TWEEK THE VALUES and you get different graphs. This little applet allows you to change the value of r (the "birth rate") and then see how the population changes over the years.

Oh, I'm sorry. You don't appear to have a Java-enabled browser. If you did have a Java browser, you'd see a little application here which allows you to change the "birth rate" parameter and see a graph of the population changing over time. This is the kind of thing you could have seen as you changed r:
Period 2 graph Period 4 graph Chaos graph
Period 2 Period 4 Yikes!

Setting r to be above 3.0 and you'll start to see periodic behaviour: the graph doesn't settle down to a single value. Inch it up further and you'll see periods of 4 (r=3.5), 8, and so on. The doubling of periods start to come faster and faster, and then, at about r=3.7 you're into chaos. Mess around with a few values. My favourite is r=3.88989. (Oh, don't set r below 1 because you'll kill off all the fish).

Ecologists at the time assumed that the numbers were fluctuating around some underlying equilibrium point. (Those folk didn't have PCs to crank the calculations out...ouch). Gleick says: " the back of the ecologist's minds was always the assumption that an erratic string of numbers probably meant that the calculator was acting up or, or just lacked accuracy."

Contact Richard Dallaway