Shit-sifting

More on making a living from cutting grass. We really haven't a clue about the complexity of the world but science is making baby-step progress to understanding who interacts with whom in an ecosystem. We were talking at the June meeting of the Wexford Science Cafe, about the complex micro-biological ecosystem which is the soil underneath a wheat-field. The top 10cm is filled with aerobic microbes - bacteria and fungi - as well as earthworms and nematodes and plant roots. Below that, a different dramatis personae holds the stage.  Everyone is happy or at least has come to an accommodation with the neighbours.  Then, sometime in October, Bob the Farmer comes with a 10 ton tractor-and-plough and turns the top 30cm of soil upside down killing all the anaerobic nitrogen-fixers with a blast of toxic oxygen and suffocating their upper-storey neighbours in the murk. This practice suppresses weeds, yes, but also forces a dependence on bought-in nitrogenous fertilisers.  See, we haven't a clue.

But the classic complex ecosystem that we do understand a little better is the African plains of the Serengeti. We know more because the actors are bigger-than-a-breadbox and have been studied for the last 100 years through binoculars and rifle-sights. We think we understand but there are still mysteries like - how does this seemingly parched and unproductive soil support such a huge and diverse biomass from antelope to zebra. A study by Tyler Kartzinel and Rob Pringle of Princeton described by National Geographic goes some way to explaining how everyone makes a living.  It turns out that they are all eating different food. It's no longer necessary or sufficient to roughly bin the mammals into grazers (zebras eat grass) and browsers (giraffes eat trees and goats eat the shrubbery).  How do we know the details of who eats what?  Now I've some small experience sifting crocodile shit to raise a few shillings towards my education. But a swat team from Princeton with binoculars and plastic bags has been scooping up the poop of known mammals and then running the samples through a DNA analysis that identifies the species eaten.  And whoa, the two species of Zebra Equus quagga burchellii and Equus grevyi eat grass, yes, but different species of grass. Now we know; but we didn't until Kartzinel & Pringle had the smarts to think of a technique to sort the input stream at the end of the digestive process. We've known for a long time that you can raise more tons of cattle and sheep on a hectare if you have them graze the field at the same time. I don't think anyone knew the details of how the pasture is partitioned, now we have the tools to answer that question.  Thanks Princeton.

The original scientific paper.