About crackers, peanut butter and marmite …

I’m a sucker for good metaphors and analogies when I’m teaching. These are great for linking the ideas that I am trying to communicate with things with which the students are already familiar. One of my favourite analogies for stream ecology comes from a 1974 review paper by the US ecologist Kenneth Cummins. He was describing the process by which leaves which fall into streams at this time of year are broken down by the organisms that live in the stream in order to release their energy. There are a number of aquatic invertebrates, termed “shredders”, whose mouthpieces are specially adapted to tearing apart these leaves. They gain their nutrition from the leaves, so the theory goes, with the partially-digested leaf material emerging from their intestines, in due course, as “fine particulate organic matter”. That itself is a euphemism. Go figure.
But leaves alone do not make a particularly nutritious diet. In fact, the shredders are not living solely on these leaves. As soon as a leaf falls from the tree it is vulnerable to attack from bacteria and fungi. Like the invertebrates (like humans eating spinach, too), they can gain nutrition from this leaf, and the enzymes they produce help to soften up the tissues making it easier for the shredders to tear apart. Once in the water, the dead leaf will also be colonised by algae whose photosynthesis will produce oxygen which will replace that used by the various bugs as they break the leaf down. The combination of fungi, bacteria and algae also add to the nutritional content of the leaf. Cummin’s great analogy was that the leaf was akin to a ‘cracker’ whilst the microbial life was akin to ‘peanut butter’. A single cracker, as you know, is not itself greatly nutritious, but we tend to use crackers as ‘carriers’ for protein- and energy-rich foods such as cheese or, in Cummin’s example, peanut butter. An even better analogy for a UK reader is a cracker spread with Marmite which really is microbial-based nutrition.
Metaphor and analogies have their limitations, of course. But in an age where science is increasingly quantitative, the importance of having strong mental images of systems before you start taking them apart and counting and measuring the various components must be emphasised. It is a tradition that goes back at least as far as Leonardo da Vinci, and possibly further.


Cummins, K.W. (1974). Structure and function of stream ecosystems. Bioscience 24: 631-641.

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