Studies of predation by roach (Rutilus rutilus) on zebra mussel (Dreissena polymorpha) in a large, eutrophic lake showed that there was a clearly marked size threshold (∼ 160 mm SL) above which roach began to feed on mussels. Roach preying on various sizes of mussels selected them in proportions different from their abundance and accessibility in the habitat. The mean size of mussels ingested by roach of 220 mm and larger, which fed predominantly on Dreissena, closely followed the pattern expected for a constant ratio of mean prey size to mean predator mouth size = 0.59. To explain the size selection we applied an optimal foraging approach, based on the ability of different-sized fish to crush (cost) mussels of different sizes, and hence crushing resistance, and energy contents ('benefit'). We found that fish smaller than 160 mm, which showed no inclination to eat Dreissena, would only be able to take small mussels with a very high cost/benefit ratio. The real switch to Dreissena would be expected in fish of 230-240 mm that could take most of their prey from highly profitable, numerous, and easily accessible size classes while keeping the mean prey size at the optimal level relative to mean predator mouth size.
. 1993. Spatial distribution of brown hare Lepus europaeus populations in habitats of various types of agriculture. Acta theriol. 38: 435 -442.In 1987 -1990 an investigation was made on the spatial structure of brown hare Lepus europaeus Pallas, 1778 populations using the transect routes method. The investigation was carried out in two different parts of Poland that differed in the character of agriculture: I -northeastern with large fields of state farms, and II -eastcentral with small fields of individual farmers. The results indicate that independently of the agrarian structure, the distribution of hares in crops corresponds to their percentage share in the region, with exception of rape fields, which show a tendency to be avoided. There was also a significant relationship between the spatial distribution of the hare and the habitat diversity -highly variegated fields are more consistently inhabited by hares. Besides, hares show a distinct tendency to prefer areas close to the field margins. Positively oblique distance distributions between the individuals and the margins of field crops are particularly strongly expressed in the region of a highly monotonous crop species. It may be expected that a change in type of farming by passing to the large-fields and continuous cropping is one of the factors that make it difficult for the hare to utilize the space within the habitat.
The paper summarizes the results of a long-term study on the ecology of Dreissena polymorpha (Pall.) in several dozen lakes of different size, depth and trophy. The abundance of adults, planktonic larvae, and settled postveligers was found to vary widely between the lakes, localities within a lake, and years. The temperature influences the period of appearance and abundance dynamics of planktonic larvae; wind affects their vertical and horizontal distribution in the water column; winds and water currents displace settling postveligers. Settling postveligers display preferences for various natural substrates: most often they settle on submerged plants, especially perennials. The age structure of D. polymorpha settled on plants (dominance of young individuals, at most 3 years old) differs from that in colonies settled on the littoral bottom. The abundance of mussel populations in lakes is primarily determined by the mortality at the transition from planktonic to sedentary life, and by the mortality of individuals settled on submerged plants.
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