Tuesday, 25 October 2011

Endemics, or why are some species common?

This Saintpaulia (African Violet), like most others is probably endemic to the Eastern Arc
As you might have guessed, I've been away again. This time I've enjoyed a few days exploring the Amani Nature Reserve in the East Usambara Mountains. It's a fantastic bit (or, really, bits) of forest, perched high above the north coast of Tanzania, and forms a key part of the fames "Eastern Arc" mountains that stretch from northern Tanzania (just catching southern Kenya in the Taita Hills) around and down the coast, cutting back inland through the equally famous Uluguru forests and down to the Udzungwa mountains. And if biodiversity is your thing, then the Eastern Arc has it - forget the savannah, the real wildlife is in these Eastern Arc forests. They form an important part of one of only 34 global biodiversity hotspots (2.3% of the global land surface, but hosting over 50% of all plant species!) identified by Conservation International (NB there are 8 of these hotspots in Africa, the same number as in Europe, Central Asia, North and Central America combined - no wonder I love to be here!). There are at least 96 vertebrate species that are endemic to the Eastern Arc forests - 10 mammal, 19 bird, 29 reptile and 38 amphibian species. A far, far greater degree of uniqueness than you'll find in any savannah. And that brings me to one of my favourite scientific questions - what makes some species common and widespread, and others rare and local? I think we'll leave for now the question of why there should be so many rare species all concentrated into such a small area (why the Eastern Arc is a centre of endemism) and focus on this more general pattern of common and rare species (mostly because it's something I've published on myself and I won't need to look so much up!).
Usambara Pitted Pygmy-chameleons Rhampholeon temporalis are incredibly restricted in range
Most people tend to ask why is such and such a species rare and then struggle to find an answer. Why should these rather cute Usambara Pitted Pygmy-chameleons be completely restricted to the Usambaras, and not hop onto the Pare Mountains just a few kilometers away, or even the forests of Kilimanjaro? No reason at all really - expect they're not there. So recently some people have started asking a slightly different question - why are some species common and widespread? This might seem a trivial piece of semantics, but I don't think it is. In fact, if you look at the areas of occupancies of species in any taxonomic group, you discover that it's not small ranges that are unusual, but large ranges - in other words, rare species are common, and it's commonness that's rare!

This Mt Kilimanjaro Two-horned Chameleon is common in Arusha and clearly related to the Usambara species, but still incredibly local in distribution
Now start looking at the problem this way and you might start wondering where a species starts from in the first place. Let's assume we have an ancestral species that's on the verge of speciation. However we want that new species to split from the ancestral population (by geographical isolation, or together with the other species by, perhaps, chaning breeding season), we're going to start with at least one rather small distribution - perhaps the founders made landfall on some distant island and evolved in isolation into a new species, restricted to this new range. But then along comes an earthquake, and the island is no longer isolated, the species is free to come back to the mainland (where, of course, it's ancestors have also continued a process of evolution and might be rather different by now). Coming back into contact with the descendants of this ancestor the new species can either compete happily and spread in range, or just might not spread at all. It looks like, for most species, they don't bother to spread, just stay nice and localised and rare. But every now and again, one of them makes the grade and becomes common and widespread. What makes this difference isn't yet clear and there's certainly no one simple answer. Rather, it seems likely that to make it big you've got to pass several tests simultaneously - you need to disperse well, breed fast, etc.
The Usambara Bush Viper is so localised and rarely seen that I can't find another photo of a juvenile to be sure of the identification here! This might be some sort of Egg-eater instead.
 On the level of individual species, though, it's still a mystery - why should Rufous-tailed Weavers have such a small range between Serengeti and Tarangire? What's wrong with the savannah that, to me, looks identical just north, south and east of this range that keeps them out? Is it competition? With what? Why can't the evolve just that teeniest bit more to let them spread further? Why is the range changing now (they're recently made it to Kenya)? It seems to be climate, but how and again, what stops that tiny bit of evolution that's needed from happening? Hmmmm. All puzzling questions really, but great to discuss whilst you're looking at some of Tanzania's endemics (especially as many of them, like the rufous-tailed weaver and ashy starline, aren't that inspiring to look at!). And, as ever, if you've got any ideas of your own, feel free to pass them on!

Oh, and do check out the Eastern Arc mountains if you want a truly unique wildlife experience!

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