Monday, 3 October 2011

Dung Beetles

Continuing the theme of small but rather important pieces of the savannah jigsaw puzzle, I thought I'd write a little about dung beetles. As usual, I'll try and follow my three questions for interpreting wildlife sightings - what is it? What's it doing? And what's it's role in the ecology of the environment?
Scaning for the route?!

So, let me first confess that I've never even identified a single dung beetle to species level. Unless you're a real specialist, I think you can forget it. Our dung beetles are are insects of the order Coleoptera and, as a rather prominent biologist (J.B.S. Haldane) once (may have) said when asked what we can learn about the Creator through studying His works: "He must have an inordinate fondness for beetles". In fact, the latest estimates  - published here in August - of terrestrial biodiversity are around 8.7 million species, of which about 7.8 million are animals (we've only described about 1.7, though). It's estimated that about 80% of all species are insects, and of these about 40% are Coleopteran beetles, which would suggest there are about 2.5 million species of beetles out there. In one tiny corner of the Serengeti plains alone, over 100 species were recorded in a relatively small study. As only a tiny proportion of these are already described I conclude (a) if you want to discover a species new to science, look at beetles, and (b) there are far too many beetles to spend time trying to identify them specifically. Still, most of our dung beetles belong to the Scarabaeidae family, and most people will have heard of Scarabs, especially if they know anything about the ancient Egyptians, who considered them sacred (holy), since it's clear that the world must be kept in motion by a giant dung beetle rolling it about.

So, that's what they are, identified as far as I feel the need. But what are they doing? Well, when we usually notice them they're rolling balls of dung along the track. Why? Because they eat it. Yumm. Most of the ruminants feeding on grass only extact about 50% of the nutrients from their forage, and obviously much less for hind-gut fermenters like elephants, so there's still significant resource left in the dung of these animals. And dung beetles love it for everything - they eat it themselves (some species eat it on site, some under the dung and some roll it off to snack on elsewhere), they roll it off and eat it as part of their courtship procedure (if you find two on the same ball, they might off on honeymoon with a nice snack to keep up their energy...), and they take it away to provision their young. For breeding, often the males dig large holes where they'll store several dung balls, a female laying a single egg on the top of each one (and in some cases coating the balls with a layer of clay that hardens around the dung ball. These broods are, in turn, a favourite food of honey badgers and some mongooses.
Dung beetle nest predated by honeybadger or mongoose, Lake Manyara, July 2010

The really exciting thing about dung beetles (I promise!) is, however, the impact they have on the ecology. First, let's appreciate the task they perform in tidying up dung. Your average zebra produces about 4.1 kg of dung per day, and a Grant's Gazelle about 0.75kg (never let it be said I'm not full of useful facts!), so let's assume about 200,000 zebra and 1.4M wildebeest for Serengeti, and guess that wildebeest, being bigger than Grant's Gazelles do about 2kg per day, and we're looking at a massive 1.3 Million tons of dung per year in Serengeti - nearly 3620 tons per day! It's just as well there's an army of dung beetles out there, just waiting for their meals (as the Australians learnt, when they started cattle ranches and the Australian dung beetles, used to a fine quality product from kangaroos, turned their noses up at the offerings from cows, with a massive fly problem the result - and they had to import African dung beetles to clean the mess up!). And, of course, we all know that dung is a pretty good fertiliser - whilst most of what they bury they also eat, the sheer numbers ensure that huge amounts of nutrient cycling are carried out by these beasts. What's more, they a bt picky about where they dig - it must be moist enough for them to dig, so that's why they're out there rolling balls long distances, looking for somewhere suitable. And as not everywhere is suitable, they tend to concentrate the dung in certain areas, creating a nutrient hotspot. Which, of course, attracts more wildebeest, to produce yet more dung, which is immediately returned locally - a major source of heterogeneity in the Serengeti plains. So, absolutely critical for nutrient cycling in the savannah - in fact, the huge volumes of dung involved alows you to realise that, thanks to dung beetles removal and burying, almost all the soil you walk on in Serengeti must, at one stage not that long ago, have been a dung ball. Lovely thought...

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