In our recent publication in the Journal of Biogeography, we assembled a comprehensive distribution map of all reptiles in Africa in order to quantify their geographical overlap with the other vertebrate groups, and to assess the environmental correlates underlying these patterns.
The latitudinal gradient of increasing biological diversity towards the equator is one of the best recognized patterns in biogeography, and has been acknowledged for some time. The naturalist, Alexander von Humboldt wrote of his travels over 200 hundred years ago, that as we approach the tropics, "the greater the variety of structure, form, colour, youth and vigor of organic life." A number of well-known hypotheses explaining this pervasive pattern of the increasing number of different species towards the equator have since proliferated. These include elevated ambient energy and precipitation, the number of different habitats or niches, higher plant productivity, and many more.
| Until now reptile diversity gradients have remained largely unmapped and the least studied of the terrestrial vertebrates, especially in Africa. This is an important distinction because reptiles are an extremely diverse class of terrestrial vertebrates (over 10,000 species and counting), and as ectotherms, which often thrive in arid regions, their diversity patterns are thought to differ from the classic latitudinal gradient of the other land vertebrates (amphibians, birds, and mammals). In addition, the distinct reptile lineages - amphisbaenians, crocodiles, lizards, snakes, and turtles are likely to respond differently to environmental variables. |  Vipera palaestinae (photo: Uri Roll) |
To create our geographic distribution map of reptiles in Africa, we obtained data from a variety of field-guides and atlases, museum databases, the primary literature, IUCN assessments, and maps based on expert knowledge of reptile species and the habitats they occupy. A challenging aspect of the project was to ensure that our maps remained current with respect to new species discoveries and taxonomic name changes (which are constantly being revised), and we also had to confirm the validity of type specimen identifications and localities, especially those referenced from obscure sources and archaic museum specimens. We used GIS software to digitize and overlay the maps of each individual African reptile species (1,601 species in total!) one on top of the other, which allowed us to count the number of species present in a given area - which we call “species richness”.
| Here is the product of all of that hard work - the first comprehensive richness map of all reptile species in Africa. The colour codes correspond to the number of species from low (blue) to high (red). It shows that the reptile richness map is largely congruent with previously mapped amphibian, bird, and mammal richness showing the classic species latitudinal gradient, including high richness in the arid regions not seen in the other vertebrates. But when you look at the reptile groups distinctly you see that while the overall reptile richness map mostly resembles snakes, lizards in particular are qualitatively very different. Lizard richness hotspots are widely dispersed with high diversity in tropical regions, as well as arid and mountainous areas, where the distribution of the other reptile and non-reptile groups is relatively low. |  |
When we looked at which environmental predictors best explained these species richness maps we found that net primary productivity (the amount of photosynthetic activity by plants) and precipitation explain most of the variation in reptile and other vertebrates. This explains the clear latitudinal pattern seen in their respective maps, which reflects a strong correlation with plant productivity and rainfall as you move closer to the equator. But again, lizards are unique in that none of these environmental correlates explain their distributions. This is because lizards are well adapted to a wide range of habitats including the tropics as well as the harsh conditions of the desert where plant productivity and rainfall are low. We also showed that individual lizard species on average occupy smaller geographic distributions, reflecting their ability to occupy diverse niches.
Our findings that the distribution of lizard species in Africa is unique when compared to the other vertebrate groups now confirms a pattern that has been seen elsewhere in previous studies (i.e. Australia) and most recently by our paper on the global distribution of reptiles. This shows the importance of studying the diverse reptile groups distinctly instead of lumping them all together, and will have bearing on large-scale conservation efforts that do not represent all reptile groups.
Author: Amir Lewin
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