Unusual Weather and Insect Population Dynamics: Lygaeus equestris during an Extinction and Recovery Period

Abstract

A metapopulation of the seed feeding bug Lygaeus equestris has been censused from 1977 to 1990 in an area in SE Sweden with several small and scattered host-plant patches. Here I describe weather induced population changes during the last four years. During this period the population underwent a large scale extinction followed by recolonization and recovery, a pattern which differs radically from the previous ten years when fluctuations had been moderate and patch occupancy high. In the summer of 1987, with unusually cool weather and an exceptional deficit in sunshine hours, L. equestris populations declined to an unprecedented low level. Total extinction ensued in the following winter. Because weather effects are correlated over large areas the extinction coincided with low population density and patch occupancy in adjoining areas. Hence, following the extinction colonization rates were evidently at a minimum. The study area remained unpopulated until the summer of 1989. when some colonists arrived from outside areas. Because colonists and their offspring were so few establishment would most likely have failed had this been an ordinary year. However, unusually sunny weather resulted in a second generation, something that had not happened during the earlier years of study. This extra reproduction in combination with new immigration from growing populations outside the study area gave rise to a large autumn population, which survived the winter. However, at the end of 1990 population density was still lower than in an adjoining area which had not faced total extinction.