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.


For want of a nail …

How much mercury does it take to kill a fish?   A quick search on Google reveals  just over 270000 hits for the search terms “mercury”, “toxicity” and “fish”, with one reputable-looking source close to the top of the list suggesting just under one part per million is all it takes to kill a rainbow trout within 24 hours.   Exposure to a lower concentration might also have the same effect, albeit over a longer period, and even very low concentrations might be enough to disrupt metabolism and impair breeding potential, or to kill delicate life stages such as fry.    Much more could be written about the effects of mercury but this is not, actually, my subject for this post.

We often find the term “parts per million” in discussions about harmful chemicals in the environment but what exactly does this mean?  How can we visualise what one part per million actually looks like?   Here are two examples:  one gram of rice consists of about 30 grains.  Now imagine being asked to look for three grains of rice in one bag of sugar.   A bag of sugar weighs a kilogramme, so this is equivalent to searching for one part in 10,000.   One part of rice per million would be searching for three grains of rice in 100 bags of sugar.   Alternatively, you could think of one part per million as the equivalent of emptying a “tall” Starbucks coffee into an Olympic-sized swimming pool (50 metres x 25 metres x 2 metres).

Metaphors and analogies such as these are useful means of conveying information about toxicity and risk but they introduce a further problem of making the quantities seem so infinitesimally small that they trivialise serious issues.   So let’s try two further analogies: first, a single screw may cost a few pennies and, therefore, be just one or two “parts per million” of the total cost of a car.  Yet, the absence of that screw might have serious implications for the performance of the car as a whole.   And a chilli con carné needs only a tiny amount of chilli powder to impart the spicy heat that characterises the dish.   However much meat and rice you serve, it is the tiny quantities of spices that define dishes such as this.

Many of the nutrients and minerals that sustain our ecosystems exist at concentrations well below one part per million in the natural world, sometimes as low as a few parts per billion (a thousand times smaller than “parts per million” – imagine a pinch of salt in ten tonnes of potato crisps).   Yet these, in turn, provide vital nuts and bolts in the machinery of life.   Toxic metals such as mercury interfere with these nuts and bolts without which the performance of whole organisms can shudder to a halt.

For want of a nail, the shoe was lost; for want of a shoe, the horse was lost, and so on ….