Seasonal variability in the palatability of freshwater macrophytes: a case study

The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.     

Does habitat-specific variation in trematode infection risks influence habitat distribution of two closely related freshwater snails?

Parasitism may be an important factor determining the geographic distribution of closely related species. A habitat-specific risk of parasitism may lead to exclusion of susceptible host types from parasite-rich environments, and promote speciation if it leads to reproductive isolation between susceptible and resistant types. We surveyed populations of the freshwater snail Lymnaea peregra for differences in habitat distribution and trematode parasitism between its two distinct shell morphs, L. ovata and L. peregra. We surveyed 58 populations (43 L. ovata, 15 L. peregra). At each location we recorded an array of habitat characteristics that were summarized using a nonlinear principal components analysis. This yielded two orthogonal habitat score variables. Discriminant analysis with these habitat dimensions indicated that the snail morphs differed in their habitat distribution. L. ovata preferred larger, more permanent natural habitats surrounded by forests, while L. peregra was found more often at a higher altitude, in nonpermanent habitats, often surrounded by meadows. The snails were parasitized by four cercarial types of castrating trematodes. The morphs had a similar prevalence of infection by each of the parasite types, with one exception: monostomid cercariae were found at a higher prevalence in L. ovata than in L. peregra. However, monostomes were rare parasites, and the difference in prevalence of infection was not significant when only populations with monostomes were compared. Our results indicate that variation in the overall prevalence of infection seems to be independent of snail morph, and do not support the idea that a difference in the rate of parasitism might explain differences in the habitat distribution of these snail morphs. Received: 4 January 1999 / Accepted: 30 June 1999     

The impact of the 2003–2008 weather variability on intra-annual stem diameter changes of beech trees at a submontane site in central Slovakia

The link between the dynamics of beech (Fagus sylvatica L.) diameter changes, recorded by mechanical dendrometers, and the weather activity was studied at a submontane site in central Slovakia between 2003 and 2008. A new approach was presented by constructing seasonal increment chronologies, dividing them into shorter periods and joining them across several seasons to analyse the main influencing factors. For this purpose, response functions and correlation functions and their intra-seasonal moving variants were calculated. The photoperiod appeared to be synchronising the culmination of the seasonal dynamics of diameter changes; however, the main factors influencing the seasonal dynamics were weather variables. At the beginning of the season, the diameter increase was significantly correlated with temperature and its accumulation over a longer period. In summer, the diameter increase was hampered by long-term heat waves and the positive influence of precipitation became more pronounced. Precipitation towards the end of summer and in autumn significantly positively impacted the variability of the dendrometer signals. In 2003, the unusual seasonal dynamics were caused by a hot May and a hot and dry summer.In view of the expected climate change, including warmer springs and a higher frequency of summer droughts and heat waves, we expect a shift in the culmination of beech seasonal diameter increase towards May. Intensive drought and heat periods during summer will probably stress the beech populations growing close to their lower altitudinal limit.     

Seasonal and ontogenetic variations in resource use by two sympatric Arctic charr morphs

The study compares the resource utilization of two sympatric Arctic charr morphs over an annual period in a subarctic lake. The two morphs are reproductively isolated in time and place of spawning, and are referred to as the littoral and profundal morphs (L-morph and P-morph) according to their spawning habitats. Fish were sampled monthly (ice-free season) or bimonthly (winter) using gillnets in the main lake habitats. The spatial range of the P-morph was restricted to the profundal zone throughout the whole annual period. The L-morph in contrast utilized all main habitats, exhibiting distinct seasonal and ontogenetic variations in habitat distribution. In the spring, the whole L-morph population was located along the bottom profile of the lake, in profundal and littoral habitats. During summer and autumn, habitat segregation occurred between different life-stages, juveniles mainly utilizing the profundal, pre-adults the pelagic and adult fishes the littoral zone. During winter the whole population was assembled in the littoral habitat. The L-morph also had large seasonal and ontogenetic variations in their feeding ecology, with littoral zoobenthos, zooplankton and surface insects being important prey. The P-morph had a narrower diet niche mainly consisting of chironomid larvae and other profundal zoobenthos. Hence, the two Arctic charr morphs exhibited a consistent resource differentiation during all annual seasons and throughout their life cycles, except for a dietary overlap between P-morph and juvenile L-morph charr in the profundal during summer. The findings are discussed in relation to resource polymorphism and incipient speciation.     

Evidence for divergent natural selection of a Lake Tanganyika cichlid inferred from repeated radiations in body size

Divergent natural selection is thought to play a vital role in speciation, but clear, measurable examples from nature are still few. Among the many possible sources of divergent natural selection, predation pressure may be important because predators are ubiquitous in food webs. Here, we show evidence for divergent natural selection in a Lake Tanganyika cichlid, Telmatochromis temporalis , which uses burrows under stones or empty snail shells as shelters. This species contains normal and dwarf morphs at several localities. The normal morph inhabits rocky shorelines, whereas the dwarf morph invariably inhabits shell beds, where empty snail shells densely cover the lake bottom. Genetic evidence suggested that the dwarf morph evolved independently from the normal morph at two areas, and morphological analysis and evaluation of habitat structure revealed that the body sizes of morphs closely matched the available shelter sizes in their habitats. These findings suggest that the two morphs repeatedly evolved through divergent natural selection associated with the strategy for sheltering from predators.     

Spatial density patterns of herbivore response to seasonal dynamics in the tropical deciduous forest of central India

Resource dynamism in seasonal forests leads ungulates to differential habitat preference; hence, their distribution aligns with environmental covariates across spatial and temporal scales. Seasonal patterns of four species of ungulates, namely sambar, chital, nilgai, and wild pig, were investigated and identified as the environmental variables driving the density gradient across two seasons, summer and winter, in the tropical dry deciduous forest of Panna Tiger Reserve, central India. Distance sampling data were analyzed using density surface modeling for ungulates with a survey effort of 518 km in winter and 356 km in summer in a generalized additive modeling framework. We found that season significantly affected the spatial densities of all ungulates, with sambar, chital, and nilgai congregating in summer and wild pig in winter. All ungulates showed a clear seasonal shift to the valley in summer and preferred plateaus in winter. The spatially explicit map outputs draw attention to the seasonal hot spots for ungulates abundance and the species and season-specific roles of environment variables in defining their distribution. These results provide a scientific basis for direct conservation efforts to the spatially prioritized habitats for cost-effective management interventions.     

Activity and weight loss in relation to solar radiation in the polymorphic land snail Cepaea nemoralis

Shell spectral reflectance, activity and weight loss in the land snail Cepaea nemoralis were examined to test whether solar radiation has different effects on the behavioural and physiological responses of different morphs. A uniform area from the empty shells of different morphs was cut and a spectrophotometer was used to measure the spectral reflectance at wavelengths from 220 nm to 800 nm. Snails were put into cages made of mesh hardware cloth, and their activities were observed from dawn onwards on two hot, dry days. Weight loss of snails under shade and under direct sunlight was also measured.
The results show that unbanded morphs reflect more light than five-banded ones, but the yellow morphs reflect only a little more than pink morphs. Five-banded morphs became inactive more quickly than unbanded ones from dawn onwards, but the differences in activity patterns were not significant between pink and yellow morphs. There are significant differences in percent weight loss between snails placed under direct sunlight and under shade. The weight loss of different morphs under shade was not statistically different. But when exposed to sunlight, five-banded morphs lost more weight than pink and yellow unbanded morphs.
Because of their lower reflectance of solar radiation, the five-banded morphs are, in a hot, arid climate, at a selective disadvantage compared to unbanded morphs. However, they can avoid such selective disadvantage by actively seeking a shaded area, as all other morphs do in the field. Such behavioural habitat selection could help the maintenance of genetic polymorphisms in natural populations.     

Analysis of polymorphic variation in different types of habitat

A correlation between the distribution of an organism and features of its environment can be taken as indirect evidence of natural selection. Biologists may therefore collect samples from polymorphic populations at a number of locations, classify the locations into habitat types, and consider whether the distribution of morphs varies with the habitat. Statistical aspects of this type of study are discussed in this paper. A randomization test for habitat effects is proposed and a negative binomial model is suggested for the distribution of morphs from random locations within one type of habitat. Data on the distribution of Cepaea hortensis and C. nemoralis snails in southern England provide an example. For both species there is clear evidence of differences between habitats, although the morph distributions are rather variable within habitats. The negative binomial model suggests that, for the snail data, variation in morph proportions is mainly due to location differences. The binomial sampling error is relatively unimportant unless the sample size at a location is very small. Therefore it is reasonable to analyse morph proportions by standard methods without giving different weights to data from different locations. The snail data are analysed in this way. Discriminant function analyses are used to test for habitat effects. The relationships between C. hortensis and C. nemoralis morph frequencies within one habitat are examined by a canonical correlation analysis.     

Habitats of relict terrestrial snails in southern Siberia: lessons for the reconstruction of palaeoenvironments of full‐glacial Europe

Aim Shells of fossil molluscs are important for palaeoecological reconstructions. However, the habitat requirements of snail species typical of central European full‐glacial loess sediments are poorly known because most of them became very rare or extinct in Europe. The recent discovery of an almost complete extant assemblage of such species in mountainous regions of central Asia enables more precise characterization of their habitats, which may significantly improve reconstructions of Pleistocene environments. Location Altai Mountains, Russia. Methods Terrestrial snail assemblages, vegetation composition and selected environmental variables were recorded at 118 sites along a gradient of climatic continentality in the Russian Altai. Habitat characteristics of sites where species typical of the full‐glacial period occurred were described using a classification tree. Results Seven of the eight species that are typical of central European full‐glacial loess sediments were found in the study area. They were confined to cool areas with January mean temperatures below ?17 °C, but occurred mainly in sheltered habitats with a warmer microclimate, such as scrub or open woodland. Pupilla loessica and Vallonia tenuilabris had the broadest habitat range, occurring from woodland to dry steppe. Unexpectedly, Columella columella, Pupilla alpicola, Vertigo genesii, V. parcedentata and V. pseudosubstriata were found mainly in wooded fens and shrubby tundra rather than in open steppe. Most of these seven species were recorded in base‐rich wooded fens. Very dry open steppe habitats usually supported no snails. Main conclusions Habitat ranges of the studied snails in the Altai indicate that the full‐glacial landscapes of central European lowlands that harboured these species were not completely dominated by open and dry loess steppe. Most probably they contained a significant component of shrubby vegetation, patches of wet habitats, and probably also areas of woodland at sites with a favourable mesoclimate.     

The evolution of phally polymorphism in Bulinus truncatus (Gastropoda,Planorbidae): the cost of male function analysed through life-history traits and sex allocation

In the hermaphrodite freshwater snail Bulinus truncatus, two sexual morphs, euphallic (regular hermaphrodites) and aphallic individuals without a male copulatory organ, co-occur at various ratios in natural populations. Both aphallic and euphallic individuals can reproduce by selfing, but when outcrossing aphallic individuals can only play the female role. A comparison of life-history traits and sex allocation in these two forms provides the opportunity to investigate the evolution and maintenance of sexual polymorphisms. This study was performed to test whether a reallocation of resources from the lost male function to the female function occurs in aphallic snails at the level of both sex organs (sex allocation) and life-history traits. In a first experiment we compared life-history traits over a whole life-cycle under selfing between the two sexual morphs. In a second experiment, the sex organs were weighed to test for a difference in sex allocation between the two morphs. No difference in resource allocation to female function between the two morphs was observed in either experiment. This is in contrast to patterns frequently observed in sexually polymorphic plants, and in a previous study performed on aphally in the same snail species. We discuss the genetic and physiological hypotheses that could explain these results, and their consequences for the evolution and maintenance of phally polymorphism in B. truncatus.     

Waterfowl of a subtropical African floodplain

Many water bodies in arid southern Africa are seasonal and waterfowl habitats become restricted during the dry winters. Nevertheless, some tropical species, such as the white-faced duck, are expanding their ranges into more temperate regions where summer habitats are available, thus increasing pressure on overwintering habitats. The subtropical Pongolo River floodplain is an important overwintering area for a diverse water-fowl community that includes 13 of the 16 species that breed in South Africa and one of the few regular migrants. Historical evidence suggests that numbers of white-faced duck, which represent over 70% of the floodplain community, have increased 10-fold over the last two decades. An abundant, nutritious food (Potamogeton crispus turions) attracts waterfowl to the Pongolo floodplain during winter and an increase in bird fat reserves suggests that they leave for summer breeding grounds in good condition. Unseasonal flood releases from an upstream reservoir may cause low food supplies in late spring before the onset of the summer rainy season when breeding habitats become available. This study emphasizes the need for a better understanding of the role of winter habitats in maintaining waterfowl populations.     

Histochemical Localization of Phenolic Deposits in Leaf Blades of Eragrostis racemosa

The distribution of phenolic deposits in the leaves of Eragrostisracemosa from different habitats was investigated during cultivationunder uniform environmental conditions. The plants retainedtheir leaf phenolic distribution pattern over a period of 8months. Plants from dry habitats had phenolic inclusions inboth epidermal layers, whereas phenolic deposits were absentor only occurred in the abaxial epidermal layers of plants occupyingmoist habitats. Eragrostis racemosa, narrow-heart love grass, Poaceae, phenolic deposits, epidermis, histochemistry, anatomy     

Using environmental effects on fecundity to compare the adaptive characteristics of the morphs in a hybrid zone of Littorina saxatilis

We studied the role of ecological processes in the maintenance of a hybrid zone for the intertidal snail Littorina saxatilis by comparing the fecundity responses of the females from different morphs to local environmental variation. This procedure makes it possible to discern if the environments maximizing fecundity are the same in these morphs, and therefore to study adaptive differences between them. We found that the pure morphs were adapted to their respective shore levels, thus confirming that this hybrid zone is associated with the occurrence of an ecotone between two habitats. However, fecundity differences did not explain the precise spatial location and width of the hybrid zone. We also found that the adaptive characteristics of hybrids were very different from those of the pure morphs, in sharp contrast with any expectations based on their intermediate phenotype. The comparison of fitness responses to environmental variation might be an useful procedure in the field study of adaptive differences between groups of individuals.     

Feeding activity and dietary composition of roe deer at the southern edge of their range

We studied feeding activity and dietary components of hand-reared European roe deer (Capreolus capreolus) in Israel. Our ultimate goal was to assess habitat suitability for future reintroduction of the species, which has been locally extinct for nearly a century. Activity patterns, diet composition, and body mass of four does were monitored in two (fenced) typical east Mediterranean habitats: mature forest and scrubland recovering from fire. Food supplements were provided between trials. Throughout the year, the deer exhibited diurnal and nocturnal activity, mostly at dawn and dusk. Diet composition varied considerably between seasons and habitats, demonstrating the opportunistic flexibility of the deer. In both habitats, the deer fed on over 85% of the plant species but preferred a particular plant species or parts. In summer and early autumn, fruits and seeds became the dominant portion of their diet. In our semi-natural experimental setup, deer maintained body mass through the winter and spring. Weight loss occurred as the dry season advanced, but the animals rapidly regained mass when annuals and grasses became available following the first rains. In the east Mediterranean habitats, water availability seems more problematic for deer survival than food availability.     

Global changes and animal phenotypic responses: melanin-based plumage redness of scops owls increased with temperature and rainfall during the last century

The ecological effects of global climate changes include shifts of species'' distribution and changes in migration strategies and phenotype. Colour polymorphism, which can be envisaged as a species'' evolutionary response to alternating conditions or to a wide range of habitats, may be affected by climate changes as well. The scops owl (Otus scops) shows two main colour morphs, dark- and pale-reddish, as well as intermediate morphs. We investigated temporal trends in an index of plumage colour of Italian scops owls from museum collections (1870–2007). We found a significant increase in plumage redness over the last century, which was correlated with an increase in temperature and rainfall of the years before specimen collection. However, the temporal increase in plumage redness persisted after controlling for climatic variables, suggesting that other environmental factors could be involved. Our study indicates that ongoing climate changes might have either shifted the selective balance between colour morphs, or differentially affected migration and movement patterns of colour morphs.     

The control of Galba truncatula (Gastropoda: Lymnaeidae) by the terrestrial snail Zonitoides nitidus on acid soils

Field investigations on the control of Galba truncatula by Zonitoides nitidus were carried out over the past 30 years in the different types of lymnaeid habitats located in central France. When a layer of mowed vegetation was used to cover G. truncatula habitats at the end of June, the introduction of adult Z. nitidus (20/m²) eliminated lymnaeid populations after 2 years of control in habitats located in swampy meadows, around the heads of intermittent springs, in areas trampled by cattle, and along river or pond banks. In the case of wild watercress beds, 3 years were necessary. The best results (elimination of G. truncatula after a single year of control) were obtained using an association of snails (Z. nitidus + Oxychilus draparnaudi), or a mixed control (a first application of 0.1 mg/l CuCl₂, followed 3 months later by the introduction of Z. nitidus). Recolonization of treated habitats by G. truncatula coming from downstream populations was noted 3 years following the last application of biological control. Apart from the regular introduction of Z. nitidus in several watercress beds, the use of this snail to control G. truncatula has not become generalized in cattle- and sheep-breeding farms. The reasons for this situation are probably the complexity of applying this control in the field by nonspecialists and the difficulty of selecting the period of snail control at the end of June due to frequent local rainfall at this time.     

Disruptive selection on female reproductive characters in a hybrid zone of Littorina saxatilis

We studied natural selection in a hybrid zone of the intertidal marine snail Littorina saxatilis located in Galicia (NW Spain) by measuring the number and average size of the embryos carried by females. We related these characters with the females' position on the phenotypic and environmental gradients between the two pure morphs of the hybrid zone. In contrast with previous interpretations of studies made in this hybrid zone, we found a depression in both embryo number and size for phenotypically intermediate females. This disruptive natural selection could play an important role in the maintenance of the population's bimodal phenotypic distribution and in that of the assortative mating between the pure morphs. We found also that intermediate environments tended to be unfavorable for all phenotypes. Although the precise causes for the found depression in female reproductive characters remain to be determined, these results serve to emphasize the importance of studying whole fitness surfaces in hybrid zones, across the complete phenotypic and environmental ranges, instead of merely comparing the fitness averages of the two pure morphs and the intermediate individuals, taken as three discrete phenotypic classes.     

Response of the dung beetle community to different climatic zones: Does the land use system matter?

The conversion of Brazilian savanna into exotic pastures leads to the loss of dung beetles and a decrease in their contribution to ecological functions. We hypothesized that the dung beetle communities from exotic pastures would show greater significant differences between climatic zones, when contrasted to communities from Brazilian savanna in the same region, since dung beetle assemblages in pastures are more simplified. We assessed which variables (purpose of production, type of management, percentage the habitat per buffer, soil penetration resistance, pasture area and herd size) affect more the dung beetle community in exotic pastures. We carried out this study in 48 areas of native Brazilian savannas and exotic pastures distributed across four bioclimatic zones: BZ1, hot with three dry months; BZ2, hot with 4–5 dry months; BZ3, sub-hot with 4–5 dry months and BZ4, meso-thermal with 4–5 dry months of Minas Gerais State, Brazil. In each BZ, six areas of Brazilian savannas and six areas of exotic pasture were selected. In the Brazilian savanna areas, the species richness, abundance and biomass of dung beetles did not differ between the bioclimatic zones, unlike the exotic pastures. The composition of the dung beetle community was different between land use systems and between bioclimatic zones; the interaction between the two factors was also significant. Our results provide evidence that dung beetle communities active in exotic pastures are more susceptible to climatic environmental variations than communities from more complex and stable habitats, such as savannas. Finally, the best model suggested that all the six variables combined explained about 91% of the total variability in species composition observed between sampling sites.     

Responses to drought stress among sex morphs of Oxyria sinensis (Polygonaceae), a subdioecious perennial herb native to the East Himalayas

It is generally accepted that dioecious plants occur more frequently in dry and nutrient‐poor habitats, suggesting that abiotic stress factors could contribute to evolution of dioecy from hermaphrodite. Therefore, experimental investigations on the responses of subdioecious species, a special sexual system comprising male, female, and hermaphrodite plants, to abiotic stress factors could quantify the contribution of selective pressure on the evolution of dioecy. In this study, we evaluated the physiological responses of different sex morphs of Oxyria sinensis Hemsley, a perennial herb native to the East Himalayas, to drought stress. Male, female, and hermaphrodite plants of O. sinensis were subjected to low, moderate, and high drought stress conditions in a glasshouse. Generally, with increasing water stress, the values of most measured variables slightly decreased, whereas water‐use efficiency slightly increased. Furthermore, there were no significant differences in most of the measured parameters among the sex morphs under each drought stress treatment, indicating that O. sinensis might be well‐adapted to drought stress conditions as its typical habitat is the dry and hot habitats of xerothermic river valleys. However, nitrogen‐use efficiency was significantly higher in male and female plants than in hermaphrodite plants under high drought stress conditions, suggesting that that nitrogen‐use efficiency under conditions of drought stress might have contributed to the evolution of dioecy from the hermaphrodite to some degree.     

Availability,uptake and regeneration of phosphate in mesocosms with varied levels of P deficiency

A population of cyclomorphic Bosmina coregoni was studied in Lake Östersjön, southwestern Sweden and results from field samples collected in 1988, 1989, 1990 and 1991 are presented. Animals collected in summer have remarkably higher carapace and prolonged antennule compared to what we call the normal morph. In 1991 the extreme morph reach its maximum body length, body height and antennule length in July to September. The occurrence of the extreme morph coincide with the hatching of the predaceous cladoceran Leptodora kindtii.The two morphs fluctuate in abundance and in relation to each other. In early spring only the normal morph occurred in the samples followed by a period of about two months when the two morphs were found together, in July only the extreme morph was found. In September the two morphs were again present in the lake. As has been shown for other cladoceran, the conspicuous carapace and antennule could be an adaptive response that decreases mortality due to invertebrate predation. Spectacular features like these are likely also accompanied by some sort of costs.