|Serengeti Lions eat a diversity of mammal species.|
|Topi are definitely on the menu, but need enough food too. Moru, Dec. 2011|
From a knowldge of physiology we can expect that smaller grazers have a smaller mouth and therefore lower intake rates than larger animals. What's more, smaller animals generally have a higher metabolic rate than larger ones, so need more food. They also have a smaller space for digestive organs and consequently what little they do eat can't stay inside very long, meaning they can only get the easily digested nutriment from what they do eat. By contrast, larger animals will have higher intake rates, relatively lower energetic demands and a longer digestive tract that allows for some fermentation of the food even among ruminants, meaning they can get more nutriment from the same food than smaller animals can. They are bigger though, so still do need more food overall. So you'd expect that small animals will be looking for higher quality grass (with more of the important nutrients like Nitrogen) than larger ones, which will just be looking for large amounts of grass. In essence, you can suggest with some certainly that both food quality and food quantity are going to be important to some degree, with perhaps animals favouring one over the other.
|Everyone eats a Tomson's gazelle, but they don't eat much! Grumeti, Sep 2010|
|Buffalo are less troubled, but need lots of food, Seronera, Dec 11|
|Elephants have escaped most predation, but eat continuously. Moru, Dec 2011|
In particular, buffalo don't care about anything but food abundance, the gazelles were the only things to chose the high nutrious, low biomass areas with low predation risk, and the hartebeest avoided both high biomass and high quality areas, perhaps searching for low quality grazing in areas with low predation risk, though they were the 'wrong' way round relative to topi, given their body mass. But still, body size alone and it's interaction with food quality, quantity and predation risk can explain some of the patterns of distribution in these species. The key here, of course, is that some - and it's a rather small amount of variation that the authors explain. That's not surprising to me - masses of things affect animal distributions, especially in herding species like these. The analysis can't really consider social effects and there's also bound to be some massive random variation in just where the animals are when the surveys happen - try again a day later and you'd have different patterns to explain. So not a bad effort at least! (Anyone who knows my gripes about distribution modelling, however, will know I've one or two issues with some of the statistics, but nothing that will make a huge influence to the overall results, I guess...)
And another question for Grant - why not include wildebeest? Beacuse you knew they wouldn't fit the pattern? Or what?!
Anyway, the summary is that, to survive in the Serengeti, you need to know how big you are. As for me, I'm somewhere between a Grant's Gazelle and a Topi - that means I should definitely be looking to avoid places of high predation risk, and I should probably be looking for my grass to be short and nutritious - if it doesn't taste good, I'd better not bother... Sticking to the short grass plains definitely sounds like a good survival strategy to me!
Hopcraft, J., Anderson, T., Pérez-Vila, S., Mayemba, E., & Olff, H. (2012). Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers Journal of Animal Ecology, 81 (1), 201-213 DOI: 10.1111/j.1365-2656.2011.01885.x