Answers to Review Questions
1. The largest spatial scale is the global scale, which covers the entire world, over which there are major differences in species diversity and composition with latitude and longitude.
These patterns are controlled by speciation, extinction, and dispersal. The next scale down is the regional scale, defined by areas of uniform climate and by species that are bound by dispersal limitation to the region. Within a region, species diversity and composition depend on dispersal and extinction rates across the landscape. The regional species pool (also called gamma diversity) has an important influence on the species present at the next scale down, the local scale (also called alpha diversity). The relationship between regional and local species richness can help us to determine the extent to which the regional species pool or the local effects of species interactions and physical conditions determine local species richness.2. Wallace identified six terrestrial biogeographic regions, which represent distinct biotas that vary in species diversity and composition. Wallace believed that these biogeographic regions reflect the evolutionary isolation of species due to the movements of the continents. Thus, the ancestors of many modern species may have occurred together in the evolutionary past, but since Pangaea began breaking up into the continents we know today, they have evolved separately. Recent research suggests that the biogeographic regions are more subdivided than previously thought, suggesting more isolation than simply the movements of the continents. There are also impediments to dispersal within oceans, such as currents, thermal gradients, differences in water depth, and the continents themselves, so it is assumed that the oceans could be divided into biogeographic regions, but that effort has received considerably less attention.
3.
The three main hypotheses focus on (1) species diversification rate, (2) species diversification time, and (3) productivity. The first hypothesis proposes that both the large geographic land area and the thermal stability of the tropics might promote higher speciation rates and lower extinction rates, thereby increasing the population sizes and geographic ranges of species. Speciation rates should increase because larger geographic ranges should lead to greater reproductive isolation. Extinction rates should decrease because larger population sizes should lower the risk of extinction due to chance events while larger species ranges should spread extinction risk over a larger area.The second hypothesis suggests that the tropics have had a longer evolutionary history than the temperate or polar zones because of their greater climatic stability. This stability may have allowed more species to evolve without the interruption of severe climatic conditions that would have hindered speciation and increased extinctions in the temperate and polar zones.
The third hypothesis suggests that the high productivity of the tropics increases species diversity by promoting larger population sizes, which should lead to lower extinction rates and overall higher species richness.