Wednesday, 25 January 2012

How to survive the Serengeti: predation, food and body size

Serengeti Lions eat a diversity of mammal species.
Another paper describing one of my favourite talks from the TAWIRI conference back in December has just been published, this time in the Journal of Animal Ecology (available to some here). Grant Hopcraft and colleagues are interested in why different grazing animals use different parts of Serengeti in different ways when they all eat grass. Why, for example, do you usually find buffalo around riverine areas, whereas gazelles tend to be on the open plains? At some level it's obvious that where you find an animal is the place that it survives best and there are two aspects to survival that most ecologists would agree are crucially important: where will you eat? And where will you be eaten? Although both buffalo and gazelles eat grass, and both are eaten by large cats, it's quite possible that grazers of different size will be affected differently by these same two processes. And that's what Grant and colleagues set out to test.

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
Predation risk similarly differs with body size. Very large animals like elephants and rhino at least as adults don't ahve to worry about predators at all, whilst smaller animals certainly do. If you're a large herd of buffalo you're not going to worry about much except, perhaps, a particularly hungry pride of lion - on the other hand the Thomson's Gazelle is pretty much the universal snack for all the large predators in Serengeti and is going to be very, very concerned about where predators might hide! So, what we've called the landscape of fear will have different influences on different animals too, in a way that might well be directly related to body size.

Buffalo are less troubled, but need lots of food, Seronera, Dec 11
Now Grant and his colleagues set about testing this through the known distribution of animals (from areal survey data of buffalo, topi, Coke's hartebeest, Thomson's and Grant's Gazelles) using statistal modelling methods. They (sensibly) decided there might be different factors that explain presence or absence in a particular census square to the factors that determine if there are any animals there at all. (For example, for some animals that drink a lot presence of water within, say, 2km might be essential to presence, but as long as there's a little bit of water, more water won't mean more animals, whilst more food might well.) So they set about modelling in a two stage way - one to predic presence or absence, then another to predict the relative abundance of each species. Having done that they set up a model that isn't often used by ecologists, called a Structural Equation Model, which allowed them to link things they could actually measure in the field (like bush cover, nitrogen content of grass, grass biomass, etc.) into the three main sets of processes that they considered likely to influence abundance: those factors to do with food quantity (like rainfall), those affecting food quality (like nitrogen content) and those affecting predation risk (like presence of bushes), and then set off to see if their predictions held: did smaller grazers really select high quality areas and avoid areas with high predatin risk? Did buffalo just focus on places with lots of grass, irrespective of predation risk and foraging? And where topi and hartebeest somewhere in between? And I guess they were pleased when they found the answers, broadly speaking, were yes.
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!

Main reference: Hopcraft, J., Anderson, T., PĂ©rez-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

No comments:

Post a Comment