Bo W. Svensson

Assemblages of dytiscid predators and culicid prey in relation to environmental factors in natural and clear-cut boreal swamp forest pools

Assemblages of diving beetles (Dytiscidae) and mosquito immatures (Culicidae) were studied during 1987–1988 in 40 small, more or less temporary, snowmelt pools in spruce swamp forest and clearings at the Arctic Circle in Sweden. Larger pools were warmer than smaller ones, and clearing pools were warmer than forest pools. Temperature differences between pools remained high until late July. Twenty-one dytiscid species, representing three guilds, occurred in the pools, and individual pools had 1–13 species. Ten species occurred in both habitats. A few stenotopic species of boreal swamp forests had in clearing pools apparently been replaced by some species with a preference for more productive, often man-made habitats. Six Aedes species were collected in the clearing pools. Five of these were found in the forest pools, of which three had no mosquito larvae. Dytiscid assemblages in both habitats and culicid assemblages in clearing pools showed strong nested patterns.Abundance and species richness of both culicids and dytiscids were higher in clearing than in forest pools with an area >2 m2 after that the effects of pool area had been accounted for (MANCOVA). In both habitats, abundance and species richness of both culicids and dytiscids were strongly and positively correlated with a linear combination of pool area, depth and temperature (Canonical Correlation). Increasing drought frequency of pools had a negative, less significant effect on the biota. In the clearing, the abiotic correlations with abundance were somewhat weakened chiefly by the relatively low abundance values from the largest pool.Distribution and mean abundance of individual dytiscid species were positively related in the clearing pools. Flying dytiscids were trapped in the larger (1.6 m2), but not in the smaller (0.07 m2) artificial pools, and the immigration rate was markedly higher on clearings than in forest. Dug pools were colonized faster on clearings than in forest. Even the flightless Hydroporus melanarius colonized dug pools during the first year.

Changes in occupancy, niche breadth and abundance of three Gyrinus species as their respective range limits are approached

Three widely distributed Gyrinus species were studied in seven different habitat types over a wide range of latitudes in northern Europe. All three species have a broad habitat range and are opportunistic exploiters of temporary and fluctuating habitats. The distribution range of the two southern species, G. natator and G. substriatus, overlaps with that of the boreal-subarctic G. opacus in south-central Sweden, creating an inland area of sympatry, where all three species coexisted in 10% of the waters sampled. For the two southern species occupancy increased with distance from their respective range limits. The occupancy level of G. opacus in its southernmost inland zone was higher than those of the other two species in their respective northernmost zones. Levels of maximum occupancy were similar for all species. Patterns of habitat occupancy partly differed within and between species. Although temporary pools, which represented the most unpredictable habitat, were highly frequented in the south by G. substriatus, occupancy levels associated with them declined sharply before reaching the northern range limit. The niche breadth of the two southern species increased as the distance from their respective northern range limits increased and was broadest for G. substriatus in southern Sweden, north of its geographical range centre (southern Poland), where pollution has no doubt severely affected the original distribution. The habitat range of G. opacus did not change as its southern range limit was approached, but fewer habitat types appeared to be used in the very far north. Abundance of all three species in temporary pools increased with distance from their respective range limits, and there was a regional correlation between occupancy and abundance for G. substriatus and G. natator in this habitat.

Environmental heterogeneity in space and time : patch use, recruitment and dynamics of a rock pool population of a gyrinid beetle

A population of the whirligig beetle Gyrinus opacus inhabiting a cluster of 108 rock pools on a small peninsula in south-central Sweden was studied over a 3-year period. In terms of recruitment of new adults and population size, the study period included two good years, one dry (1990) and one wet (1991), and one bad, dry year (1992). Mean occupancy of pools was around 40% during the two good years and 18% during the bad year, i.e. most pools were vacant. As expected from theory, turnover rates were highest for pools with the lowest average population size. While new adults emerged from about half the pools, only 7.4% were successful in all three years. Beetles attempted to hibernate in about half the pools and succeeded in 41%. Most activities, such as local movements, recruitment of new adults and hibernation, involved the 19 major pools, but no pool was outstanding. Dispersal between pools was frequent and, as expected, breeding pools were in contact with a higher number of pools than those where no successful breeding took place. Surprisingly, during the dry good year, both recruitment number and reproductive success were positively related to the length of the dry period of pools, which may possibly reflect an escape from predators on eggs and larvae. During the two first years when the population was stable, pools that dried out in 1990 but not in 1991 produced significantly more new adults in the dry 1990. The size of the total population showed a seasonal bimodal pattern with a minimum in summer. The number of beetles in different pools tended to vary synchronously within and among years, but there were many exceptions. Beetles survived drought periods by transferring to neighbouring, permanent refuge pools, or by seeking refuge for up to a week under a boulder in the desiccating pool. This illustrates the great importance of environmental heterogeneity for local population survival, i.e. normally unimportant elements in the habitat may assume critical importance under stressful periods.

Effects of predation on clutch size and egg dispersion in the codling moth Laspeyresia pomonella

ABSTRACT. 1 The effects of predation on clutch size and egg dispersion in the codling moth Laspeyresia pomonella (L.) were experimentally studied in an orchard by recording survival of eggs glued onto different substrates. 2 Egg mortality due to sucking insects was readily distinguishable from that due to chewing insects. 3 In both the field and laboratory, females preferred to deposit eggs singly on the uppersides of leaves. 4 Eggs on the uppersides of leaves and on fruit survived significantly better than did those on the undersides of leaves or on twigs. 5 Egg losses were highest during the early part of the flight season. 6 The smallest (one egg) and largest (nine eggs) clutches, associated with one egg‐bearing leaf per twig, both suffered the lowest rates of predation. Factors selecting against oviposition on fruits and production of large clutch sizes are presented and discussed.

The possible role of volatile secretions as intra- and interspecific alarm signals in Gyrinus species

Gyrinid beetles have paired pygidial glands producing a secretion consisting of both a high molecular weight fraction, rendering them toxic for predatory fish, and usually also volatile fractions (8 of 10 north European Gyrinus species), which act as alarm signals. The presence of multi-species aggregations on the water surface raises the question whether the pygidial secretion of one species can elicit an escape response in other species. There were no differences in escape reaction to signals between two strongly scented species (G. aeratus and G. substriatus) and one species without obvious scent (G. opacus ). Furthermore, signals were received across the species borders, although only the two species emitting volatile components were able to alarm other individuals. The two scented species (G. aeratus and G, substriatus) often aggregate in large groups on open water, whereas the unscented species (G. opacus and G. minutus) rarely do so, but preferably occur singly or in loose groups among the emergent marginal vegetation of various standing waters. This suggests that the presence of alarm signals is correlated with behaviour and habitat choice. Furthermore, the putatively weak competitor G. opacus spends significantly more time below the water surface than G. substriatus, perhaps as a result of it lacking an alarm function. The occasional presence of individuals of the two unscented species in large aggregates of G. aeratus on open surface, may indicate that they may take advantage of the alarm signals produced by individuals of another species and as result escape approaching danger. It is not known whether G. opacus and G. minutus have lost the ability to produce volatile alarm substances or whether they never have had it. Comparisons with the ca 11000-year-old subfossil localities of G. opacus, however, indicate that at least from that date this species has inhabited localities similar to the present with respect to predation risks and other properties, i.e. mainly small, predatory fish-free waters.

Morphological variation in Gyrinus sericeolimbatus Rég. in New Guinea and description of a new Gyrinus species (Coleoptera: Gyrinidae)

Abstract. Samples of Gyrinus sericeolimbatus from twenty‐six localities in New Guinea vary greatly in metric body characters as well as in elytral reticulation patterns. The variation is smaller within than between populations. Although the characters of adjacent populations are usually similar, certain proximate populations exhibit significant differences, probably resulting from some kind of dispersal barrier. Similarities between populations at comparable latitudes suggest that some of the morphological variation is altitude‐related. In addition, evidence is found for habitat‐related morphological differentiation. Changes in phenotypes in response to possible environmental factors in ontogeny and constraints imposed by various selection pressures are discussed.