The Relationship Between Distribution and Abundance
- Chapter 8

Spatial
Scale of
Geographic
Ranges

Variations in Geographic Range size
• Within a taxonomic group, most species have a small geographic range:

Geographic Ranges Vary With Latitude
The geographic range size of mammals increases with latitude

1) Climatic variability is higher at high latitudes
–
Only organisms that can survive a broad range of climates will survive
– Thus, they can occupy a broader geographical range
– This works generally works out for terrestrial animals, but is a bit different for marine organisms.

Water temperature is more stable at the equator and at the poles.
Because there is a large temperature difference in the temperate latitudes, we would expect to see an adaptive difference to temperature variation in the middle latitudes.
Thibodaux
Critical temperature limits for shallow water marine fish. Blue = upper lethal limit , red = lower lethal limit .

2) Product of Glaciation, particularly in the
Northern Hemisphere.
– Only those animals with a high dispersal capacity were able to colonize northern areas, thus having a large geographical range.
– Does not explain for Southern Hemisphere
–
However, glaciation is probably a contributing factor

3) Lack of competition in polar areas.
–
Because fewer species, level of competition may be smaller
– Not yet tested

• Can be abrupt or gradual

• There is a positive correlation between distribution and abundance – Hanski’s Rule .
Distribution = number of traps scattered around
Britain that collected that species.
Abundance = average across all sites for all years
263 species of British moths

• Sampling Model – more rare (or hard to catch) species may not show up in all traps.
• Ecological Specialization Model (Brown’s Model) –
Species able to exploit a wide range of resources become both widespread and common.
– Generalists versus a specialists
• Local Population Model (metapopulation) – populations are found in discrete patches.
– Species differ in their capacity to disperse
– Species that disperse more are likely to be more common and more widespread