice (another misnomer) all are small nocturnal burrowing seed-eaters of arid habitats.  In another example, three kinds of lungfish all have organs to breath air during the tropical dry season, when rivers and ponds often totally dry up.  These various creatures separately acquired their traits in separate ranges through evolutionary convergence, meaning independent development in response to similar environments at widely separate places.

Coincident Ranges

Very often the ranges of dissimilar organisms are quite similar.  Usually this is symptomatic of some degree of interdependence.  Various species of spruce trees dominate the boreal forests of North America, in cold regions of short growing season and infertile soils.  Migratory kinglets do not feed on spruce (they are insectivores), but instead rely on spruce for nest sites and materials; hence kinglet summer range coincides quite closely with spruce range.  

Redback voles are herbivorous mammals, but feed mostly on vegetation other than spruce.  As they are non-migratory hibernators, they eat almost constantly during their waking hours in the short summers.  Spruce is suitable habitat for voles so long as grasses are available, but they are less dependent on spruce than kinglets and vole range accordingly extends farther into tundra and grasslands.  Because of their constant food needs, however, redback voles have been unable to cross the Straits of Belle Isle barrier to the island of Newfoundland, even though spruce has.

Other Spatial Descriptors

Although range remains the most common means of describing distributions, others also receive occasional discussion.  At more of an local scale there are territories, such as the area "claimed" by an individual organism or a small group such as a wolf pack.  

At a broader  ecological scale there are the classic biomes, and within these are more specific formation groups.  These regions have definition principally on the basis of climate and vegetation.

One final means of characterizing distributions of life, although somewhat arbitrary and archaic, is the concept of biotic realms, or provinces.   Essentially these are continent- or ocean-scale regions nominally having distinct assemblages of organisms.  Some authors split them into floral realms and faunal realms, and there is some inconsistency with the labels and boundaries for particular realms (e.g., the South African realm is about the same as Capensian, but its precise boundaries may or may not include the Namibian deserts).  The concept of realms blurs further as a result of vicariance and other disjunctures, and because of biotic relocations.

Nevertheless, because there are occasional references to some realm or other, the maps below provide a rough depiction of the major floral and faunal realms.

References:

Brockman, C. Frank. Trees of North America. LOC 68-23532 1968 ISBN 0-307-47001-6

Brown, J. H. and Gibson, A. C.  Biogeography.  LOC QH84.B76 1983  ISBN 0-8016-0824-4

Burt, W. H. and Grossenheider, R. P.  A Field Guide to the Mammals of North America North of Mexico.  LOC QL715.B8 1976  ISBN 0-395-24084-0

Seddon, B.  Introduction to Biogeography.  1971.  ISBN 06-496140-0