by Greg Mayer

There’s been a lot of media attention the last few days about the prospect of a derelict Russian passenger ship, the Lyubov Orlova, crossing the Atlantic from Canada (where it was last berthed) and crashing into Ireland or Britain, spilling disease-ridden, inbred, cannibal rats on their shores. The ship was being towed to the Dominican Republic when it broke away and floated off last year. The Sun has had perhaps the most dire take on the story, headlining their piece “Ship of Ghouls“.

That the ship, laden with cannibal rats, might fetch up on shore, however, seems very unlikely. As the Guardian reports, both the Irish and British coastguards downplay the possibility. In particular, Irish officials note that they have looked for it, can’t find it, and suspect that it may have sunk.

But there’s another reason that it is highly unlikely, having nothing to do with whether or not the ship is still afloat, and because it reveals an important biological principle, it’s worth a mention here at WEIT: an ecosystem without primary producers cannot be sustained except for very brief periods of time. Unless those rats have been raising crops on the ship, the last one, cannibal or not, died some time ago.

There are very few “laws” in biology in the strong sense with which the term “law” is used in physics. Mendel’s “laws” of heredity, for example, have numerous exceptions and limiting conditions. When I teach genetics, I refer to them not as “laws”, but as Mendel’s “useful generalizations”. But one of the biological principles that does have law-like status is that primary producers must be at the base of every food web, and that energy is lost at every step in the food chain. Perhaps the reason these principles are law-like is because they derive so directly from the laws of thermodynamics: energy must be put in to a system to avoid entropy increase (so we must have producers to capture energy) and no energy transfer is 100% efficient (so there’s less energy available further up the food chain than at the bottom).

These ecological principles are often expressed in terms of a “trophic pyramid“, a visual representation of the fact that primary producers (usually green plants that capture energy by photosynthesis, but also photosynthetic and chemosynthetic bacteria) outnumber herbivores (primary consumers), who outnumber carnivores (secondary consumers), who outnumber top carnivores (tertiary consumers; there are rarely more than four or five levels in a food chain).

The exact shape of the pyramids depends on the efficiency of energy transfer between levels, and whether the pyramid is based on number of individuals, biomass, or energy flow units. As a rough guideline, efficiency of transfer between levels is often estimated at about 10%; the efficiency of capture of solar radiation is much lower. These important details can be investigated here and here, for example, but they do not alter the fundamental principles.

Viewing ecosystems in this manner owes much to an extremely influential paper, “The trophic-dynamic aspect of ecology“, by the young limnologist Raymond Lindeman, who tragically died of an obscure form of hepatitis a few months before the paper was published. The very interesting story of how the paper was almost not published, first being rejected by Ecology for being too theoretical, but finally accepted, has been told by Bob Cook.

So, what does this all mean for the people of Ireland and Britain? You can relax. Although rodents will resort to cannibalism when food supplies run low (though I’ve never seen it in rats myself, which, by the way, are much friendlier and make better pets than mice), an ecosystem based on cannibalism cannot persist, because there is no energy input to the system, and there cannot be a 100% efficient transfer, so the rat-level in the “food chain” will continually decrease in number and biomass. How long a bunch of rats resorting to cannibalism would last depend on details such as the number of rats, their caloric needs, etc., but suffice it to say that no reasonable numbers for those details could get the rats to last nearly a year. The caloric needs of mammals are quite high, and they need to eat a lot. (This may not be a comforting thought, but a ship full of anacondas and reticulated pythons could last a year or more at sea, without even having to eat one another, because of their low caloric needs. They would come ashore hungry, though.) The reason I say “probably”, rather than “definitely”, is because it is conceivable that large stores of food were left behind, and depending on the quantity, these could support a rat population for some time (recall that in most zombie movies the survivors rely on canned goods for survival, although in the last season of the Walking Dead they did begin farming). But since the ship was being towed for salvage, I doubt the kitchens and larders were full.

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Cook, R.E. 1977. Raymond Lindeman and the trophic-dynamic concept in ecology. Science 198:22-26. pdf

Lindeman, R.L. 1942. The trophic-dynamic aspect of ecology. Ecology 23:399-418. pdf

Sterner, R.W. 2012. Raymond Laurel Lindeman and the trophic dynamic viewpoint. Limnology and Oceanography Bulletin 21:38-50. pdf