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

Rapoport’s Rules

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.

Temperature Tolerance Range

Critical temperature limits for shallow water marine fish. Blue = upper lethal limit, red = lower lethal limit.

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

Rapoport’s Rules

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

Rapoport’s Rules

3) Lack of competition in polar areas.– Because fewer species, level of competition may

be smaller

– Not yet tested

Boundaries of Geographical Ranges

• Can be abrupt or gradual

Relationship Between Distribution and Abundance

• There is a positive correlation between distribution and abundance – Hanski’s Rule.

263 species of British moths

Distribution = number of traps scattered around Britain that collected that species.

Abundance = average across all sites for all years

Hanski’s Rule Explained

• 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