This dichotomy has its uses, but it can cause problems if taken tooliterally. For one thing, no population can be driven entirely bydensity-independent factors all the time. No matter how severely orunpredictably birth, death, and migration rates may be fluctuating around theirlong-term averages, if there were no density-dependent effects, the populationwould, in the long run, either increase or decrease without bound . Put another way, it may bethat on average 99 percent of all deaths in a population arise fromdensity-independent causes, and only one percent from factors varying withdensity. The factors making up the one percent may seem unimportant, and theircause may be correspondingly hard to determine. Yet, whether recognized or not,they will usually determine the long-term average population density.
In order to understand the nature of the ecologists investigation, we may think ofthe density-dependent effects on growth parameters as the signal ecologists are trying to isolateand interpret, one that tends to make the population increase from relativelylow values or decrease from relatively high ones, while the density-independenteffects act to produce noise in the population dynamics. For populations that remain relativelyconstant, or that oscillate around repeated cycles, the signal can be fairlyeasily characterized and its effects described, even though the causativebiological mechanism may remain unknown. For irregularly fluctuatingpopulations, we are likely to have too few observations to have any hope ofextracting the signal from the overwhelming noise. But it now seems clear thatall populations are regulated by a mixture of density-dependent anddensity-independent effects in varying proportions.
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