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Seasonal Migration (seasonal + migration)

Distribution by Scientific Domains

Life Sciences	61%
Earth and Environmental Science	23%

Selected Abstracts

Struggling to Save Cash: Seasonal Migration and Vulnerability in West Bengal, India

DEVELOPMENT AND CHANGE, Issue 4 2003
Ben Rogaly
This article concerns an important but overlooked means by which able-bodied poor people get hold of lump sums of cash in rural West Bengal: seasonal migration for agricultural wage work. Drawing on a regional study of four migration streams, our main focus here is on the struggle to secure this cash by landless households in just one of those streams, originating in Murshidabad District. Case studies are used to illustrate the importance for women in nuclear families of maintaining supportive networks of kin for periods when men are absent. A parallel analysis is made of the negotiations between male migrant workers and their employers, at labour markets, during the period of work, and afterwards. The article then briefly discusses some of the contrasting ways in which remittances are used by landless households and owners of very small plots of land, in the context of rapid ecological change, demographic pressure and growing inequality. [source]

Seasonal migration and land-use change in Ghana

LAND DEGRADATION AND DEVELOPMENT, Issue 1 2004
A. K. Braimoh
Abstract When farmers migrate yearly to a village to carry out intense farming during the rainy season, and thereafter return to a more permanent place of abode this is referred to as seasonal migration. The impact of such migration on land-use/land-cover change in an area within the Volta Basin of Ghana was examined using satellite image analysis and socioeconomic surveys. The most drastic land-cover change involved the conversion of woodland to agricultural land, while there was also a general transition to less vegetation cover. Socioeconomic surveys revealed that most of the migration occurred during the post-structural adjustment period in Ghana with declining soil fertility accounting for the highest per cent of causes of migration. Multiple regression results highlighted the role of population size and distribution, marketing of agricultural produce and technological evolution of the household in determining agricultural land-use change. Policy initiatives that could lead to environment conservation are suggested. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Struggling to Save Cash: Seasonal Migration and Vulnerability in West Bengal, India

Nocturnal migration of dragonflies over the Bohai Sea in northern China

ECOLOGICAL ENTOMOLOGY, Issue 5 2006
HONG-QIANG FENG
Abstract 1.,A sudden increase and subsequent sharp decrease of catches of dragonflies in a searchlight trap, with Pantala flavescens Fabricius (Odonata: Libellulidae) predominating, observed at Beihuang Island in the centre of the Bohai Gulf, in 2003 and 2004, indicated a seasonal migration of these insects over the sea during the night in China. The movements were associated with the onset of fog. 2.,Simultaneous radar observations indicated that the nocturnally migrating dragonflies generally flew at altitudes of up to 1000 m above sea level, with high density concentrations at about 200,300 or 500 m; these concentrations were coincident with the temperature inversion. 3.,During early summer, the dragonflies oriented in a downwind direction, so that the displacement direction varied between different altitudes. In contrast, during late summer, the dragonflies were able to compensate for wind drift, even headwind drift, so as to orient south-westward no matter how the wind changed, and thus the displacement direction was towards the south-west. 4.,The duration of flight, estimated from the variation of area density derived from radar data and hourly catches in the searchlight trap through the night, was about 9,10 h. The displacement speed detected using radar was ,5,11 m s,1. Therefore, the dragonflies might migrate 150,400 km in a single flight. 5.,The dragonflies were thought to originate in Jiangsu province and they migrated into north-east China to exploit the temporary environment of paddy fields in early summer. Their offspring probably migrated back south during late summer and autumn. [source]

Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies

ECOLOGICAL ENTOMOLOGY, Issue 2 2000
Sonia M. Altizer
Summary 1. Monarch butterflies Danaus plexippus (L.) (Lepidoptera: Nymphalidae) are susceptible to infection by the obligate protozoan parasite Ophryocystis elektroscirrha (McLaughlin and Myers) (Apicomplexa: Neogregarinida). Because monarchs form resident and migratory populations in different parts of the world, this host,parasite system provides the opportunity to examine how variation in parasite prevalence relates to host movement patterns. 2. Parasite prevalence was evaluated using 14 790 adult monarchs captured between 1968 and 1997. Comparison of three populations in North America indicated that parasite prevalence is associated negatively with host dispersal distances. A continuously breeding, nonmigratory population in southern Florida showed high prevalence (over 70% heavily infected). The western population migrates moderate distances to overwintering sites on the Pacific Coast and has intermediate prevalence (30% heavily infected). The eastern migratory population, which travels the longest distance to Mexican overwintering sites, has exhibited less than 8% infection throughout the past 30 years. 3. Variation in parasite loads within North American migratory populations was investigated to determine whether the prevalence of heavy infection and average parasite loads declined during migration or overwintering. Average parasite loads of summer-breeding adults in western North America decreased with increasing distance from overwintering sites. This suggests that heavily infected monarchs are less likely to remigrate long distances in spring. No differences in the frequency of heavily infected adults were found among eastern or western North American monarchs throughout the overwintering period, however, suggesting that this parasite does not affect overwintering mortality. 4. Changes in the prevalence of monarchs with low parasite loads demonstrate that spore transfer occurs during migration and overwintering, possibly when adult butterflies contact each other as a result of their clustering behaviour. 5. This study of geographical and temporal variation in O. elektroscirrha among populations of D. plexippus demonstrates the potential role of seasonal migration in mediating interactions between hosts and parasites, and suggests several mechanisms through which migratory behaviour may influence parasite prevalence. [source]

Life-history traits of the edible stinkbug, Encosternum delegorguei (Hem., Tessaratomidae), a traditional food in southern Africa

JOURNAL OF APPLIED ENTOMOLOGY, Issue 9-10 2009
C. M. Dzerefos
Abstract Little is known of the life history of the edible stinkbug, Encosternum delegorguei, although it is an important food for people living in north-eastern South Africa and southern Zimbabwe. The present study aimed to establish key elements influencing long-term sustainable harvesting. Outdoor insectaries of two sizes were constructed to observe: daily activity, utilization of plants, copulation, oviposition, eclosion and survival from May 2006 to February 2007. The rest of the annual life cycle was observed in the field in March and April 2007 and identified as univoltine. In autumn (May) E. delegorguei entered reproductive diapause and aggregated within the escarpment mist-belt where it survived the winter on vapour condensation without feeding. Monthly dissections showed that abdominal fat content was highest in June. In spring (September) E. delegorguei fed on sap of the trees Combretum imberbe, Combretum molle, Peltophorum africanum, to a lesser degree on Dodonaea viscosa and the grass Pennisetum clandestinum. Copulation occurred in October and November. An overall total of 1752 E. delegorguei eggs were laid by 103 females and incubation time averaged 18.7 ± 9.0 days (range 7,37) at outdoor temperature ranging from 11°C to 25°C. The mean number of eggs in 64 egg masses was 27.4 ± 13.9 (range of 2,56 eggs). Shade cloth (68.8%) was the most commonly used substrate for depositing eggs followed by P. clandestinum (12.5%), C. imberbe (7.8%), P. africanum (6.3%), D. viscosa (1.6%), C. molle (1.6%) and C. erythrophyllum (1.6%). The parasitoid wasp, Anastatus sp. (Hymenoptera: Eupelmidae) infected 57% of the eggs deposited by captive females. Availability of food plants in combination with parasitoid threat may be a reason for seasonal migration between overwintering sites within the mist-belt and summer oviposition sites. Diminishing harvests could be attributed to fuelwood harvesting of food plants in the summer sites. [source]

Seasonal migration and land-use change in Ghana

Nuclear and mitochondrial markers reveal distinctiveness of a small population of bottlenose whales (Hyperoodon ampullatus) in the western North Atlantic

MOLECULAR ECOLOGY, Issue 11 2006
MEREL L. DALEBOUT
Abstract Small populations at the edge of a species' distribution can represent evolutionary relics left behind after range contractions due to climate change or human exploitation. The distinctiveness and genetic diversity of a small population of bottlenose whales in the Gully, a submarine canyon off Nova Scotia, was quantified by comparison to other North Atlantic populations using 10 microsatellites and mitrochondrial DNA (mtDNA) control region sequences (434 bp). Both markers confirmed the distinctiveness of the Gully (n = 34) from the next nearest population, off Labrador (n = 127; microsatellites ,FST= 0.0243, P < 0.0001; mtDNA ,,ST = 0.0456, P < 0.05). Maximum likelihood microsatellite estimates suggest that less than two individuals per generation move between these areas, refuting the hypothesis of population links through seasonal migration. Both males and females appear to be philopatric, based on significant differentiation at both genomes and similar levels of structuring among the sexes for microsatellites. mtDNA diversity was very low in all populations (h = 0.51, , = 0.14%), a pattern which may be due to selective sweeps associated with this species' extreme deep-diving ecology. Whaling had a substantial impact on bottlenose whale abundance, with over 65 000 animals killed before the hunt ceased in the early 1970s. Genetic diversity was similar among all populations, however, and no signal for bottlenecks was detected, suggesting that the Gully is not a relic of a historically wider distribution. Instead, this unique ecosystem appears to have long provided a stable year-round habitat for a distinct population of bottlenose whales. [source]

Migration, Diet, or Molt?

BIOTROPICA, Issue 4 2010
Interpreting Stable-Hydrogen Isotope Values in Neotropical Bats
ABSTRACT Migratory behavior in bats is poorly described, particularly in the Neotropics. Stable-hydrogen isotope (,D) analysis may allow tracking of altitudinal movements of bats but has not been explored. ,D values in rainwater (,Dp) deplete linearly with altitude and are reflected in the keratinous tissues of animals through diet. A mismatch between keratin ,D (,Dk) and that expected at the capture site based on ,Dp can indicate prior migration. We collected rainwater, claws and hair from eight bat species at two lower-montane forest sites in Nicaragua. Claw ,D for Carollia brevicauda and hair and claws for Desmodus rotundus (known to be non-migratory) fell within the predicted range based on rainwater (,17 to ,60,) suggesting these tissues were synthesized at the study site. ,D tissue values for Artibeus toltecus, Sturnira lilium, Glossophaga soricina, Anoura geoffroyi, and hair for C. brevicauda were more negative than predicted for the capture site (,60,) suggesting tissue synthesis at higher elevation and migration downslope to the capture site. However, our study area represents the highest elevation in the region; the nearest appropriate higher elevations are 350,500 km away and seasonal migration is expected to be<200 km. Thus we consider that seasonal shifts in ,Dp (9 to ,45,) may result in differences in species which molt at different times, and that diet may have driven differences in ,D. Our results suggest that the effects of molt timing and diet may first need to be understood before ,D may be successfully used to track bat movements. [source]

Seasonal spatial dynamics and causes of nest movement in colonies of the invasive Argentine ant (Linepithema humile)

ECOLOGICAL ENTOMOLOGY, Issue 5 2006
NICOLE E. HELLER
Abstract 1.,Colony organisation and movement behaviour of the Argentine ant (Linepithema humile) was studied over 3 years in field populations in California and in captive colonies in the laboratory. This invasive species is highly polydomous and unicolonial; colonies consist of expansive and fluid networks of nests and trails. The spatial and temporal organisation of colonies may contribute to ecological dominance. 2.,Argentine ant nests and inter-nest trails shift in size, abundance, and location, so that colony networks are spatially contracted in the winter and expanded spring to autumn. Colonies occupy permanent sites; ants migrated to and from the same winter nest locations year after year, and occupied 30% of the same nests repeatedly during seasonal migrations. 3.,Nests were moved on average 2,3 m. Forty-two per cent were occupied less than 1 month, 4% the entire study, and the other 54% lasted 3.9 ± 2.3 months (mean ± SD). 4.,Nests were located within 2,4 m of woody plants, in warm sites in the winter and cool sites in the summer. Both humidity and food availability influenced nest-site choice in laboratory colonies. However, when faced with a trade-off between factors, the ants chose humid nest boxes over nest boxes near food, and ants moved nests only in response to changes in humidity and not distance to food. 5.,The results indicate that L. humile colonies are seasonally polydomous, and that nest movements are driven by changes in microclimate. Colony organisation maintains high local density and increases food supply, which may improve the competitive ability of L. humile colonies and reduce opportunities for species coexistence. [source]

Seasonal variation in the migration strategies of the green lacewing Chrysoperla carnea species complex

ECOLOGICAL ENTOMOLOGY, Issue 4 2006
JASON W. CHAPMAN
Abstract 1. Insect migration strategies are generally poorly understood due to the propensity for high-altitude flight of many insect species, and the technical difficulties associated with observing these movements. While some progress has been made in the study of the migration of important insect pests, the migration strategies of insect natural enemies are often unknown. 2. Suction trapping, radar monitoring, and high-altitude aerial netting were used to characterise the seasonal migrations in the U.K. of an assemblage of aphid predators: three green lacewings in the Chrysoperla carnea species complex. 3. Chrysoperla carnea sens. str. was found to be very abundant at high altitudes during their summer migration, and some individuals were capable of migrating distances of , 300 km during their pre-ovipositional period. In contrast, high-altitude flights were absent in the autumn migration period, probably due to a behavioural adaptation that increases the probability that migrants will encounter their over-wintering sites. The other two species in the complex, C. lucasina and C. pallida, were much rarer, making up , 3% of the total airborne populations throughout the study period. 4. The summer migration of C. carnea sens. str. was not directly temporally associated with the summer migration of its cereal aphid prey, but lagged behind by about 4 weeks. There was also no evidence of spatial association between aphid and lacewing populations. 5. The results show that to understand the population ecology of highly mobile insect species, it is necessary to characterise fully all aspects of their migration behaviour, including the role of high-altitude flights. [source]

Variation in the diet of the Patagonian toothfish with size, depth and season around the Falkland Islands

JOURNAL OF FISH BIOLOGY, Issue 2 2003
A. Arkhipkin
The ontogenetic and seasonal variations in the feeding spectrum were studied in 756 specimens of the Patagonian toothfish Dissostichus eleginoides (16,159 cm total length, LT) collected on the shelf, continental slope and bathyal waters (67,1960 m, depth range) around the Falkland Islands between April 1999 and August 2002. On the shelf, small toothfish (<40 cm LT) were active predators taking mostly one relatively large prey item at a time (mainly near-bottom Patagonotothen ramsayi and Loligo gahi). Medium-size toothfish (40,60 cm LT) fed on the same prey, but the number of prey items increased to 1,2 items per fish. Large toothfish (>60 cm LT) switched their diet to other large pelagic fishes occurring near the bottom (Macruronus magellanicus and Micromesistius australis australis), again taking mostly one prey item at a time. The diet of medium-size D. eleginoides on the shelf varied seasonally depending on the abundance and migrations of the major prey species. Patagonotothen ramsayi was abundant in the diet throughout the year, whereas L. gahi appeared only from February to October during its offshore seasonal migrations to the depth range of D. eleginoides. During November to January, L. gahi migrated inshore to spawn and disappeared from the toothfish diet, being substituted by M. australis australis which dispersed on the shelf after spawning. After its ontogenetic descent to the lower part of the continental slope (500,1000 m depths), toothfish took less active (than on the shelf) fishes such as Antimora rostrata whilst also feeding on active near-bottom macrourids and skates. In their deepest habitat (>1000 m depths), toothfish became a typical opportunistic predator, feeding mainly on relatively small and inactive fishes, squids and prawn-like crustaceans Acanthephyra pelagica and Thymops birsteini. Decrease in hunting activity with depth could be related to a specific adaptation to keep neutral buoyancy by increase of lipid content in white muscles of D. eleginoides with size. [source]

Seasonal variation in terrestrial resource subsidies influences trophic niche width and overlap in two aquatic snake species: a stable isotope approach

OIKOS, Issue 7 2010
John D. Willson
Quantifying diet is essential for understanding the functional role of species with regard to energy processing, transfer, and storage within ecosystems. Recently, variance structure in the stable isotope composition of consumer tissues has been touted as a robust tool for quantifying trophic niche width, a task that has previously proven difficult due to bias in direct dietary analyses and difficulties in integrating diet composition over time. We used carbon and nitrogen stable isotope analyses to examine trophic niche width of two sympatric aquatic snakes, banded watersnakes Nerodia fasciata and black swamp snakes Seminatrix pygaea inhabiting an isolated wetland where seasonal migrations of amphibian prey cause dramatic shifts in resource availability. Specifically, we characterized snake and prey isotope compositions through time, space, and ontogeny and examined isotope values in relation to prey availability and snake diets assessed by gut content analysis. We determined that prey cluster into functional groups based on similarity of isotopic composition and seasonal availability. Isotope variance structure indicated that the trophic niche width of the banded watersnake was broader (more generalist) than that of the black swamp snake. Banded watersnakes also exhibited seasonal variation in isotope composition, suggesting seasonal diet shifts that track amphibian prey availability. Conversely, black swamp snakes exhibited little seasonal variation but displayed strong ontogenetic shifts in carbon and nitrogen isotope composition that closely paralleled ontogenetic shifts in their primary prey, paedomorphic mole salamanders Ambystoma talpoideum. Although niche dimensions are often treated as static, our results demonstrate that seasonal shifts in niche dimensions can lead to changes in niche overlap between sympatric species. Such short-term fluctuations in niche overlap can influence competitive interactions and consequently the composition and dynamics of communities and ecosystems. [source]