Messing with food chains

One of the less-travelled roads in an O.R. education is biological models.  Perhaps the best-known family of models is that developed by Volterra.  At their simplest, these models describe the interaction of two species, one of which is a predator on the other.  (Hence the name "Predator-prey models")  There is a nice student paper about them here.

This week, two stories related to predator-prey models came my way.  The first is a tragic story from the Everglades in Florida.  Apparently, Burmese pythons are breeding successfully in the region and eating the local mammals, fish and birds.  The newspaper story didn't mention anything that might be a predator on these snakes, and the evidence is that human intervention so far has made little impression on the population.

Looking at the picture, it is hard to imagine what might be a predator on such a large snake.  But if the biological models are to be believed, the ecosystem is heading for a new stable solution - and sadly, in that solution, there won't be as many mammals in the Everglades as there were pre-pythons.

The second story is local and much happier.  About a mile from here, the Devon Wildlife Trust (DWT) has its headquarters in an old water-mill just outside the city walls.  The mill dates from the time when the area between the city walls and the river Exe was given over to the wool industry.  Exe Island, as the area was known, was criss-crossed with mill-leats powering the mills, and wool was hung out to dry on numerous racks.  The DWT has a night-vision camera watching the mill-wheel, and recently this recorded a mother otter and young otter playfully learning the way through the water.  Otters are gradually returning to the river Exe, after years of absence.  There hasn't been human intervention aimed at the otters, simply changes to other parts of their food chain, making the fish that they need more abundant in the river.

 An otter seen by day by DWT photographer)

What happens next?  It is extremely unlikely that we will see growth in numbers of otters at the rates seen among pythons in the Everglades.  Mammals breed less profusely than snakes.  So the move to a new stable state will be slow.   And that state is almost certainly sustainable, as it will be reverting to one which was stable several decades ago before pollution affected the food chain that provides for otters.

So two stories of the food chain.  One where messing about has led to serious and undesirable ecological problems; the other, much more benign and generally desirable.

(Written as a contribution to the INFORMS challenge "O R and food")


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