Stevens 1989

Stevens GC. 1989. The latitudinal gradient in geographical range: how so many species coexist in the tropics. The American Naturalist 133: 240-256.

This paper is an essay that provides an overview of the evidence suggesting a relationship between Rapoport’s rule (larger species geographic ranges at higher latitudes) and the global latitude biodiversity gradient. Both phenomena show coincident exceptions of taxa, indicating a common underlying cause. This is also the paper that coins the term “Rapoport’s rule”.

The original explanatory mechanism for Rapoport’s rule invokes the range of temperatures or other climatic conditions experienced by individuals during their lifetimes at different latitudes. High latitude locations have wider annual ranges of temperature, for example a spruce tree in an Alaskan forest may experience lows below -50°C and highs above 30°C in a single year, while no tropical sites below mountain tops experience that range. Within the geographic range of a species in the tropics, such ranges of environmental variation may occur, but few or no individuals would experience the full range. In addition, bands of climate (such as between annual mean temperatures) that lie along mountain slopes are narrower in the tropics, providing less space for meso-habitat adapted populations to establish and persist, such that population sizes are smaller and long term persistence and local adaptation are less likely.

Narrower tolerances for abiotic conditions in the tropics results in smaller geographic ranges and higher diversities because areas of diversity measurement will encompass a larger number of distinct climate zones than comparable measurements made in temperate or polar latitudes. Further increasing diversity estimates is a predicted strong-in-the-tropics “rescue effect” (Brown and Kodric-Brown, 1977) that produces sink populations in areas a species is poorly adapted to, maintained by immigration from nearby populations in more suitable habitats.

I find these arguments convincing, but some of the exceptions described here are problematic. One key exception that Stevens (1989) mentions more than once is the hymenopteran family Ichneumonidae. Ichneumonid species richness peaks at temperate, not tropical latitudes (Owen & Owen, 1974). Stevens (1989) explains this exception to the general latitude-diversity pattern by invoking the summer-only activity pattern of these parasitoid wasps. Stevens (1989) claims that because they are inactive during all but the warmest part of the year, ichneumonids “… in sense, live in the tropics, no matter what latitude they call home.” I see two critical problems with this argument as applied to Ichneumonidae as an informative exception to Rapoport’s rule and the latitude-diversity pattern. First, many other organisms, plants and animals and presumably other kingdoms and phyla, show severely reduced winter activity in temperate regions. Why do they not similarly show exceptional species-richness patterns? Second, “inactive” is not a synonym with “immune to environmental factors”. Overwintering in places that have winter (i.e. distinctly lower temperatures in one season compared to other times of the year) requires adaptations, sometimes extreme adaptations. The large body of literature concerning overwintering strategies of animals, and the various adaptations that constitute “freeze-tolerant” and “freeze-resistant” forms, comes readily to mind. Overwintering ichneumonids must survive the conditions of winter, even if they do not move around or show high metabolic rates during winter. As a further consideration, another section of this paper clarifies that Rapoport’s rule does not describe a pattern of increased species numbers per genus, it describes changes in the geographical distribution of functional groups of organisms, and includes the example of willows. Willows do not have higher diversity in the tropics; they are absent from tropical regions. But, willows are not an exception to either pattern because they are replaced by several other genera of morphologically and ecologically similar plants at lower latitudes. Why does Stevens (1989) not apply this functional-group replacement criterion to the example of the Ichneumonidae? There are other families of parasitoids, surely one or more of those taxa have high tropical diversity.