Local Extinction and Re‐Immigration of Whirligig Beetles (Coleoptera, Gyrinidae)

The adult population dynamics and ecology of Gyrinus substriatus in one of several small water—filled quarries on an island in southeast Sweden were studied over several years. The entire population was censused with a mean interval of 14 d. Individuals were marked to indicate the first date they were observed and tenerals were distinguished from older beetles. These methods provided records of longevity, mortality, and reproductive success. The maximum population varied by a factor of 2.5 over six normal years. During two consecutive years the population went extinct, when low water level plus overgrowth of the perennial macrophyte Juncus bulbosus caused greatly reduced water surface area. During the extinction years, the beetles were attacked by a parasitic water mite (Eylais sp.) and several dead beetles were found. There was a high late spring—early summer immigration, but no exchanges could be shown between adjacent populations. This, in combination with records in the quarry ofspecies that did not breed in the vicinity of the surveyed locality, suggests regular long—distance dispersal. My data suggest that the size of the new generation generally is not proportional to the size of the locally hibernated fraction. This may or may not indicate some sort of density—dependent regulating factors. With the high winter mortality, the between—year population fluctuations were much affected by large—scale recruitment of spring colonizers from remote localities. G. substriatus had a long life—span and 4—13% of the immigrants overwintered 2 yr as adults. But winter mortality was severe; 47—86% of late—autumn beetles died by the following spring. Immigrated 1—yr adults had a higher survival rate during the 1st mo than tenerals. Survival of tenerals was related to season; early emergers had much higher survival than very late emergers. Each female produced between 11 and 16 adult offspring per year. The experimentally introduced G. aeratus, with which G. substiatus ofen co—occurs in mainland localities, had high adult survival. It had higher survival during a year when all G. substriatus were removed experimentally, perhaps indicating competitive release. This is supported by preliminary findings of identical prey choice for adults ot these two species. G. aeratus reproduced in both years when it was introduced, but only °30% as successfully as did G. substriatus. The young of G. aeratus were unaffected by the presence of G. substriatus. High death rate of teneral G. aeratus led to extinction °1 yr after each experimental introduction. The local but scarce G. natator did not increase after introduction.