See figure 18.14 , which summarizes hypotheses for why species richness is higher in the tropics. Panel B has an error that has confused students in previous years. What is the error? The blue temperate line should be above the red tropical line. There is no problem with this panel If only time available for diversification differs, both temperate and tropical species diversity should be plotted on the same curve. Tropical should be plotted on the curve at younger times. If only time available for diversification differs, both temperate and tropical curves should have the same asymptote (reach the same level of species richness). FIGURE 18.14 Hypotheses Proposed to Explain the Latitudinal Gradient in Species Richness (A) The tropics have a higher diversification rate (speciation rate - extinction rate) than temperate areas do, so they have accumulated species faster. (B) The tropics have had more time for diversification than temperate areas have, so they have accumulated more species. (C) Because their productivity is higher, the tropics have a higher carrying capacity than temperate areas, so more species can coexist there. (After G. G. Mittelbach et al. 2007. Ecol Lett 10: 315-331.).