Carry-over effects,sequential density dependence and the dynamics of populations in a seasonal environment

Most animal populations have distinct breeding and non-breeding periods, yet the implications of seasonality on population dynamics are not well understood. Here, we introduce an experimental model system to study the population dynamics of two important consequences of seasonality: sequential density dependence and carry-over effects (COEs). Using a replicated seasonal population of Drosophila, we placed individuals at four densities in the non-breeding season and then, among those that survived, placed them to breed at three different densities. We show that COEs arising from variation in non-breeding density negatively impacts individual performance by reducing per capita breeding output by 29–77%, implying that non-lethal COEs can have a strong influence on population abundance. We then parametrized a bi-seasonal population model from the experimental results, and show that both sequential density dependence and COEs can stabilize long-term population dynamics and that COEs can reduce population size at low intrinsic rates of growth. Our results have important implications for predicting the successful colonization of new habitats, and for understanding the long-term persistence of seasonal populations in a wide range of taxa, including migratory organisms.     

Simple signals indicate which period of the annual cycle drives declines in seasonal populations

For declining wild populations, a critical aspect of effective conservation is understanding when and where the causes of decline occur. The primary drivers of decline in migratory and seasonal populations can often be attributed to a specific period of the year. However, generic, broadly applicable indicators of these season‐specific drivers of population decline remain elusive. We used a multi‐generation experiment to investigate whether habitat loss in either the breeding or non‐breeding period generated distinct signatures of population decline. When breeding habitat was reduced, population size remained relatively stable for several generations, before declining precipitously. When non‐breeding habitat was reduced, between‐season variation in population counts increased relative to control populations, and non‐breeding population size declined steadily. Changes in seasonal vital rates and other indicators were predicted by the season in which habitat loss treatment occurred. Per capita reproductive output increased when non‐breeding habitat was reduced and decreased with breeding habitat reduction, whereas per capita non‐breeding survival showed the opposite trends. Our results reveal how simple signals inherent in counts and demographics of declining populations can indicate which period of the annual cycle is driving declines.     

Seasonal changes in testosterone and corticosterone levels in four social classes of a desert dwelling sociable rodent

Animals have to adjust their physiology to seasonal changes, in response to variation in food availability, social tactics and reproduction. I compared basal corticosterone and testosterone levels in free ranging striped mouse from a desert habitat, comparing between the sexes, breeding and philopatric non-breeding individuals, and between the breeding and the non-breeding season. I expected differences between breeders and non-breeders and between seasons with high and low food availability. Basal serum corticosterone was measured from 132 different individuals and serum testosterone from 176 different individuals of free living striped mice. Corticosterone and testosterone levels were independent of age, body weight and not influenced by carrying a transmitter. The levels of corticosterone and testosterone declined by approximately 50% from the breeding to the non-breeding season in breeding females as well as non-breeding males and females. In contrast, breeding males showed much lower corticosterone levels during the breeding season than all other classes, and were the only class that showed an increase of corticosterone from the breeding to the non-breeding season. As a result, breeding males had similar corticosterone levels as other social classes during the non-breeding season. During the breeding season, breeding males had much higher testosterone levels than other classes, which decreased significantly from the breeding to the non-breeding season. My results support the prediction that corticosterone decreases during periods of low food abundance. Variation in the pattern of hormonal secretion in striped mice might assist them to cope with seasonal changes in energy demand in a desert habitat.     

Non-invasive administration of 17β-estradiol rapidly increases aggressive behavior in non-breeding,but not breeding,male song sparrows

17β-Estradiol (E₂) acts in the brain via genomic and non-genomic mechanisms to influence physiology and behavior. There is seasonal plasticity in the mechanisms by which E₂ activates aggression, and non-genomic mechanisms appear to predominate during the non-breeding season. Male song sparrows (Melospiza melodia) display E₂-dependent territorial aggression throughout the year. Field studies show that song sparrow aggression during a territorial intrusion is similar in the non-breeding and breeding seasons, but aggression after an intrusion ends differs seasonally. Non-breeding males stop behaving aggressively within minutes whereas breeding males remain aggressive for hours. We hypothesize that this seasonal plasticity in the persistence of aggression relates to seasonal plasticity in E₂ signaling. We used a non-invasive route of E₂ administration to compare the non-genomic (within 20 min) effects of E₂ on aggressive behavior in captive non-breeding and breeding season males. E₂ rapidly increased barrier contacts (attacks) during an intrusion by 173% in non-breeding season males only. Given that these effects were observed within 20 min of E₂ administration, they likely occurred via a non-genomic mechanism of action. The present data, taken together with past work, suggest that environmental cues associated with the non-breeding season influence the molecular mechanisms through which E₂ influences behavior. In song sparrows, transient expression of aggressive behavior during the non-breeding season is highly adaptive: it minimizes energy expenditure and maximizes the amount of time available for foraging. In all, these data suggest the intriguing possibility that aggression in the non-breeding season may be activated by a non-genomic E₂ mechanism due to the fitness benefits associated with rapid and transient expression of aggression.     

Strong migratory connectivity and seasonally shifting isotopic niches in geographically separated populations of a long-distance migrating songbird

Whether migratory animals use similar resources during continental-scale movements that characterize their annual cycles is highly relevant to both individual performances and population dynamics. Direct knowledge of the locations and resources used by migrants during non-breeding is generally scarce. Our goal was to estimate migratory connectivity of a small Palaearctic long-distance migrant, the common nightingale Luscinia megarhynchos, and to compare resources used in non-breeding areas with resources used at the breeding grounds. We tracked individuals of three geographically separated populations and characterised their stable isotope niches during breeding and non-breeding over 2 years. Individuals spent the non-breeding period in population-specific clusters from west to central Africa, indicating strong migratory connectivity at the population level. Irrespective of origin, their isotopic niches were surprisingly similar within a particular period, although sites of residence were distant. However, niche characteristics differed markedly between breeding and non-breeding periods, indicating a consistent seasonal isotopic niche shift in the sampled populations. Although nightingales of distinct breeding populations migrated to different non-breeding areas, they chose similar foraging conditions within specific periods. However, nightingales clearly changed resource use between breeding and non-breeding periods, indicating adaptations to changes in food availability.     

Seasonal expressions of GPR41 and GPR43 in the colon of the wild ground squirrels (Spermophilus dauricus)

G-protein-coupled receptor 41 (GPR41) and G-protein-coupled receptor 43 (GPR43) are important short-chain fatty acids (SCFAs) receptors. Previous studies indicated that GPR41 and GPR43 are involved in the secretion of gastrointestinal peptides, and glucose and lipid metabolism, and are closely related to obesity and type II diabetes, and other diseases. The purpose of the study was to explore the relationship of the GPR41 and GPR43 with seasonal breeding, and provide new prospects for further exploring the nutritional needs of breeding. We identified the localization and expression levels of GPR41 and GPR43 in the colon of the wild ground squirrels (Spermophilus dauricus) both in the breeding season and non-breeding season. The histological results revealed that the lumen diameter of the colon had obvious seasonal changes, and the diameter of the colonic lumen in the non-breeding season was larger than that in the breeding season. Immunohistochemical staining suggested that GPR41 and GPR43 are expressed in the simple layer columnar epithelium. In addition, compared with the breeding season, the mRNA and protein expression levels of GPR41 and GPR43 in the colon were higher during the non-breeding season. In general, these results indicated that GPR41 and GPR43 might play a certain role in regulating seasonal breeding.Key words: GPR41, GPR43, colon, wild ground squirrel, seasonal breeding     

Population priorities: the challenge of continued rapid population growth

Rapid population growth continues in the least developed countries. The revisionist case that rapid population could be overcome by technology, that population density was advantageous, that capital shallowing is not a vital concern and that empirical investigations had not proved a correlation between high population growth and low per capita income was both empirically and theoretically flawed. In the modern world, population density does not play the role it did in nineteenth-century Europe and rates of growth in some of today''s least developed nations are four times than those in nineteenth-century Europe, and without major accumulation of capital per capita, no major economy has or is likely to make the low- to middle-income transition. Though not sufficient, capital accumulation for growth is absolutely essential to economic growth. While there are good reasons for objecting to the enforced nature of the Chinese one-child policy, we should not underestimate the positive impact which that policy has almost certainly had and will have over the next several decades on Chinese economic performance. And a valid reticence about telling developing countries that they must contain fertility should not lead us to underestimate the severely adverse impact of high fertility rates on the economic performance and prospects of many countries in Africa and the Middle East.     

Distribution of Pratylenchus scribneri between Root and Soil Habitats

The abundance of Pratylenchus scribneri in soil and root habitats was compared in potato and corn plots during 1986-88. Nematodes were extracted from 100-cm³ soil samples and the roots contained within the samples. The percentage of the population recovered from soil, similar among years and crops, averaged ca. 50% at the beginning and end of the growing season and ca. 20% from early to late season. Proportionately more adults and fourth-stage juveniles than younger stages were located outside roots until harvest. In a related study, nematodes were isolated from the roots, root surfaces, and soil associated with roots of whole corn and potato plants sampled from the field. Nematode population estimates calculated from the whole plant samples were generally lower than those based on soil cores, but showed similar patterns of population growth. Nematode density per gram dry weight was highest in roots, intermediate on root surfaces, and lowest in soil. Estimates of the absolute abundance of nematodes in each of the three habitats were highest in roots or soil, depending on the sampling date, and lowest on root surfaces. This study demonstrates that P. scribneri inhabits soil environments even when host roots are present and illustrates the importance of considering all possible habitats when estimating the size of Pratylenchus spp. populations.     

Continent-wide tracking to determine migratory connectivity and tropical habitat associations of a declining aerial insectivore

North American birds that feed on flying insects are experiencing steep population declines, particularly long-distance migratory populations in the northern breeding range. We determine, for the first time, the level of migratory connectivity across the range of a songbird using direct tracking of individuals, and test whether declining northern populations have higher exposure to agricultural landscapes at their non-breeding grounds in South America. We used light-level geolocators to track purple martins, Progne subis, originating from North American breeding populations, coast-to-coast (n = 95 individuals). We show that breeding populations of the eastern subspecies, P. s. subis, that are separated by ca. 2000 km, nevertheless have almost completely overlapping non-breeding ranges in Brazil. Most (76%) P. s. subis overwintered in northern Brazil near the Amazon River, not in the agricultural landscape of southern Brazil. Individual non-breeding sites had an average of 91 per cent forest and only 4 per cent agricultural ground cover within a 50 km radius, and birds originating from declining northern breeding populations were not more exposed to agricultural landscapes than stable southern breeding populations. Our results show that differences in wintering location and habitat do not explain recent trends in breeding population declines in this species, and instead northern populations may be constrained in their ability to respond to climate change.     

Better off alone! Reproductive competition and ecological constraints determine sociality in the African striped mouse (Rhabdomys pumilio)

1.?While the reasons for group-living have been studied for decades, little is known about why individuals become solitary. 2.?Several previous experimental studies could demonstrate that group-living can arises as a consequence of ecological constraints. 3.?It has been argued that reproductive competition between group members leads to significant costs of group-living, being a main reason of solitary-living. However, so far, no studies tested experimentally whether reproductive competition can explain solitary-living. 4.?Using a socially flexible species, the African striped mouse (Rhabdomys pumilio), we tested experimentally in the field whether dispersal and solitary-living are more likely to occur when reproductive competition is present. 5.?We investigated ecological constraints, here expressed as a function of population density, by removing groups of striped mice and creating vacant territories. To control for the effect of reproductive competition, which occurs only during the breeding season, we performed experiments during both the breeding and the non-breeding season. This is the first removal experiment performed in a species with communal breeding during the non-breeding season. 6.?During the breeding season, when population density was low, more striped mice from experimental groups moved into the vacant territories and became solitary than striped mice from control groups. This is in support of the ecological constraints hypothesis. 7.?During the non-breeding season, striped mice remained group-living despite the availability of free territories. Significantly, more striped mice became solitary-living during the breeding than during the non-breeding season. This is the first experimental support for the reproductive competition hypothesis explaining solitary-living. 8.?Analysis of the sexual maturity of males showed that males which became solitary had a higher reproductive potential than males that remained group-living. Analysis of the body mass data of females showed that more solitary females reproduced than group-living females. These results indicate that by becoming solitary individuals of both sexes avoided costs of reproductive competition within groups. 9.?Our study provides experimental evidence that reproductive competition within groups can lead to dispersal and solitary-living.     

Songbird frequency selectivity and temporal resolution vary with sex and season

Many species of songbirds exhibit dramatic seasonal variation in song output. Recent evidence suggests that seasonal changes in auditory processing are coincident with seasonal variation in vocal output. Here, we show, for the first time, that frequency selectivity and temporal resolution of the songbird auditory periphery change seasonally and in a sex-specific manner. Male and female house sparrows (Passer domesticus) did not differ in their frequency sensitivity during the non-breeding season, nor did they differ in their temporal resolution. By contrast, female house sparrows showed enhanced frequency selectivity during the breeding season, which was matched by a concomitant reduction of temporal resolution. However, males failed to show seasonal plasticity in either of these auditory properties. We discuss potential mechanisms generating these seasonal patterns and the implications of sex-specific seasonal changes in auditory processing for vocal communication.     

Temporal Changes in the Vertical Distribution of Pratylenchus penetrans under Raspberry

Population densities of Pratylenchus penetrans and the biomass of fine roots of raspberry at depths of 0-5, 5-10, 10-20, and 20-30 cm were determined every 2 weeks for 2 years. The vertical distribution of P. penetrans varied from season to season, but the seasonal changes were not similar for the 2 years. In most seasons, the greatest population density was in the 5 to 10-cm-depth interval. Population densities of P. penetrans were not consistently correlated with the vertical distribution of raspberry roots in any season.     

Circannual variation in habitat use of the White-winged Snowfinch Montifringilla nivalis nivalis

High mountain areas are subject to strong seasonal fluctuations, and species inhabiting these particular environments show a high degree of habitat specialization to cope with extreme abiotic conditions. Estimates of habitat use are influenced by the spatial and seasonal scales at which they are evaluated, so studies at multiple scales are important in order to explore adaptive responses to seasonal environments. In the present study, we assessed habitat use of the White-winged Snowfinch Montifringilla nivalis subsp. nivalis (henceforth Snowfinch) during breeding and non-breeding seasons at three different spatial scales (diameters of 100, 250 and 500 m). Although Snowfinches clearly used high-elevation habitats in both seasons, there was evidence that they are less specific during the non-breeding period: the variance explained by habitat and topographical factors was lower in winter than in the breeding season. Moreover, our results suggest that the use of habitat is scale-dependent. This pattern was especially relevant in the breeding season, perhaps because habitat use might be more related to nest-site selection and specific foraging sites to provide food for nestlings. Snowfinches use high mountain habitats throughout the year, probably as a consequence of physiological and morphological specializations typical of high-elevation species, but in winter they show a certain flexibility in habitat use. Snowfinches might thus adopt a flexible specialist strategy. This could represent a trade-off to overcome possible effects on survival, condition and fitness, which can be particularly strong in harsh, unpredictable environments.     

Non-migratory stonechats show seasonal changes in the hormonal regulation of non-seasonal territorial aggression

In many birds and mammals, male territorial aggression is modulated by elevated circulating concentrations of the steroid hormone testosterone (T) during the breeding season. However, many species are territorial also during the non-breeding season, when plasma T levels are basal. The endocrine control of non-breeding territorial aggression differs considerably between species, and previous studies on wintering birds suggest differences between migratory and resident species. We investigated the endocrine modulation of territorial aggression during the breeding and non-breeding season in a resident population of European stonechats (Saxicola torquata rubicola). We recorded the aggressive response to a simulated territorial intrusion in spring and winter. Then, we compared the territorial aggression between seasons and in an experiment in which we blocked the androgenic and estrogenic action of T. We found no difference in the aggressive response between the breeding and the non-breeding season. However, similarly to what is found in migratory stonechats, the hormonal treatment decreased aggressive behaviors in resident males in the breeding season, whereas no effects were recorded in the non-breeding season. When we compared the aggressive responses of untreated birds with those obtained from migratory populations in a previous study, we found that territorial aggression of resident males was lower than that of migratory males during the breeding season. Our results show that in a resident population of stonechats T and/or its metabolites control territorial aggression in the breeding but not in the non-breeding season. In addition, our study supports the hypothesis that migratory status does modulate the intensity of aggressive behavior.     

5. GENERAL NOTES

Olive Thrushes Turdus olivaceus olivaceus in Grahamstown, South Africa, were present in their territories throughout the year. Mist-net captures showed no seasonal fluctuation in the population density of adults. The breeding season had a main peak from August to November and a minor peak in April. Song output was greatest at the beginning of the breeding periods, when territorial boundaries were most keenly contested. There was a positive correlation between seasonal variation in breeding intensity and song output. The adult survival rate was estimated at 80%, and 56% of the fledglings were alive in their natal territories at the age of independence. In this study area, only females built nests and brooded nestlings. Only males sang. Both males and females defended their territories.     

Infestations of Ornithonyssus bursa,a haemotophagous mite,on starlings over the non-breeding season

Abstract

Ornithonyssus bursa (Berlese) (Dermanyssidae) is found in nests and neotboxes and on nestlings of the starling (Sturnus vulgaris L.), sometimes in large numbers, during the breeding season. During the non-breeding season the nestboxes and nest material are devoid of live mites. O. bursa overwinters ectoparasitically on starlings, and is present on approximately 25% of the population at the beginning of the breeding season. These nucleus populations build up rapidly in nest boxes during the breeding season.     

Changes in ovarian morphology and hormone concentrations associated with reproductive seasonality in wild large Japanese field mice (Apodemus speciosus)

Wild large Japanese field mice (Apodemus speciosus) responses to cyclic seasonal changes are associated with physiological and behavioral changes. However, the detailed regulation of oogenesis in the ovary during the seasonal reproductive cycle in wild large Japanese field mice has not been studied. We assessed the dynamics and changes in ovarian morphology and hormone concentrations associated with reproductive seasonality throughout the year. The stages of the ovarian morphological breeding cycle of wild large Japanese field mice were classified as breeding, transition, and non-breeding periods during the annual reproductive cycle. Measurement of blood estradiol concentrations throughout the year showed that the levels in September and October were higher than those in other months. It is presumed that follicle development starts from a blood estradiol concentration of 38.4 ± 27.1 pg/mL, which marks a shift from the transitional season to the breeding season, followed by the transition to the non-breeding season at 26.1 ± 11.6 pg/mL. These results suggest that seasonal follicle development in wild rodents is correlated with estradiol regulation. We consider this species to be an alternative animal model for studying seasonal reproductive changes and the effects of environmental changes.     

Population-specific migratory strategies of Twite Linaria flavirostris in Western Europe

Migratory connectivity describes the extent to which different individuals from specific breeding populations use similar non-breeding sites, with important consequences for the demography and population structure of birds. We used mark-recapture data from the EUring data bank to describe the non-breeding range of Twite Linaria flavirostris within Western Europe. We quantified the strength of migratory connectivity across all European breeding populations, and for the British and continental breeding populations separately. Twite from different parts of the British breeding range used different non-breeding areas from one another, whereas Twite from different parts of the continental breeding range used similar non-breeding areas. There was essentially no overlap during the non-breeding season between British and continental Twite.     

Annual cycle of the Sertoli cell population in adult stallions

Stereological methods were employed in two experiments with adult stallions: to confirm seasonal variation in number of Sertoli cells and to characterize the annual cycle of the Sertoli cell population. In the first experiment, testes from 28 adult (4-20 years old) horses obtained in the non-breeding season (December-January) were compared to testes from 28 adult horses in the breeding season (June-July). Sertoli cell numbers were calculated from the nuclear volume density, parenchymal volume, and volume of an individual Sertoli cell nucleus determined by reconstruction of serial sections or from average height and width measurements. The number of Sertoli cells per testis was significantly greater in the breeding season. In a second experiment involving 43-48 adult horses in each 3-month period, the Sertoli cell population was higher (P less than 0.05) in May-July than other periods and higher (P less than 0.01) than in November-January. These combined studies confirm seasonal differences in the Sertoli cell numbers per testis and define the annual cycle of the Sertoli cell population in adult stallions.     

Neural correlates of behavioural olfactory sensitivity changes seasonally in European starlings

Background

Possibly due to the small size of the olfactory bulb (OB) as compared to rodents, it was generally believed that songbirds lack a well-developed sense of smell. This belief was recently revised by several studies showing that various bird species, including passerines, use olfaction in many respects of life. During courtship and nest building, male European starlings (Sturnus vulgaris) incorporate aromatic herbs that are rich in volatile compounds (e.g., milfoil, Achillea millefolium) into the nests and they use olfactory cues to identify these plants. Interestingly, European starlings show seasonal differences in their ability to respond to odour cues: odour sensitivity peaks during nest-building in the spring, but is almost non-existent during the non-breeding season.

Methodology/Principal Findings

This study used repeated in vivo Manganese-enhanced MRI to quantify for the first time possible seasonal changes in the anatomy and activity of the OB in starling brains. We demonstrated that the OB of the starling exhibits a functional seasonal plasticity of certain plant odour specificity and that the OB is only able to detect milfoil odour during the breeding season. Volumetric analysis showed that this seasonal change in activity is not linked to a change in OB volume. By subsequently experimentally elevating testosterone (T) in half of the males during the non-breeding season we showed that the OB volume was increased compared to controls.

Conclusions/Significance

By investigating the neural substrate of seasonal olfactory sensitivity changes we show that the starlings'' OB loses its ability during the non-breeding season to detect a natural odour of a plant preferred as green nest material by male starlings. We found that testosterone, applied during the non-breeding season, does not restore the discriminatory ability of the OB but has an influence on its size.