Size In Autumn Research Articles

Determinants of interspecific variation in season length of perennial herbs.

Perennial plants in seasonal climates need to optimize their carbon balance by adjusting their active season length to avoid risks of tissue loss by adverse conditions. As the season length is determined by two processes, namely spring growth and senescence, it is likely to vary in response to several potentially contrasting selective forces. Here we aim to disentangle the cascade of ecological determinants of interspecific differences in season length. We measured size trajectories in 231 species in a botanical garden. We examined correlations between their spring and autumn size changes and determined how they make up the season length. We used structural equation models (SEMs) to determine how niche parameters and species traits combine in their effect on species-specific season length. Interspecific differences in season length were mainly controlled by senescence, while spring growth was highly synchronized across species. Structural equation models showed that niche parameters (light and moisture) had stronger, and often trait-independent, effects compared to species traits. Several niche (light) and trait variables (plant height, clonal spreading) had opposing effects on spring growth and senescence. The findings are indicative of different drivers and potential risks in growth and senescence. The strong role of niche-based predictors implies that shifts in season length due to global change are likely to differ among habitats and will not be uniform across the whole flora.

Agroecological measures in meadows promote honey bee colony development and winter survival

AbstractThe homogenization of agricultural landscapes has led to a decrease in pollinator diversity and abundance. In response to this decline, farmers have implemented agroecological measures, which, in meadows, aim at providing more floral resources. These measures are the availability of unmown floral strips, delayed mowing, and discouraging the use of the conditioner, a device known to harm insects. The aim of our study was to investigate the cascade of effects of these agroecological measures on honey bee colony development and winter survival. We (1) determined the effect of these measures on colony size during the nectar and pollen collecting season in spring and summer, (2) evaluated the effect of spring and summer colony sizes on autumn size, and (3) described the effect of colony size in autumn on winter mortality. In this study, 300 honey bee colonies were monitored over three years in three cantons of Switzerland. Colony size was defined by the numbers of brood cells and of adult workers. Honey bee colony size in summer and autumn was improved by agroecological measures on meadows and likely contributed to the increased overwintering success. This study is a first step toward the targeted identification of viable agroecological measures on temporary meadows that can be implemented to promote honey bee colonies health in the agricultural landscape.

Rodent fertility in commercial orchards in relation to body mass and body condition

Although rodents are recognized as pests, they are still an important part of agricultural ecosystems. In 2018–2020, we snap-trapped small mammals in Lithuania in 18 commercial orchards (apple and plum orchards, currant and raspberry plantations) and in adjacent control habitats. We aimed to discover whether the litter size of the six most abundant rodent species depended on season, on female body mass and/or on fitness. Litter size in common and root voles was smaller in habitats with a higher intensity of agricultural practices. Compared to summer, observed litter size in autumn significantly decreased in common voles (from 5.6 to 4.8) and yellow-necked mice (from 5.8 to 4.6). In autumn, litter size and female body mass was positively correlated in all rodent species, while in summer this was characteristic to root voles (r = 0.67, p < 0.05, 45% of variation of the litter size explained) and yellow-necked mice (r = 0.53, p < 0.005, 27% respectively) only. Female body condition index and litter size correlations were weak. Potential litter size exceeded the observed rate and breeding failures were observed in all species, with the highest percentage in root, bank and common voles.

Physiological and behavioral responses of phytoplankton communities to nutrient availability in a disturbed Mediterranean coastal lagoon

Short-term bioassays were conducted in Biguglia lagoon (Corsica) to study the physiological and behavioral responses of phytoplankton to N- and P-availability. Natural communities were collected in two stations representative of the two sub-basins, at three periods with contrasting environmental characteristics to address the impact of seasonal variability. These samples were separately enriched with a full N and P enrichment, and with enrichments minus N or minus P. Phytoplankton size structuration, diversity, and growth of the total phytoplankton, the micro-, nano- and ultraphytoplankton were evaluated using spectrofluorimetry, and optical microscopy. Results showed that the communities were fueled by NO3− in the wet periods (autumn and spring) and NH4+ in summer. The phytoplankton communities displayed highest cell size in autumn, with high abundances of nanoflagellates, and smallest cell size in summer with a large dominance of phycocyanin-rich picocyanobacteria. Blooms of dinoflagellates also occurred during the wet periods, coinciding with high N:P ratios. The full enrichment has not stimulated phytoplankton growth in autumn, suggesting the importance of other controlling factors such as light, a possible NH4+ inhibition or the use of mixotrophic abilities. In spring, communities have displayed single P-limitation in the northern basin and different N and P co-limitations in the southern basin. In summer, the full enrichment consistently stimulated the growth of all cell sizes. The communities showed high N and P co-limitations, which is consistent with growing observations in aquatic ecosystems, and reflects the different functional responses of phytoplankton communities to the nutrient availability.

Recolonization and Facilitation in Baltic Salt Marsh Vegetation

Vegetation gaps are considered to be important for germination and establishment of species, which are weak competitors but have long-lived seeds in the soil. Vegetative growth is a colonization strategy especially important in regularly disturbed grasslands. In a salt marsh on the Baltic coast of Germany, we studied (i) the role of seedling recruitment and vegetative growth in recolonization of gaps, (ii) if gaps are necessary for seedling recruitment and (iii) whether gaps contribute to species diversity and composition of the salt marsh. We carried out a two-factorial field experiment during two years. We created 48 gaps of 0.04 m2 in size in autumn 2006. We eliminated the seed bank by sterilization and prevented vegetative growth into these gaps with dense meshes. We used a full-factorial design with four treatments: (1) seed bank and vegetative growth eliminated, (2) seed bank eliminated and vegetative growth intact, (3) seed bank intact and vegetative growth eliminated, (4) both intact. Seedlings an...

Non-deep complex morphophysiological dormancy in seeds of the Iberian Peninsula endemic geophyteMerendera montana(Colchicaceae)

AbstractHeretofore, no detailed account was available on seed dormancy and germination of a member of the Colchicaceae (Liliales). Thus, the primary aim of this study was to do an in-depth investigation of the temperature requirements for dormancy break and germination in seeds ofMerendera montana(Colchicaceae) at the embryo and whole-seed levels under near-natural temperatures in a non-heated frame shade-house and under controlled conditions in the laboratory. Mean embryo length in fresh seeds wasc. 0.57 mm and embryos had to grow to at least 1.30 mm before radicle emergence. Embryos grew to full size and seeds completed germination (radicles emerged) when they were stratified at 28/14°C for 60 d followed by a cool temperature for 60 d and then incubated at a cool temperature for 30 d. The optimum cool stratification temperature for dormancy-break was 10°C. Thus, after the moist pretreatment at 28/14°C+10°C, radicle emergence was&gt;93% at all incubation temperatures (5, 15/4 and 20/7°C). In its natural habitat,M. montanaseeds are dispersed in June, the embryo elongates to full size in autumn and radicles emerge from early November to early February. Although the shoot does not emerge until March and April, it is not physiologically dormant. The shoot emerged from 80% of the radicle-emerged seeds in 13 d at 20/7°C without a previous cold pretreatment. Seeds ofM. montanahave non-deep complex morphophysiological dormancy, C1b1aB-C1a. This is the first study on seeds with complex MPD to show a delay in shoot emergence following root emergence despite the shoot being physiologically non-dormant.

Reduced daylength stimulates size-dependent precocious maturity in 0+ male Atlantic salmon parr

To study relationships between fish length, photoperiod and the onset of precocious maturity in 0+ parr during intensive rearing of Atlantic salmon (Salmo salar L.), 0+ juveniles of 55–89mm length were individually tagged with Passive Induced Transponders (PITs) in May and exposed to reduced daylength (LD12:12) in May, June or July, or kept as control fish under continuous light (LD24:0). Relationships between the length of the fish and maturity were studied by the use of probabilistic maturation reaction norms. The incidence of mature males and the proportion of fish of lower modal group size in autumn were highest in the groups exposed to short days in May, and lowest under continuous light. In contrast with the expectation that high growth rates promote maturation, the future mature male parr were smaller than the immature males at the start of the experiment, and they also grew more slowly during the subsequent maturation process. Variability in condition factor was low until autumn, when the condition factor of maturing males rose well above that of immature males during autumn as the fish grew from ~80 to ~95mm length. Reduced daylength increased the probability of the onset of precocious maturity in the fish smaller than, but not above a threshold length of ~90–100mm (9 – 11g). Intensively reared parr of 60–90mm, and possibly also smaller fish, may be particularly sensitive to photoperiod manipulations that may influence the probabilities of fish adopting one of the three life-history alternatives, to enter lower or upper mode or to mature precociously.

Space use patterns of mountain hare (Lepus timidus) on the Alps

Populations on the limits of species’ distribution can show different behavioral adaptations to strong ecological pressure than in the central part of the range. We investigated space use patterns of alpine mountain hare (Lepus timidus) at two areas on the southern edge of the species’ range. We monitored 34 hares between 2005 and 2008, estimating home range size, overlap, and site fidelity, and compared our results with space use in Scottish and North-European populations. Home ranges of mountain hares did not differ between two study areas with different habitat types. Subadult animals used larger ranges than adults and both age groups reduced home range size in autumn, a period that might be critical for hares due to changes in diet and/or high energy expenditure during the previous breeding season. Home ranges in these alpine populations were smaller than in Scandinavian populations but within the range of populations in different habitat types in Scotland. Seasonal home ranges overlapped considerably, but differed among the sexes: male–female overlap was higher than same sex (male–male and female–female) spatial overlap. Seasonal shifts of home ranges were small, and site fidelity remained high over the seasons, suggesting that resource distribution remained constant throughout the year and that the knowledge of an intensively frequented area is an important element of habitat quality. We concluded that habitat structure and availability of mates interact in affecting mountain hare space use in alpine habitats.

Influence of growth history on the accumulation of energy reserves and winter mortality in young fish

In seasonal environments, accumulated energy reserves are important to avoid starvation mortality during periods of low resource levels. Here we investigated patterns of energy accumulation and the importance of growth history for winter survival in young-of-the-year Eurasian perch ( Perca fluviatilis ). Under simulated winter conditions in aquaria, we showed that high winter mortality most likely relates to the depletion of energy reserves in small perch. Correspondingly, in a field study, using four lakes covering 3–6 lake-years each, overwinter survival within cohorts was positively related to individual size. However, average size in autumn did not explain the variation in overwinter survival between cohorts. Instead, we showed that seasonal growth history is an important factor. High growth rates late in season may increase cohort survival over winter, irrespective of average size, related to a positive growth-dependent increase in allocation to energy reserves when approaching winter. Mechanisms regulating within-season temporal dynamics of growth rates are therefore suggested to be important for overall cohort performance.

Density‐dependent growth and survival in a natural population of the facultative biennial Digitalis purpurea

Summary 1 Density-dependent effects on vital rates may vary in both magnitude and direction at different stages of the life cycle. In monocarpic perennials, however, it is often assumed that recruitment is the stage most affected by density. 2 The spatial pattern of newly emerged individuals of the facultative biennial Digitalis purpurea was recorded and followed in five 0.5-m 2 plots censused twice during each of three seasons. 3 To examine effects of local density on growth and survival, a plant’s neighbourhood was defined as the number of individuals inside a circle of fixed radius around it. Species identity of the nearest neighbour was established to see whether the effects of intra- and interspecific interactions differed. 4 Relative growth rate (RGR) was negatively related to local density during both of the first two summers, but not during winter. In 1998 the negative effects were stronger when the closest neighbour was conspecific. 5 Contrasting patterns of size-dependent growth were found during summer and winter, with summer RGR an increasing, and winter RGR a decreasing, function of size. 6 Summer survival was generally high, and was positively related to spring size. There was a significant negative effect of increasing local density on survival probability in 1998. No effect of neighbour identity was detected. 7 Winter survival was low, and mainly dependent on autumn size, with no significant effects of local density in any season. During the first winter, individuals with a conspecific as the nearest neighbour had lower survival probabilities. 8 There was no evidence of a tradeoff between summer RGR and survival probability. 9 Density-dependent effects may be significant beyond the recruitment stage in monocarpic perennials such as D. purpurea . Density dependence was strongest at early life stages although, because effects on growth persisted into the second year, it is likely that local density may influence timing of reproduction in this population. The use of detailed studies investigating the timing and magnitude of density dependence across the entire life cycle may provide new insight into life history evolution.

Interactions between population processes in a cyclic species: parasites reduce autumn territorial behaviour of male red grouse

The causes of population cycles fascinate and perplex ecologist. Most work have focused on single processes, whether extrinsic or intrinsic, more rarely on how different processes might interact to cause or mould the unstable population dynamics. In red grouse (Lagopus lagopus scoticus), two causal mechanisms have been supported: territorial behaviour (changes in autumn aggressiveness) and parasites (parasite induced reduction in fecundity). Here, we report on how these two regulatory processes might interact, by testing whether the parasite suspected to cause the grouse cycles, the nematode Trichostrongylus tenuis, reduces male autumn territorial behaviour. We either treated males with an anthelmintic, to remove parasites (dosed or D-males), or challenged them with infective T. tenuis larvae, to increase parasite intensity (challenged or C-males). We first show that dosing was effective in removing T. tenuis parasites, while parasite intensities increased in challenged birds during the autumn. Because old males initially had more parasites than young males, the treatments generated greater differences in parasite intensity in old than in young males. We also show that various aspects of territorial behaviour (increase in testosterone-dependent comb size in autumn, territorial call rate, likelihood of winning territorial interactions and over-winter survival) were significantly higher in dosed than in challenged males, but in old birds only. Our data thus supported the hypothesis that parasites reduce male aggressiveness during the autumn territorial contests, and could thereby influence recruitment. Our results also highlight that the territorial behaviour of young males, which have fewer parasites, is not as limited by parasites as that of old, previously territorial males. We discuss the implications of these findings for our understanding of the processes regulating red grouse populations and causing their complex, unstable population dynamics.

PEOPLE

Short-term bioassays were conducted in Biguglia lagoon (Corsica) to study the physiological and behavioral responses of phytoplankton to N- and P-availability. Natural communities were collected in two stations representative of the two sub-basins, at three periods with contrasting environmental characteristics to address the impact of seasonal variability. These samples were separately enriched with a full N and P enrichment, and with enrichments minus N or minus P. Phytoplankton size structuration, diversity, and growth of the total phytoplankton, the micro-, nano- and ultraphytoplankton were evaluated using spectrofluorimetry, and optical microscopy. Results showed that the communities were fueled by NO3− in the wet periods (autumn and spring) and NH4+ in summer. The phytoplankton communities displayed highest cell size in autumn, with high abundances of nanoflagellates, and smallest cell size in summer with a large dominance of phycocyanin-rich picocyanobacteria. Blooms of dinoflagellates also occurred during the wet periods, coinciding with high N:P ratios. The full enrichment has not stimulated phytoplankton growth in autumn, suggesting the importance of other controlling factors such as light, a possible NH4+ inhibition or the use of mixotrophic abilities. In spring, communities have displayed single P-limitation in the northern basin and different N and P co-limitations in the southern basin. In summer, the full enrichment consistently stimulated the growth of all cell sizes. The communities showed high N and P co-limitations, which is consistent with growing observations in aquatic ecosystems, and reflects the different functional responses of phytoplankton communities to the nutrient availability.

Moose Alces alces hunting in Finland ‐ an ecological risk analysis

The moose Alces alces population in Finland has been managed for sustained harvest since 1970 by regulating annual hunting quotas. However, against all expectations the population size declined in the 1990s. An ecological risk analysis approach was used to build a growth model with annual harvest for the moose population. In the model there is stochasticity in the parameters representing population dynamics. We shall address: 1) whether the population decline could be due to a mismatch between harvest and anticipated population growth rate, and 2) to what extent hunting the moose population down to a much lower target size succeeds. A central element in this is the assumption that the estimate of the pre‐hunting population size errs. First, the probability of a population decline due to hunting increases from values close to 0% up to 100% in a very narrow range (15–25%) of harvest rates. Even with high birth rates the risk of a population decline was substantial when the hunting rate exceeded 25% for cows and 37.5% for calves and bulls. The 1974–1994 moose harvest rate was, on average, ca 45% of the population size in autumn. The high rate suggests that the harvest might have been too intense in that period to keep the population stable. Second, we set the target to reduce the moose population drastically (to say 50% of the existing population size). Assuming that the estimates of the population size may err, our analysis shows that the achieved population size after the severe harvest is far below the size we aimed at.

Contest behavior in territorial male butterflies: does size matter?

Although intrasexual contests generally favor bigger or stronger individuals, the relevance of body size to war of attrition-type disputes between weaponless animals such as butterflies is unclear. In this study I aimed to investigate the significance of size in this context by studying territorial contests in Hypolimnas bolina (L.), a species that exhibits consistent seasonal plasticity in body size. In this species adult age is positively correlated with large size in spring but with small size in autumn. This shift allowed independent evaluation of the relevance of each variable (size and age) to intrasexual contest success. Observation of a population of marked individuals indicated that only age appeared important, with the winners of pair-wise contests significantly older than losers in both seasons, and with contests lasting longer when the age difference between the combatants was small. Age was also linked to residency; residents won 99% of all contests. This study suggests that size does not matter in these aerial disputes, but age and residency do. It is not yet possible to determine whether older butterflies are intrinsically better competitors, or whether they simply have greater opportunity to find a vacant territory.

Population structure change of Salpa thompsoni from austral mid-summer to autumn

Changes in the population structure of Salpa thompsoni from austral mid-summer to autumn off the Adelie Coast, Antarctica, were examined using cluster analysis based on reproductive states. S. thompsoni was the dominant macrozooplankton species through the research periods although the abundance decreased markedly in autumn. In both summer and autumn we identified three geographically coherent subpopulations, which were considered to be either a mating swarm or physical aggregation. Many aggregate forms with an empty (dead) embryo were observed, and the ratio of newborn solitary forms decreased in autumn, indicating sexual reproduction of the salps was deactivated. The significant reduction in size of mature solitaries suggested that the capacity of asexual reproduction was also reduced in autumn. In addition to water temperature decline, the phytoplankton shortage (which was partly caused by high grazing pressure of S. thompsoni during summer) might be the factor causing decline in their own population size in autumn.

Grazing effects of two freshwater snails on juvenile Fucus vesiculosus in the Baltic Sea

Fucus vesiculosus and Fucus serratus are the only large, structuring perennial brown macroalgae in the low salinity waters of the Swedish coast of the Baltic Proper. Since the 1970s there are reports of declining Fucus vesiculosus stands from several locations around the Baltic Proper, but also indications of recoveries e.g. in Finland, in the Asko area of the northern Baltic proper and in the south east coast of Sweden. There are probably several causes for these changes. This thesis focuses on how, among biological and other factors, grazing may be the proximate factor to structure the rocky phytal zone of the Baltic Sea. Destructive effects on F. vesiculosus may be connected to extreme densities of grazers. In a field survey large densities of the potential mesograzer Idotea baltica correlated positively with grazing injuries on and reduced depth penetration of Fucus vesiculosus. In a grazing experiment biomass of F. vesiculosus was halved within two weeks at a grazer density found in the field (80 I. baltica per 100 g of Fucus wet weight). In a second survey large numbers of the gastropods Theodoxus fluviatilis and Lymnaea peregra coincided with unsuccessful recruitment of F. vesiculosus. In an experiment these gastropods grazed germlings of F. vesiculosus up to sizes of 0.8 and 1.0 mm respectively. Abundances of gastropods were highest in autumn, indicating that autumn reproductive F. vesiculosus might be more influenced than spring reproductive F. vesiculosus as the latter would have surpassed the critical size in autumn. Grazing may be modified by environmental factors. Comparatively small densities of Theodoxus fluviatilis correlated positively with abundances of Fucus vesiculosus and negatively with abundances of filamentous algae. Grazing effects advantageous to F. vesiculosus were tested in a field experiment with manipulation of grazers, nutrients and propagules of filamentous algae. At low nutrient loads grazers tended to clear the substrate from filamentous algae and strengthen the competitive ability of F. vesiculosus. At high nutrient loads the grazers could not prevent dominance of filamentous algae with detrimental effects to F. vesiculosus. Physical factors like wave-action may also modify grazing effects. In a wave-exposed coastline with mixed stands of perennial brown algae and frequent occurrences of grazing isopods, F. vesiculosus declined faster than F. serratus. In an experiment F. vesiculosus was more heavily grazed than F. serratus, but only at intense water motion. Thus F. serratus seem to have a competitive advantage to F. vesiculosus in wave exposed coasts where severe grazing occurs. The fact that the grazer I. baltica often has been connected with F. vesiculosus, might indicate that this animal would be dependent on F. vesiculosus as habitat and/or food. In a wave-exposed habitat on the eastern coast of Oland, probably earlier dominated by F. vesiculosus but now holding discrete patches of either red algae {Polysiphonia fucoides) or saw-wrack (F. serratus) I. baltica used both species as habitat and food. In addition to the two macroalgae faecal pellets contained 30-40 % microalgae, indicating that mixed diets may be important for this species. In monitoring programs of rocky bottom communities fixed sites may be revisited on a yearly bases to estimate the development of the community. There is an underlying assumption that the observations at a fixed site are representative to surrounding areas. This assumption was tested along a 350 km coastline in SE Sweden. 18 years of observations were evaluated and compared to complementary sites. Within areas of similar wave exposure there was good agreement between fixed sites and surrounding areas. When all fixed sites within wave-protected areas were analysed together for long-term trends, there was an agreement in development between sites over the whole geographic area. The Fucus community increased its distribution during the 1980s, decreased during the early ]990s (probably from grazing as described above), to increase again in 2000-2001, that is an oscillating pattern is suggested. In wave-exposed sites, along a 100 km coastline, the F. vesiculosus stands were destroyed, seemingly by grazing as described above, around 1992-1994, with no signs of recovery as late as in 2002. The results are discussed in the context of grazing, regulation of grazing by predation, eutrophication and physical factors. The importance of the lack of fish predators is suggested for further research.

Life cycles and food habits of mayflies and stoneflies from temporary streams in western Oregon

SUMMARY1. Field data and results from laboratory rearing are combined to describe life cycles and food habits of the mayflies Paraleptophlebia gregalis and Ameletus n. sp., and the stoneflies Soyedina interrupts, Ostrocerca foersteri, Sweltsa fidelis and Calliperla luctuosa.2. P. gregalis, A. n. sp., S. interrupts and O. foersteri have univoltine life cycles which are characterized by a high degree of plasticity. S. fidelis and C. luctuosa have semivoltine life cycles which are more tightly synchronized.3. Laboratory feeding trials and field observations characterize P. gregalis as a collector, A. n. sp. as a scraper, O. foersteri and S. interrupta as shredders and C. luctuosa as a predator mainly of midge larvae. Late‐instar larvae of S. fidelis are believed to be scavengers.4. Laboratory rearing yielded a negative correlation between growth rates (Y) and larval size in autumn (X) for S. interrupta. This indicates compensatory growth by small larvae in order to achieve synchronized emergence. The correlation can be described by the equation: Y = 0.0053–0.0036X (R2= 0.82; P &lt; 0.01; n = 22)5. The field and laboratory data indicate that photoperiod mainly determines the rate of development and size of emerging subimagos in P. gregalis.

Role of spawning condition in the determination of the reproductive traits of spring- and autumn-spawning Atlantic herring from the southern Gulf of St. Lawrence

Mature spring-spawning (May–June) and autumn-spawning (August–September) herring differ in total weight. Autumn herring have both heavier gonads and higher somatic weights at length. Both the residual levels of storage lipids and somatic weight at length, corrected for storage lipid, are higher in autumn than in spring. Within spawning seasons, males and females deplete the same amount of storage energy. Percent somatic lipid content is positively correlated with body size during spring but shows no relationship to body size in autumn. These patterns indicate that the relative allocation of storage energy to reproduction differs with spawning season. Spring spawners (lengthy gonad maturation period) allocate proportionally more storage energy to routine and active metabolism than autumn spawners (brief gonad maturation period). The low intercept and steep slope of the gonad weight – length relationship for spring spawners reflect the interaction between decreasing metabolic rate with body size and increasing storage energy capacity with body size. Egg weight is unrelated to the parent's somatic lipid content for either spawning group. Spawning history (recruit versus repeat spawning) has only a minor influence on egg weight.

SEASONAL CHANGE IN THE DISTRIBUTION OF CELL SIZE OF COCCONEIS SCUTELLUM VAR. ORNATA (BACILLARIOPHYCEAE) IN RELATION TO GROWTH AND SEXUAL REPRODUCTION1

ABSTRACTGrowth and sexual reproduction of the marine littoral diatom Cocconeis scutellum Ehrenb. var. ornata Grun. were investigated at 30 different combinations of temperature (5, 10, 14, 18, 22° C), irradiance (20, 60, 100 μE·m−2·s−1) and daylength (14:10 and 10:14 h LD cycle). Growth occurred at all combinations. The optimal growth was observed at 14–18° C, long daylength and highest‐to‐moderate irradiance, and at 18° C, short daylength and highest irradiance. Sexual reproduction on the other hand occurred between 5 and 18° C, and the optimal condition was 10–14° C and short daylength.Annual cyclic, and sesonal changes in the distribution of cell size (valve length) were observed in a field population. These changes were characterized by an annual minimum in mean cell size in autumn, an annual maximum in winter, a slight decrease from the mean in spring–middle summer, a rapid decrease from the mean in late summer–early autumn, and appearance of bimodal distribution of cell size in winter. These changes were caused by sexual reproduction in autumn, rapid growth in late summer–early autumn and slow growth in other seasons, and poor viability of small cells near the lower end of the size range.

Adaptation of Drosophila simulans in Japan.

Drosophila simulans has been found for more than 10 years in Japan, mainly in suburban areas whereas its sibling species D, melanogaster tends to be found in houses. Seasonal activities of the two species were clearly different; In a typical hot summer the frequency of D. simulans decreased temporarily in August making two peaks in July and September. But in a cool summer the two peaks disappeared making a single peak in September or October. On the other hand, D. melanogaster showed higher activity in August in the hot summer but it occasionally shifted the active peak during the season avoiding competition with D. simulans. A nine year survey of D. simulans around the area of Mt. Fuji revealed that the species gradually invaded from the Pacific coast to the inland. Population size in autumn was strongly affected by the temperature of the Preceeding winter. Cold winters such as in 1977 and 1981 decreased the size drastically at every collection site.