Qualitative Changes in Populations in Unstable Environments

Abstract

Inactive moths of Malacosoma pluviale (Dyar) oviposit near their birthplaces, and most of their offspring also are inactive. More active moths can travel farther before they oviposit, and always have a higher proportion of vigorous individuals among their progeny.Such polymorphism allows the insect to cope with environmental diversity; e.g., inactive residents exploit favourable habitats, and active migrants colonize more severe habitats, or replenish the vigour of other populations.Because the most active moths usually export the most vigorous progeny, the population left behind becomes less vigorous during successive generations. This steadily decreasing vitality eliminates local populations that are not replenished by vigorous immigrants.Qualitative changes in Malacosoma populations follow this basic pattern, but the rate of deterioration is affected by the habitat. Departing migrants fly too high to be stopped by small trees in farmland, bur many are stopped near their source by tall trees in forests. Deterioration therefore is slower in forests. Forests also delay return migration to nearby farmlands, and thus allow some farmland populations to deteriorate unchecked.In a fluctuating climate, the size of the area tolerable for the species varies annually. When it begins to expand, the vigorous progeny of active moths can take immediate advantage of slight local improvements. Consequently, they are the first to exploit each marginal habitat that becomes tolerable. But while better climate persists, some less active descendants of these pioneers appear in all occupied habitats.When the regional climate deteriorates, the tolerable area contracts, and most marginal populations are totally destroyed. Moreover, even within the contracted tolerable area, the harsher climate becomes intolerable for any deteriorated stock. In the next generation, therefore, the only regional survivors are vigorous colonies deposited in the tolerable area by some of the few migrants that escaped the widespread destruction of the preceding generation in the margins. Their descendants recolonize depopulated sections of the refuge, and so preserve the species in the region while the climate remains severe.

Population fluctuations of the western tent caterpillar in southwestern British Columbia

“…Wing wear of female moths was classified for populations with different infestation histories. Means of the scores for those populations that had high caterpillar densities in 1955 showed a nonsignificant increasing trend from 1956 to 1959 whereas moths from areas not infested by tent caterpillars in 1955 showed a Wellington 1960Wellington , 1964Wellington , 1965 nonsignificant declining trend over the same time period. The trend toward greater activity of new populations in 1956 may indicate invasion of these areas by active moths.…”

“…Although there are few statistically significant data for its basis, the scenario proposed by Wellington is approximately described by the following: tent caterpillars in areas of favorable climate persist and increase when weather conditions are good (Wellington 1964). Families are composed of more and less active larvae, and this variation is partly dependent on the amount of yolk provided to the eggs.…”

Ecological outbreak dynamics and the cusp catastrophe

“…This is in spite of the great amount of data which has been collected from some outbreak insect populations, such as those of the spruce budworm (Morris, 1963). Still, some workers have claimed to account for oscillatory outbreak dynamics in certain species, for example Wellington (1957Wellington ( , 1960Wellington ( , 1964 who studied the western tent caterpillar, and their conclusions have achieved wide currency. But the ecology of outbreak species which have been long-standing objects of study often remains a matter of controversy, particularly with regard to the factors triggering and terminating outbreaks.…”

Resonant Population Cycles in Temporally Fluctuating Habitats

“…Natural populations live in changing environments; indeed, animal numbers may be intricately linked to environmental periodicities (Morris, 1963;Wellington, 1964;MacArther, 1968;Barbosa and Shultz, 1987;Tuljapurkar, 1990;Caswell and Trevisan, 1991;McFadden, 1991). Although it is widely recognized that environments are often not constant in time, the vast majority of mathematical models in population dynamics and ecology are autonomous and assume a constant environment.…”