7.1: Chapter Introduction

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In the density-limited growth examined thus far, the ecological effects of density fed back fast quickly enough that the population’s growth could adjust and the population could reach a carrying capacity, equal to −r /s . But if the growth rate is too fast compared with that feedback, the population can overshoot its carrying capacity, which can lead to highly complex outcomes.

Think about feedback in this way. Imagine driving down the road, keeping your eye on the road, instantly correcting any little deviations of your car from your lane, adjusting the steering wheel without even perceiving it, and with only normal blinking of your eyes. In this case there is very little delay in your feedback to the steering wheel, and you stay in the lane. Now suppose you close your eyes for one second at a time, perhaps every ten seconds. (Do not run this experiment; just think about it!) You may have drifted a bit to the left or right in that second and would have to turn the steering wheel further to get back in you lane. And now imagine shutting your eyes for 15 seconds every minute, then opening them and correcting your path down the road. You’ll start oscillating in your lane and precariously jerking back and forth, possibly visiting the ditch. The cause? The delay in the feedback between stimulus and response.

So it is with populations. Delays in sensing the carrying capacity can start oscillations. For example, a modeled insect population that grows and lays eggs one year and emerges the next year can suffer such oscillations. The insects are, in effect, “keeping their eyes shut” about how many insects will be produced the next year. This is in contrast to species like bacteria or humans, where the population grows more or less continuously.