Cyclic population dynamics continue to fascinate ecolo-gists, and as more long-term data are collected, more examples of population cycles are sure to be discovered. What broad conclusions can ecologists make regarding the forces responsible for the maintenance of these cycles, given the information we have at hand.? First, it is apparent that cycles are usually not caused by a single factor at a single period of time. Indeed, this is the hallmark of many ecological phenomena, since science is largely the study of complex interactions among many factors. Many agents are probably responsible for the net effects ecolo-gists have observed over time.
Given this caveat, if we examine the examples in Table 1, there seems to be a mechanistic bias toward trophic interactions, that is, predators or parasites of some kind seem to be involved in all of the examples listed. Secondly, the main drivers of the oscillations are second-order in nature, with significant time lags in density-dependent factors. This is not to say that firstorder dynamics are unimportant; first-order dynamics are often needed to stabilize cycles, but they are not sufficient to cause cycles. In general, population cycles in herbivores are rather common in nature and the cycles themselves tend to depend on significant time lags. These lags suggest that predation or other trophic processes may be operating, but only experimental manipulation over several population cycles can confirm the exact mechanism. Thus, as in most of ecological science, theories are rejected or supported based on many avenues of evidence. Since biological systems are so complex, we should expect that both the drivers of population regulation and the population trajectories themselves will be equally fascinating and multivaried.
See also: Boreal Forest; Food Chains and Food Webs;
Growth Models; Mathematical Ecology; Prey-Predator
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