Although most ecologists agree that latitudinal gradients in diversity exist, there is much less consensus as to why they exist. Dozens of hypotheses have been proposed to explain these patterns, but they are hard to evaluate—in part because there are multiple and confounding differences in such factors as geographic area, climate, and productivity with latitude. More important, because these factors operate at large spatial scales and over evolutionary time, it is impossible to conduct experiments to isolate the factors and separate correlation from causation.

Here we present three well-known hypotheses that have been advanced to account for latitudinal gradients in diversity. Corroborative evidence can be found for each of these hypotheses, varying with taxon, locality, and scale, which suggests that none of these hypotheses is mutually exclusive. Multiple factors, working at multiple spatial scales, are likely responsible for common biogeographic patterns.

SPECIES DIVERSIFICATION RATE This hypothesis proposes that in the tropics the rate of speciation is higher and the rate of extinctions is lower, resulting in an overall higher species diversity than in temperate or polar regions. Two factors could account for a higher speciation rate in the tropics: a larger geographic area (this is the widest part of the planet, so it encompasses the greatest amount of area) and a warm and stable climate. The rationale is that a larger geographic area with a consistent climate should promote speciation because species will have larger geographic ranges, giving them a greater chance of reproductive isolation (see Key Concept 22.2). In addition, these large and stable areas should reduce extinctions. The net result should be an increase in the number of species in the tropics.

SPECIES DIVERSIFICATION TIME This hypothesis proposes that the amount of time over which speciation has taken place is greatest in the tropics—in other words, that the tropics have a longer evolutionary history than temperate or polar regions. First championed by Wallace in 1878, this hypothesis proposes that tropical regions, because they are more climatically stable over time, have more time to diversify than temperate or polar regions, where severe climate conditions (such as ice ages) could have decreased speciation and increased extinction. Thus even if the rates of speciation and extinction are the same worldwide, the tropics should have accumulated more species over time simply because of the lack of dramatic changes in climate.

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PRODUCTIVITY This hypothesis proposes that species diversification is promoted by higher productivity, which allows species more resources and thus decreased risk of species extinction due to competition. The productivity hypothesis might explain the positive relationship between seabird diversity and increasing latitude, given that ocean productivity is generally higher at temperate and polar latitudes. As you will see in Chapter 56, the role of productivity can more easily be explored at smaller spatial scales, where experiments can give us a better idea of its role in promoting species diversity.