Evolution of the latitudinal gradient in species richness

Following on from the last chapter, we can refer to the same suite of evolutionary processes to explain why, at larger taxonomic and spatial scales, species richness increases as latitude declines (Figure 15.4). Remarkably, the latitudinal gradients are seldom considered in these terms,yet logically, they are likely to be formed by the same suite of processes. Clades originate somewhere and, all other processes being equal, their centre of species richness will remain

Fig. 15.4 The latitudinal gradient in species richness seen from space. (a) Amazonia, with many species.

Deforestation along a road is visible in the bottom half of the picture. (b) Antarctica (the McMurdo dry valleys) with very few species. Photos from the NASA Visible Earth image archive.

Fig. 15.4 The latitudinal gradient in species richness seen from space. (a) Amazonia, with many species.

Deforestation along a road is visible in the bottom half of the picture. (b) Antarctica (the McMurdo dry valleys) with very few species. Photos from the NASA Visible Earth image archive.

there. We must also consider how speciation and extinction vary with latitude and the nature and frequency of latitudinal shifts in geographic range (anagenesis). Just as when body size distributions could be considered to be bounded by a lower reflecting barrier, so geographical limits are placed on latitudinal range shifts, provided by the positions of land masses and oceans, for example, and on the fact that the latitude cannot vary below 0° or above 90° from the equator. Some of these geographical limits have been assessed in a number of non-evolutionary models, and have been criticized exactly because of their non-evolutionary nature. In fact the suite of evolutionary processes that I have just described are the well-known processes of evolutionary biogeography, and in a proximate sense explaining the latitudinal gradient is an exercise in evolutionary biogeography. We would expect that biased originations close to the equator as well as increased rates of cladoge-nesis, reduced rates of extinction, and biased shifts in range towards the poles would all contribute towards the observed latitudinal gradient (Figure 15.5).

Once again some progress has been made in documenting these processes. In Chapter 14, we saw that the fossil record suggests higher origination rates in the tropics, and sister-taxon comparisons of species richness suggest net rates of cladogenesis that are sometimes higher in the tropics. Taxon age also seems to vary with latitude in many instances, suggesting variation in underlying processes. In baboons, macaques, and their relatives (the Papionini), the history of range changes has been reconstructed by mapping extant distributions onto a phylogeny of species. These reconstructions show, robustly, that ancestral distributions were equatorial, that tropical regions have experienced more net cladogenesis, and that tropical regions have given

Fig. 15.5 Four ways in which evolutionary processes may have given rise to the latitudinal gradient in species richness (the circle with horizontal lines indicates the Earth with lines of latitude). (a) Speciation is more frequent in the tropics. (b) Extinction is more frequent near the poles. (c) Species move towards the equator over time. (d) The ancestor was tropical and changes in range towards higher latitudes are rare.

Fig. 15.5 Four ways in which evolutionary processes may have given rise to the latitudinal gradient in species richness (the circle with horizontal lines indicates the Earth with lines of latitude). (a) Speciation is more frequent in the tropics. (b) Extinction is more frequent near the poles. (c) Species move towards the equator over time. (d) The ancestor was tropical and changes in range towards higher latitudes are rare.

rise to more dispersal, suggesting that they actually raise the species richness of other regions (Böhm and Mayhew 2005).

In addition to documenting these processes, we will also need to find the fundamental reasons behind them; ultimate processes, and once again we will therein need to invoke ecological characteristics that have influenced them. Here there have been so many different hypotheses that I cannot do justice to them all, so I will mention one recent theory that makes quantitative predictions, and has some empirical support (Allen et al. 2002). First, assume that the total energy flux per species per unit area is relatively invariant (the energetic equivalence rule).This is an empirically derived rule that appears to hold and is derivable for some plants based on scaling relationships (Chapter 4).

Now assume that in a community of individuals made up of different species of similar type and ecology (e.g. forest trees, reptiles, beetles), the total number of individuals is determined by the number of species and the number of individuals per species. If the energetic equivalence rule is to hold then, at higher ambient temperatures, to maintain the same temperature flux per species there must be fewer individuals per species. If there are fewer individuals per species and the total number of individuals in the community is approximately invariant with respect to temperature, then the number of species must increase. This logic actually allows quantitative prediction of the relationship between species number and temperature for ectothermic organisms once the exact metabolic relationships are taken into account. The exact prediction is that a plot of ln species richness against

1000/K should give a slope of —9. A number of tree, amphibian, fish, gastropod, and parasite communities conform to this relationship across both latitudinal and elevational gradients.

The model of Allen et al. (2002) is remarkable in several ways; first, it makes quantitative predictions which can be more easily falsified than many other models. Second, it derives species richness directly from a primary environmental variable that is directly related to latitude, but actually varies in a much broader sense. It is not specific about the evolutionary processes but presumably the energetic equivalence rules could affect the likelihood of range shifts, speciation, and extinction rates primarily by allowing species to fit into a community in energetic terms or not. In many ways, the evolutionary processes can be considered to be irrelevant because as long as they produce enough species to fill up the energetic opportunities, the model will hold. It is therefore a theory based largely on ecological equilibrium, with some implicit evolutionary assumptions. Because of its novelty it has yet to be fully assessed by the scientific community, so I will tempt fate by pointing out two likely areas of criticism. The first is that it cannot predict the species richness of endotherms, because, using the same logic, their metabolic rates are invariant with ambient temperature, the number of individuals per species does not change with temperature, and the number of species should therefore remain the same. The second likely area of criticism is whether most ecological communities are at energetic equilibrium. In general, this goes counter to most ecologists' notions of community assembly.

Thus, many macroecological patterns can be derived from consideration of proximate evolutionary processes many of which have begun to be quantified. Ultimately, these processes rely on the interplay between primary ecological phenomena and the various proximate evolutionary processes. Some possible primary phenomena have been identified and some data supports their action, but the link between the ecological and evolutionary phenomena remains poorly described theoretically and totally undescribed empirically. There is much to be done to develop our understanding of how and why these ecological patterns emerge and it is essential that evolutionary biologists play a leading role.

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