First, my apologies for the long dry spell here. You know… life.
As some of you may know, I’ve just returned from spending the past month in Africa and Europe. The heart of the trip was an 11 day safari in Botswana. This was my first experience in Africa, and my first time seeing the remarkable abundance of wildlife that occurs there.
It lived up to every high expectation I had. In fact, I came away utterly shocked at the diversity and density of species. The mammals stood out of course, with dozens of species co-existing in the same areas, but the birds were a not-too-distant second in terms of abundance and diversity.
From my time in rainforests of South and Central America, I’m accustomed to exploring high biodiversity areas. But what shocked me about Botswana was that this remarkable diversity occurred not in a rainforest, but in a near-desert.
In the Savuti area of Chobe National Park, the hottest and driest area we visited, we were very rarely out of sight of one species of mammal or another. A quick (and probably incomplete) list of species encountered: Lion, African Wildcat, Leopard, Black-backed Jackal, Bat-eared Fox, Spotted Hyena, African Wild Dog, Chacma Babboon, Vervet Monkey, Hippopotamus, Giraffe, Warthog, African Elephant, Greater Kudu, Roan Antelope, Blue Wildebeest, Waterbuck, Tsessebe, Impala, Red Lechwe, Steenbok, Yellow Mongoose, Dwarf Mongoose, Banded Mongoose, Scrub Hare, Springhare, Ground Squirrel, Tree Squirrel, and probably some that I missed.
The point is that there a lot of mammals coexisting in high abundance, in a relatively small area.
This abundance and diversity raised two questions:
1. Why are there so many species?
2. How is it possible for so many large mammals to coexist in the same area?
The answer to the first question is, according to the several references I looked up, fairly clear. Africa’s large mammal diversity relates to the post-Pleistocene extinctions. During the late Pleistocene, Pliocene, and Miocene there were a series of extinctions that swept through most of the world. While sources disagree on the causes of these extinctions, two culprits seem most likely. First, was a changing climate, and second was a rapidly expanding human population. This combination was a death knell for many of the planet’s large mammals species.
Except in Africa. In Africa, due to a relatively stable, equatorial, ecosystem and a community of mammals that had co-evolved with humans, the Pleistocene extinctions left the bulk of Africa’s mammal fauna largely intact.
Dodged a bullet there.
The second question has a more convoluted answer that, when I first started looking into it, seemed to have a very unsatisfying answer.
In general, large mammals are browsers (consuming leaves and twigs from shrubs), grazers (which rely on grasses), or generalists that eat a bit of both. So, by that formula, there is room for several species. Among browsers, for example, we have the tall Giraffe plucking leaves from the hardest to reach places, the large Kudu reaching several feet off the ground, and the diminutive Steenbok, pulling leaves from the lowest shrubs. Of the generalists we have the enormous elephant and the abundant Impalas. Grasses, however, seem equally available to all of the species that rely on them: Zebra, Wildebeest, Tsessebe, Roan Antelope, etc.
Accounting for habitat preferences and foraging substrate, I can imagine room for a few co-existing species in each group, but that hardly accounts for the dozens that actually occur.
Turns out that this conundrum has stumped smarter people than me. And until a few years ago, the abundance and diversity of co-existing species in Africa was a persistent mystery.
Unsatisfying indeed.
However, the mystery seems to have been solved by a 2015 paper in the Proceedings of the National Academy of Science. The paper by Kartzinel et al looked at the DNA of the plant remains found in the scat of several co-existing species species in Kenya.
It turns out that grazers, despite sharing both habitat and foraging substrate, selected different grasses from the many species of the savannah. Kartzinel described it well in an interview with National Geographic’s “Phenomena” blog comparing the forage to a buffet. Each person passing down the line would choose some of the same foods, but the overall distribution would differ. Quite a few people (or in this case, species) could pass down that line and none would choose exactly the same meal.
And so it is in Africa. From the abundant, seasonal savannah and shrublands rise dozens of species of potential forage plants. While many species will choose the same plants, none choose exactly the same combination. And from those slight differences in diet, rise the potential for dozens of species of large mammals to co-exist.
And because there are various species of herbivores there is therefore room for the many species of predators that hunt them. And so rises the most biodiverse mammalian community in the world.
Looking at the comparatively depauperate fauna of Alaska, I can’t help but be a little jealous.
References:
Nieto et al. 2005. Historical determinants of mammal diversity in Africa: Evolution of mammalian body mass distribution in Africa and South America during Neogene and Quarternary times. African Biodiversity 287-295.
Kartzinel et al. 2015. DNA metabarcoding illuminates dietary niche partitioning by African large herbivores. PNAS 8019-8024.
