No abstract

The simultaneous hatching of Daphnia resting eggs from a number of different lakes and ponds using one set of hatching cues can be difficult to achieve as environmental conditions differ considerably between water bodies. Therefore, optimal hatching conditions for ephippial eggs originating from shallow temporary waters may differ from those found in large lakes. The aim of our study was to compare the optimal thermal conditions for hatching ephippial eggs of Daphnia found in permanent lakes and those from shallow temporary ponds. We used ephippial eggs of Daphnia from the longispina species complex originating from two temporary city ponds and two deep lakes in Poland. The ephippia were protected against overheating at all stages of the field and laboratory work to prevent activation or killing of the eggs. After a refractory period (imposed storage in cool and dark conditions), ephippia were incubated at six different temperatures (6, 9, 12, 15, 18 and 21°C) under a 16:8 L:D light regime. Our results indicate that hatching of resting eggs of Daphnia that inhabit lakes or ponds may require different thermal conditions. The hatching success of ephippial eggs originating from temporary waters was relatively high (30-56%) at all tested incubation temperatures, while for the ephippial eggs from the deep lakes it was lower (7-37%) and inversely related to water temperature. The divergent hatching responses of the ephippial eggs originating from temporary pools and lakes may reflect the typical thermal conditions during hatching in their native habitats. While in the deep lakes of the temperate zone Daphnia hatching typically occurs during the low water temperatures of early spring, in shallow ponds Daphnia hatching may occur throughout the year at varying water temperatures, from a few to over a dozen degrees Celsius.

We investigate here the mechanism of allochronic resumption of development by the dormant forms in organisms inhabiting temporary habitats. The cohorts of resting eggs of a short living freshwater crustacean Daphnia magna collected in two temporary waters at two occasions (spring and autumn) were exposed after different storage periods (0–16 weeks) spent either in wet or dry conditions to a given set of hatching stimuli announcing appearance of favourable conditions. Freshly formed resting eggs did not hatch or hatched occasionally. The resting eggs formed in autumn hatched more eagerly than the spring ones when exposed to favourable conditions after wet storage. The hatching proportion increased linearly up to 68–82% in autumn resting eggs while to 33–44% in the spring ones over 16 weeks of storage that might have covered several generations of the active forms. Dry storage of the resting eggs reduced their hatching proportion considerably. We suspect that the length variation of a refractory period (initial phase of developmental arrest when resting forms remain insensitive to hatching stimuli) followed by a reactivation period may constitute the simplest two-step physiological mechanism allowing staggering revival of the dormant forms in subsequent generations that maximise chances for survival in unpredictably changing habitats.

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A

Abstract. In 2008 on six 1-ha plots the structure and species diversity of small mammal community inhabiting a narrow belt of coastal zone of the Łuknajno Lake (Masurian Lake District, North-East of Poland) were studied. The results obtained were compared with the results of similar studies carried out in the same area in 1981, when still intensive agricultural activities were present around the lake (abandoned in 1991 by leaving the agricultural fields fallow). In comparison with 1981, a decrease in the number of species inhabiting the fringe of the lake was discovered, as well as some significant changes in the domination structure of the community. Currently, the dominant forest species -the bank vole Myodes glareolus (Schreber 1780) and the yellow-necked mouse Apodemus flavicollis (Melchior 1834) -replaced the most numerous in 1981 -the striped field mouse Apodemus agrarius (Pallas 1771). Taking into consideration the fact that environmental conditions at the coastal zone have not changed, it was suggested that the changes in the community of small mammals were caused by setting aside arable lands around the lake. The results obtained lead to the conclusion that the range of the ecological effects of local changes in the landscape mosaic may include an area much larger than the one directly affected by these changes. They constitute the basis for a discussion on the relationship between various elements of environmental mosaic in heterogeneous landscape.

In 2008 on six 1-ha plots the structure and species diversity of small mammal community inhabiting a narrow belt of coastal zone of the Łuknajno Lake (Masurian Lake District, North-East of Poland) were studied. The results obtained were compared with the results of similar studies carried out in the same area in 1981, when still intensive agricultural activities were present around the lake (abandoned in 1991 by leaving the agricultural fields fallow). In comparison with 1981, a decrease in the number of species inhabiting the fringe of the lake was discovered, as well as some significant changes in the domination structure of the community. Currently, the dominant forest species -the bank vole Myodes glareolus (Schreber 1780) and the yellow-necked mouse Apodemus flavicollis (Melchior 1834) -replaced the most numerous in 1981 -the striped field mouse Apodemus agrarius (Pallas 1771). Taking into consideration the fact that environmental conditions at the coastal zone have not changed, it was suggested that the changes in the community of small mammals were caused by setting aside arable lands around the lake. The results obtained lead to the conclusion that the range of the ecological effects of local changes in the landscape mosaic may include an area much larger than the one directly affected by these changes. They constitute the basis for a discussion on the relationship between various elements of environmental mosaic in heterogeneous landscape.