Why are there so many species in the tropics?

Abstract

Known for centuries, the geographical pattern of increasing biodiversity from the poles to the equator is one of the most pervasive features of life on Earth. A longstanding goal of biogeographers has been to understand the primary factors that generate and maintain high diversity in the tropics. Many 'historical' and 'ecological' hypotheses have been proposed and debated, but there is still little consensus. Recent discussions have centred around two main phenomena: phylogenetic niche conservatism and ecological productivity. These two factors play important roles, but accumulating theoretical and empirical studies suggest that the single most important factor is kinetics: the temperature dependence of ecological and evolutionary rates. The relatively high temperatures in the tropics generate and maintain high diversity because 'the Red Queen runs faster when she is hot'.

Keywords: Ecological interactions; Janzen–Connell dynamics; Red Queen; evolutionary rates; latitudinal diversity gradient; metabolic theory; species diversity; tropics.

Figure 1

Temperature dependence of productivity, number of trees and number of tree species on the six forest study sites of our Macrosystems project (Table1). Data are presented as Arrhenius plots, with the natural logarithm of rate as a function of inverse temperature, 1/kT (so colder temperatures are to the right). Data were fitted by ordinary least squares regression: values of E give the slopes, the measure of temperature dependence, and values of r² give the proportion of variation explained. Above: rate of net primary production (NPP); below: number of individual trees and number of tree species: data from V. Buzzard, C. Sides, A. Henderson and B.J. Enquist. Note that the temperature dependence for species richness was substantially higher than for NPP, which was substantially higher than for the number of individuals.

Figure 2

A tentative synthetic framework for the causal mechanisms that generate and maintain the latitudinal gradient of species diversity. Cause–effect relationships are indicated by arrows. Mechanisms that are at least in part temperature dependent, and hence consistent with the kinetics of metabolic theory, are in red. Other mechanisms are in green. Some emergent outcomes are shown in the graphs. The presentation is arranged in approximate order of increasing spatial scale and evolutionary time, starting with local ecological processes at the top and ending with regional species dynamics at the bottom. But the mechanisms operate both bottom-up and top-down, as indicated by the double-headed vertical arrows on the right, which indicate important feedbacks among processes and across scales. See text for additional explanation. Ŝ, carrying capacity for species; , turnover rate of species.

Figure 3

Schematic diagram illustrating a feedback between topography, temperature regime and speciation rates that plays a major role in the latitudinal diversity gradient. Above: Janzen's (1967) explanation for ‘Why mountain passes are higher in the tropics’. Organisms confined to elevational zones can more easily disperse across mountain passes at either higher or lower elevations in temperate regions (yellow and green arrows) than in the tropics (green arrows only), because temperate organisms living at any elevation experience wider seasonal temperature fluctuations and have broader thermal tolerances than tropical organisms. Below: my explanation for ‘Why marine islands are farther apart in the tropics’. Organisms that are confined to benthic habitats and disperse as planktonic larvae can travel longer distances in the temperate zones (blue arrows) than in the tropics (red arrows), because they have lower metabolic and developmental rates and longer survival in colder waters (O'Connor et al., 2007). In both cases, shorter-distance dispersal in the tropics results in reduced gene flow and allows higher rates of diversification and speciation. In both cases, in addition to the direct effect of temperature on physiology, the higher diversity and activity of enemies in warm tropical environments create more severe barriers to dispersal.