3 Variations in Geographic Range size Within a taxonomic group, most species have a small geographic range:
4 Geographic Ranges Vary With Latitude The geographic range size of mammals increases with latitude
5 Rapoport’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.
6 Temperature 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.
7 Rapoport’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
8 Rapoport’s Rules Lack of competition in polar areas. Because fewer species, level of competition may be smallerNot yet tested
9 Boundaries of Geographical Ranges Can be abrupt or gradual
10 Relationship 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
11 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 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