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1. Introduction
There is a long recognized and widely documented latitudinal gradient of species richness (SR) for terrestrial, marine, and freshwater taxa, with a general increase towards the equator [1–8]. This gradient has been maintained for at least 270 Myr [9, 10] and is the oldest and most fundamental pattern regarding life on earth [11–13]. However, the mechanisms responsible for this biodiversity gradient remain poorly understood despite great efforts by researchers in recent centuries [14], although many hypotheses have been proposed to explain it [15, 16]. Understanding the mechanisms underlying the latitudinal biodiversity gradient may be one of the most important challenges for biologists in the 21st century, given the alarming rates of biodiversity loss as a result of human activities [13, 17, 18]. Originally, a general mechanism was sought (based on climate) that would explain the latitudinal gradient in SR, but no consensus was reached and more synthetic multifactor approaches have appeared in recent decades [10].
Some authors maintain that latitude per se does not affect SR [19–21] and consider that the correlation between latitude and SR is entirely spurious, being latitude used as a variable for historical reasons and because it can be easily and objectively measured at a global scale. However, latitude is a physical characteristic of rotating planets (note that a still sphere does not have poles or latitude) and cannot be reduced to an arbitrary coordinate axis to be used in spatial analyses. Thus, both ecology and geography may influence latitudinal patterns in SR [22].
Longitude can also play a role in shaping biogeographical patterns of SR. However, the longitudinal biodiversity gradient is not as common and well documented as the latitudinal one. For example, Korpimäki and Marti [23] detected a longitudinal gradient in the dietary diversity of North American raptors related to the regional trend in prey assemblages, where the number of mammal prey species markedly increases from east to west. Longitudinal and other spatial trends may also appear depending on the characteristics of the territory analysed.
Environmental factors are responsible for most of these geographic trends [10]. Real et...