(iii) What can explain the latitudinal gradient in species diversity?

The gradient in species diversity is clearly not just an idiosyncratic characteristic of a few species. Its broad observance suggests a general shared process that consistently "produces" more species of any given taxon in the tropics than the poles.

There have been up to 35 different hypotheses proposed to explain the gradient in species diversity, many of which are not mutually exclusive (because many things co-vary with latitude, e.g., temperature, productivity, etc).

There are, however, several ways in which hypotheses can be grouped. One class is null vs. deterministic hypotheses. Null hypotheses essentially states that there is "nothing interesting going on" and that any patterns observed are artefacts of the way the data are collected or presented. The so-called "mid-domain effect" of Colwell and Hurtt (1994) is an example: this idea states that random distribution of geographic range boundaries between the "hard edges" of the poles of the earth results de facto in the greatest range overlaps mid-way between these edges, i.e., the equatoprial regions, and, consequently, greater species diversity at the equator than at either pole. Hubbell's (2001) "neutral theory of biodiversity and biogeography" is another null model against which predictions can be objectively tested. Deterministic hypotheses posit some fundamental biological or abiological process(es) are involved in structuring species diversity.

Two other general classes of hypotheses: equilibrium and non-equilibrium hypotheses. There have been alternative classification schemes proposed (time-based, habitat heterogeneity-based, evolutionary-based, etc - see Willig et al. (2003) for a nice recent review).

Equilibrium hypotheses suggest that a "steady state" has been reached among the processes that influence species diversity and that the forces that increase diversity are exactly balanced by those that reduce diversity such that species diversity remains fixed through time. In other words, if we revisited an area in 100,000 years it would have the same species diversity (although the species composition may have changed). The differences among areas in observed species diversity simply reflect the different relative magnitudes of forces that increase or decrease species diversity in a given area (imagine two containers each with a hole at a different height from the bottom. Water enters each container at the same rate, but the one with the lower hole holds less water (fewer species).)

Non-equilibrium hypotheses involve the idea that a given community has not yet reached a "steady state" and species diversity is still in the process of increasing (or decreasing) after some historical disturbance. In other words, if we came back 100,000 years later, the species diversity may have changed to reach its "true" level. For example, Pleistocene glaciations eliminated most life from North America north of about 46 degrees latutude. After the ice sheets left, animals and plants recolonized the vast area and perhaps the gradient in species diversity is simply due to the fact that not enough time has elasped to allow all species to reach or establish populations in the newly-exposed habitat. Such habitats are considered as "unsaturated" and given enough time the species diversities will increase to be the same as at more tropical latitudes.

Six major sub-hypotheses have been proposed to account for the latitidinal gradient:

1. Historical perturbations (non-equilibrium): as explained above

2. Productivity: tropical areas are more productive and there is more usuable energy available to be subdivided amongst more niches and hence more species. Opportunities for resource specialization increase species diversity. The figure below illustrates this idea. The y-axis represents population density and energy or productivity. The x-axis represents resource type (or niche type). The upper curve in each panel represents the level of productivity (or population density) in an unproductive habitat (top) and a more productive habitat (bottom). The area under each of the number curves (1-5) represents the population size of each species. Note that in the productive environment, more species are able to achieve some critical value of population size (the area under each curve) although each occupies a narrower niches (smaller portion of the x-axis). This is because of the greater height of each species curve allowed because the productivity curve is higher. This way, more productive environments may promote greater species diversity by allowing the persistence of more species that show a greater degree of resource specialization.

Alas, it's not quite so simple. Although productivity is typically correlated positively with increase species diversity (see figure below for terrestrail vertebrates in NA), it has its limits.

In fact, the productivity-species diversity relationship is often "dome-shaped", i.e., maximal species diversity is found in areas of intermediate productivity. In fact, going back to the figure of foraminiferal diversity and latitude, note how the greatest species diversity tends, in this case, to be at intermediate latitudes (i.e., those having intermediate productivities). In addition, some of the most productive environments on earth are not the most speciose (e.g., estuaries, salt marshes, hydrothermal vents, shallow eutrophic lakes).

3. Harshness: small isolated or otherwise "harsh" environments (in terms of key abiotic factors like temperature or moisture) have higher extinction rates, lower colonization potential, and less opportunity for resource specialization than mor ebenign tropical environments. This may explain why some of the most productive environments are not the mose speciose in many cases, they are too stressful (e.g., salt marches [salinity stress], hot springs [heat stress]). Stressful environments may sim0ply increase extinction rates or reduce the potential for specialization and place a limit on the numbers of species that exist in such environments because only a few possess the requisite adaptations to extreme conditions.

4. Climate stability: more variable climates prevent resource specialzation (how can you specialize on a resource that may disappear under rapid environmental change?) and hence are able to support fewer species. Again, problems with this hypothesis include the many exceptions. for instance, many regions that are quite stable (high mountain tops, deep abyssal regions of the ocean) have low species diversity. By contrast, many variable environments (e.g., the "dry" and "flood" seasons in Amazonia) are incredibly species rich. Climate stability in terms of seasonality of solar radiation, however, is likely important as it is clearly linked to latitude and productivity (less seasonal environments, i.e., the tropics, are more productive).

5. Habitat heterogeneity: diverse physical environments promote isolation, and resource specialization, and hence speciation, and the co-existence of more species. The figure below shows bird species diversity and foliage height diversity (number of layers of foliage at different heights above ground) in eastern North America (see Brown and Lomolino 1998, page 481). Similar relationships have been documented for desert rodents and some aquatic habitats. Coral reefs, for instance, are outstanding examples of complex physical environments that support some of the highest diversities of marine species.

See Davidowitz and Rosenzweig (1998) for a test of the habitat variation hypothesis in grasshoppers.

6. Interspecific interactions (2-6 are equilibrium hypotheses): More species in tropical areas create a positive feedback through increased interspecific interactions such as competition, predation or parasitism. The intensity of these interactions prevents a few species from dominating the resources, promotes specialization, and, therefore, potential co-existence of more species. There is, however, some uncertainty between cause and effect in this explanation. Do more interactions actually cause greater species diversity or are they just a necessary consequence of greater species number?

(iv) Rapoport's Rule

Rapoport in 1982 formalized the observation that subpopulations of mammals in North America (i.e., within species) tend to show larger average geographic range sizes with increasing latitude. This trend has been observed in several other taxa and is known as "Rapoport's Rule" The figures below demonstate this trend for mammals (top left), plants (top right) and breeding birds (bottom). Note the opposite trends for species richness and latitude and species range size.

(v) Species Dominance

Also noted in community comparisons is another latitudinal trend. Tropical areas tend to have more species, but these species also tend to be rarer, i.e., any individual species accounts, in terms of numerical abundance, for a lower proportion of the total individuals summed across all species in that area, than the average species in more temperate areas. This trend is shown in the figure below as a higher initial point, but steeper decline in the relationship between species abundance dominance rank (from highest to lowest, left to right on the x-axis) and proportional abundance (on the y-axis). In other words, there are fewer species in temperate areas, but those fewer species tend to be more abundant (a relatively few species "dominate" the landscape) and they have larger geographic ranges than those in tropical areas (where species are more numerous, but each one is less abundant and has a smaller average range size).

(vi) Is a synthesis possible?

All of the hypotheses above have important caveats or exceptions associated with them. Is it realistic even to think that one hypothesis can account for latitudinal gradients in species diversity and associated patterns such as Rapoport's "Rule" and species dominance? The short answer is, "probably not". In addition, there is still no real satisfactory "all encompassing" explanation (invoking multiple causes and their interactions) for the latitudinal gradient in diversity. There is some consensus, however, that once that synthesis is achieved, it will encompass inputs from two major ideas:

1. Area and age. The idea here is that time and space have a major impact on the "historical buildup" of species diversity. Essentially, larger areas support more individuals and more species (we'll see this explicitly in a few weeks). In addition, areas that have existed for longer time periods will tend to accumulate species (through greater potential for speciation and, to some extent, lower risks of extinction). The area of the earth is generally greater at lower latitudes owing to the curvature of the earth towards the poles. Hence, all else being equal, one would expect more species near the tropics as a simple function of area.

2. Productivity, stress, and biotic interactions.

Although we have discussed productivity, stress, and biotic interactions separately above, they are obviously functionally related. For instance, diversity and abundance of herbivores will be influenced by the availablity of plant life which itself is heavily influenced by abiotic factors (light, temperature, moisture) that impact primary productivity directly and indirectly through stress factors (e.g., high salinity). In general, species diversity will increase under low stress and high productivity. This will result in increasing potential for competitive, parasitic, and predatory interactions to come into play and perhaps govern the upper limits of species diversity. High degrees of interspecific interactions could also clearly limit geographic ranges and population sizes of interacting species by density dependent effects.

In this model, abiotic stresses and productivity limit species diversity towards the poles where biotic interactions (between species) are less intense, hence, increasing geographic range and species dominance. Under more benign conditions of temperature, seasonality, and moisture, biotic interactions tend to limit geographic range and species diversity. R. MacArthur used the analogy of a greenhouse in Arctic areas that could permit the growing of most plant species. Conversely, most species of plants could be grown under natural conditions of climate in the tropics as long as competitors, herbivores, etc were kept "at bay".

In sum, the latitudinal gradient of biodiversity is a fundamental observation in zoogeography that still lacks a comprehensive, predictive model to explain it. There is still a brilliant career to be launched by integrating effects of area, age, productivity, stress, and biotic interactions into a single coherent explanation for one of the most ubiquitous patterns in nature: the increase in biodiversity as one moves from the poles to the equator.

References:

Bokma Folmer. Bokma Jurjen. Monkkonen Mikko. 2001. Random processes and geographic species richness patterns: Why so few species in the north? Ecography 24: 43-49

Brown, J., and M. Lomolino. 1998. Biogeography. 2nd Ed. Chapter 15. Sinauer Assoc. Ltd., Sunderland, Mass.

Colwell, R.K. and G.C. Hurtt. 1994. Non-biological gradients in species richness and a spurious Rapoport effect. Am. Nat. 144: 570-595.

Davidowitz, G. and M. Rosenzweig. 1998. The latitudinal gradient of species diversity among North American grasshoppers (Acrididae) within a single habitat: A test of the spatial heterogeneity hypothesis. Journal of Biogeography. 25(3): 553-560.

Gaston, K.J. et al. 1998. Rapoport's Rule: time for an epitaph? Trends in Ecol. Evol. 13: 70-74.

Hubbell, S.P. 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press

Rex, Michael A. et al. 2001. Do deep-sea nematodes show a positive latitudinal gradient of species diversity? The potential role of depth. Marine Ecology-Progress Series. 210: 297-298.

Ricketts, T.H. et al. 1999. Who's where in North America. Bioscience 49: 369-381.

Ricklefs, R.E. and D. Schluter. 1993. Species diversity in ecological communities. Historical and geographical perspectives. Univ. of Chicago Press, Chicago.

Rohde Klaus. 1999. Latitudinal gradients in species diversity and Rapoport's rule revisited: A review of recent work and what can parasites teach us about the causes of the gradients? Ecography. 22(6): 593-613.

Willig, M.R., D.M. Kaufman, and R.D. Stevens. 2003. Latitudinal gradients of biodiversity: pattern, process, scale, and synthesis. Ann. Rev. Ecol. Syst. 34: 273-309.