Many ecologists take a hybrid position between the functional and environmental schools, noting the powerful influence of climate and other physical features while also acknowledging the existence of density-dependent regulation. Some have advocated a revision in terminology, arguing that, in part, the disagreement between ecologists on the nature of population regulation can be linked to poor or confusing terminology. A. A. Berryman suggests that ecology should use the vocabulary of dynamic systems theory and replace the term density dependence by negative feedback. Negative feedback occurs whenever the rate of change of a population (dynamic variable) is inversely proportional to its current or past size (states). For a population to be regulated, a negative feedback must operate. It is clear that the individual scientist's viewpoint on population regulation is heavily influenced by field experience. Those working on large mammals or birds are far more likely to emphasize biological, density-dependent possesses than those working on single-celled organisms or small fast breeding insects who frequently observe population explosions or collapses linked to a change in environmental conditions.
It is striking that much of the argument about population stability and the importance of density dependence, population regulation, and species interactions such as competition has not been based on empirical observation and has been heavily influenced by simple mathematical models. This is notable, as generally ecologists put great emphasis on the importance of field observation. More recently, there has been a reduced emphasis on gaining insights from mathematical models as it came to be realized that quite simple models could produce an astonishing richness of dynamical behaviors. In effect, everyone could find a simple model to demonstrate that his or her view had a theoretical basis. For much of the twentieth century the emphasis on theoretical considerations was inevitable as ecologists had too few long-term population dynamical time series to study. While all natural populations vary in numbers in space and time, surprisingly little was and still is known about the statistics of this natural variability. The shortage of empirical data was, in part, because ecology is a relatively new science and population data was not systematically collected until the later part of the nineteenth century so even now we have few reliable data sets extending for longer than 100 years. When it is remembered that many larger species of animals can live for 30 or more years and large plants for hundreds of years, it is apparent that our time series cover too few generations to show the full dynamical range of long-lived organisms. In part, studies of fossils have supplied the long-term perspective required; however, such studies can rarely give reliable estimates of relative abundance. A continuous stream of new observations is now rapidly increasing the database upon which ecologists can test hypotheses and measure the degree of stability actually shown. To give an impression of the typical variability and stability displayed by populations, the following section presents typical results obtained from field studies. However, before we can compare the stability of populations, it is essential to define an appropriate measure of temporal stability.
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