3Variations in Geographic Range size Within a taxonomic group, most species have a small geographic range:
4Geographic Ranges Vary With Latitude The geographic range size of mammals increases with latitude
5Rapoport’s Rules Climatic variability is higher at high latitudes Only organisms that can survive a broad range of climates will surviveThus, they can occupy a broader geographical rangeThis works generally works out for terrestrial animals, but is a bit different for marine organisms.
6Temperature Tolerance Range 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.ThibodauxCritical temperature limits for shallow water marine fish. Blue = upper lethal limit, red = lower lethal limit.
7Rapoport’s RulesProduct 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 HemisphereHowever, glaciation is probably a contributing factor
8Rapoport’s Rules Lack of competition in polar areas. Because fewer species, level of competition may be smallerNot yet tested
9Boundaries of Geographical Ranges Can be abrupt or gradual
10Relationship Between Distribution and Abundance 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 years263 species of British moths
11Hanski’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 specialistsLocal Population Model (metapopulation) – populations are found in discrete patches.Species differ in their capacity to disperseSpecies that disperse more are likely to be more common and more widespread