Small mammal populations in relation to hedgerow structure in an arable landscape

The population ecology of small mammals in hedgerows in arable farmland in eastern England is described. Features of hedgerows of importance to individual species are examined. Some 97% of the total 3042 mammals captured were wood mouse Apodemus sylvaticus , yellow-necked mouse Apodemus flavicollis , bank vole Clethrionomys glareolus and common shrew Sorex araneus . Small numbers of harvest mice Micromys minutus , field voles Microtus agrestis , pygmy shrews Sorex minutus and water shrews Neomys fodiens were also caught. Wood mouse, the most numerous species, showed a typical pattern of large numbers in autumn and winter, followed by a simultaneous decline over all hedges in early spring. Population changes were less clear in yellow-necked mouse and bank vole but the yellow-necked mouse was more scarce in the second year of study. Common shrews were most numerous in summer and declined rapidly in autumn. Hedgerow coppicing had a marked effect on yellow-necked mouse numbers but not on wood mouse. In an extensive survey of mammal numbers in relation to hedgerow features, ground cover was found to be the single largest factor influencing size of bank vole populations. Hedgerow condition (lack of gaps) was important to yellow-necked mice, which thrived only in well-established hedgerows. Wood mice appeared little influenced by the characteristics of the hedge. Common shrews were more abundant in hedgerows with adjacent permanent water.     

Population dynamics of Gyrodactylus sp. (Monogenea) infecting the sand goby in the Oslo Fjord, Norway

Prevalence and abundance of Gyrodactylus sp. on the 1993 year-class of sand gobies Pomatoschistus minutus in the Oslo Fjord, Norway, decreased slightly during the cold months, increased as temperature increased in spring, reaching a peak in June, then decreased significantly, corresponding with a decline in the number of hosts from August. Abundance and, especially, prevalence increased again in September, and decreased in late autumn. Death of the most heavily infected hosts, and/or host resistance might be responsible for the decline in parasite numbers on post-spawners in summer. The young of the year (1994 year-class) were probably infected shortly after they commenced benthic life in July, and prevalence and abundance increased significantly throughout late summer and autumn. Parasite abundance on males was significantly higher than on females at the height of the breeding season in June, but significantly lower at the end of the breeding season in July. Two-thirds of the parasites infected the gills, oral cavity and pharynx. The proportion of parasites on the body, head and fins increased on both sexes, but significantly more so on males, during the breeding season of the host, when parasite intensity was highest. Possible routes of transmission of the parasite are discussed.     

The ecology of Wood mice (Apodemus sylvaticus) on arable farmland

The ecology of Wood mice ( Apodemus sylvaticus ) was studied by live trapping and by examining the stomach contents of killed animals on an arable farm in West Suffolk. The study area included several fields but only a short length of hedgerow and was distant from other hedges and woods. Wood mice were trapped in the open fields throughout the year and their burrows were also found there. Population densities were comparable with those reported from deciduous woodland and seasonal population changes, ranging behaviour and timing of the breeding season were also broadly similar. The survival rate of juvenile mice varied greatly within and between breeding seasons tending to improve when young of the year replaced overwintered animals in the adult breeding population. Wood mice showed no marked preference for fields with different crops in winter when the main foods were grain, waste sugar beet roots, weed seeds and soil invertebrates. In spring and early summer they preferred winter wheat fields where seeding weeds provided most of their food.     

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

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.     

The dynamics of Wood mouse (Apodemus sylvaticus) populations in the Camargue

The ecology of two monospecific populations of Wood mice ( Apodemus sylvaticus ) was studied using mark-recapture techniques in a high grassland and a low grassland of the Camargue. Annual changes in population size resembled the classical pattern of variation for European Wood mice. Numbers were low in summer, and peaks in numbers occurred in autumn and decreased sharply in early spring. However, the densities of the populations in winter were higher than many other European Wood mouse populations. The Wood mouse population in the high grassland area was dense and showed regular variations throughout the year. In the low grassland area, the population was lower and showed less regular variations, especially in winter, coinciding with soil flooding. Reproduction-related differences between the demographic parameters of the two sexes pointed to the use of differential space use strategies by males and females.     

Infection with cowpox virus decreases female maturation rates in wild populations of woodland rodents

Sublethal effects of parasitic infection, such as reductions in reproductive rate, can significantly affect host population dynamics. Here we show that in wild populations of both Clethrionomys glareolus (bank vole) and Apodemus sylvaticus (wood mouse), females infected with cowpox virus are likely to delay maturation and therefore reproduction – in most cases until the following breeding season. Some infected bank voles do mature in their year of birth but still take longer than uninfected females. Together with our previous demonstration that individuals infected with cowpox virus in the summer survive better than uninfected individuals, these results support the prediction that hosts that develop an acute infection may best optimise their fitness by decreasing current reproduction to maximise the probability of surviving infection. Moreover, as the proportion of individuals infected increases with density, the reduction in host fecundity may have significant consequences for host dynamics.     

Changes in body condition and body size affect breeding and recruitment in fluctuating house mouse populations in south‐eastern Australia

Changes in body condition and body size in field populations of house mice, Mus domesticus, were examined to investigate why mouse populations do not increase rapidly in some years when favourable environmental and demographic conditions indicate they might. Mice had repeated seasonal patterns each year in breeding, growth rates and body condition that reflected the seasonal availability of food, but mean levels for each parameter varied among years. In most years mice lost body condition during summer, breeding declined and population growth slowed. Rapid population growth occurred when body condition was generally high and was maintained throughout summer. Female mice with large body length were more likely to breed than smaller mice, at all times, but changes in body condition accounted for most of the variability in female breeding activity between years and between habitats, and for the seasonal changes in the importance of body length. During rapid population growth, the recruitment rate of juveniles relative to the number of breeding females was 150–300% higher than in other years but adult survival rates were not higher. The data indicate that the ability of mice to maintain body condition, particularly when subject to moisture stress in summer, affects the proportion of females breeding, the number of juveniles weaned and their body condition at weaning, and is promoted by foraging conditions that favour maintenance of juvenile body condition after weaning. These factors, in turn, greatly affect juvenile recruitment rates and eventual population density of mice. Low juvenile survival is suggested as a reason that numbers of house mice in southern Australian cereal‐growing areas do not increase rapidly in some years when other parameters are favourable. Similar processes are likely to play a role in regulating other rodent populations.     

Seasonal changes in age class structure and reproductive status of house mice on Marion Island (sub-Antarctic)

Feral house mice on sub-Antarctic Marion Island become reproductively active (males scrotal, females with perforate vaginas or pregnant) at an age >60 days and breed until death, which may occur at more than 13 months. Breeding is strongly seasonal; pregnant or lactating females were found only from October to May. A substantial proportion of mice old enough to breed in one summer overwinters to form a significant component of the breeding population the following summer but it is unlikely that any survive a second winter. The onset of breeding is closely synchronized with increasing day length but occurs about 2 months before mean temperature at the ground surface starts to increase significantly. Cessation of breeding is more closely associated with declining temperatures in late summer. For both males and females, the best correlation between reproductive activity and any of the temperature parameters measured was with average maximum temperature 1 cm above the ground. Competition for macroinvertebrate prey increases sharply in early winter due to high mouse numbers. The breeding season in 1991/1992 and 1992/1993 was at least 2 months longer than in 1979/1980, because the mice started breeding earlier, and stopped breeding later, in 1991/1993. The later cessation of breeding in 1991/1993 was despite the fact that there was a greater competition for macroinvertebrate prey, and that mean air temperatures during the early winter months were lower, than in 1979/1980.     

The influence of congenital Toxoplasma infection on the spontaneous running activity of mice

Home-cage running-wheel activity of mice congenitally infected with Toxoplasma was recorded over 24 days. Infected mice were consistently more active than uninfected controls over the entire testing period. This finding extends previous studies and indicates that such increased activity levels occur not only in novel but also in familiar environments, and suggests that congenital toxoplasmosis tends to render mice "hyperactive'. If such behavioural alterations occur in wild mice, it is likely that infected mouse intermediate hosts would be more susceptible to predation by cats, the definitive hosts of Toxoplasma.     

On the interpretation of host-parasite ecology: Heligmosomoides polygyrus (Nematoda) in wild wood mouse (Apodemus sylvaticus) populations

This paper describes epidemiological and seasonal patterns in the interaction between wood mice, Apodemus sylvaticus and Heligmosomoides polygyrus. Data used in the analysis were collected by C. S. Elton and co-workers at Bagley Wood, Oxfordshire in the late 1920s. Heligmosomoides polygyrus was by far the most common helminth parasite with 70% of all wood mice infected and average intensity around 12 worms per mouse. Male and female mice were shown to harbour similar parasite burdens. Parasite numbers per host were highly overdispersed and were well described by the negative binomial distribution. There was little evidence for convexity in age (= weight)-intensity curves, either within or across sexes.
Host and parasite numbers showed predictable seasonal patterns, with mouse populations at their largest at the end of the breeding season, in August and September, and parasite populations at their largest in the late spring, around May. Results are discussed in relation to the ecology of H. polygyrus in wood and laboratory mice, and tentative comparison is made with human helminth infection. The interpretation of epidemiological patterns in these data was problematic. Of particular importance was the statistical distribution of parasites within the host population, and possible differences between mouse sexes in relation to growth, survival and trapping. Such difficulties are relevant to a range of similar field data.     

The role of habitat in structuring Halipegus occidualis metapopulations in the green frog

The transmission dynamics of the trematode Halipegus occidualis in its definitive host, Rana clamitans, have been examined over a 5-yr period in a North Carolina pond. The breeding season of green frogs coincides with the period of worm recruitment, during which time male frogs are territorial and females show strong site fidelity. This site fidelity allows inferences to be made regarding the suitability of a particular habitat for worm transmission based on frog infection intensities within that habitat. Four foci of infection were identified in the pond by plotting worm infrapopulation size against site of host capture. Sites within infection foci are characterized by shallow water and emergent vegetation, factors favorable for overlapping distributions of the 4 hosts in the life cycle of H. occidualis. Consistent year-to-year worm prevalences and intensities, despite fluctuations in frog population size, are thought to be the result of a relatively constant proportion of the frog population being present in infection foci each year. Removal of worms from heavily infected frogs in the fifth year resulted in further heavy worm recruitment by treated frogs suggesting that site selection can predispose a frog to heavy infection. Further, the sum of removed parasites and those recruited after parasite removal by treated frog hosts was higher than worm infrapopulations observed in previous years, indicating that worm density regulates parasite infrapopulation size in heavily infected frogs.     

Population dynamics of an outbreak of house mice (Mus domesticus) in the mallee wheatlands of Australia—hypothesis of plague formation

A plague of mice ( Mus domesticus ) in the Victorian mallee wheatlands of south-eastern Australia in autumn 1984 appeared to be generated by a sequence of rainfall events: high autumn (March), mid winter and late winter rainfall in 1983, and high summer rainfall in 1983/84. The March rainfall in 1983 ended a drought; mice began to breed and bred until the end of May. Relatively high survival of mice for 12 months after March 1983, together with early onset of breeding and high reproductive performance throughout the 1983/84 breeding season, including summer, were key demographic processes during the formation of the plague. Temporal differences in mouse abundance and breeding performance between habitats highlighted the relevance of specific habitats to the dynamics of mouse populations in the wheatlands. Fencelines were the most important habitat of mice because they were foci for breeding at the start of the breeding season, good nesting sites which were rarely disturbed, and widespread and in close proximity to crops. Cereal crops were colonized in spring 1983 and in autumn 1984; they became important habitats in 1983 when mice dispersed and bred there in early spring. Redhead's (1988) model was sufficient to explain the 1984 plague, but not the magnitude of the decline of mouse numbers in 1984, nor the absence of a further outbreak in 1985. A new model is proposed based on a sequence of rainfall events beginning at least 10 months prior to a plague.     

The initiation and maintenance of a colony of coccidia-free wood mice (Apodemus sylvaticus)

Wood mice from a population showing enzootic infection with Eimeria were trapped and bred under laboratory conditions. The maintenance regime, which freed the animals from coccidia, and the breeding procedures used over several generations are described and discussed.     

The occurrence and life history of Asymphylodora kubanicum (Platyhelminthes: Digenea: Monorchidae) in the Worcester-Birmingham canal, with special reference to the feeding habits of the definitive host, Rutilus rutilus

Two hundred roach and over 400 molluscs were examined over a one year period to investigate the occurrence and life history of Asymphylodora kubanicum in the Worcester-Birmingham canal. Larval stages, infective to the fish definitive host, were present in molluscs throughout the year but did not show a seasonal fluctuation in numbers. Parasites in the intestine of the roach showed a marked annual cycle of occurrence and maturation: low winter infection levels preceded a dramatic increase in infection during the spring and summer. Maturation of the parasite population was rapid during the spring and summer and in late summer and early autumn the parasites laid their eggs and subsequently died. The death of parasites after egg-laying resulted in the low winter infection level during which time little recruitment occurred.
Roach became infected mainly in their third year when molluscs become a dominant component of their diet. Thereafter the older fishes (<2+) are all equally susceptible to infection, but the larger (older) fishes become more heavily infected because of the greater consumption of molluscs per fish. The sex of the fish made no difference to infection with the parasite.     

Sin Nombre virus infection of deer mice in Montana: characteristics of newly infected mice, incidence, and temporal pattern of infection

Sin Nombre virus (SNV), hosted by the deer mouse (Peromyscus maniculatus), is the principal cause of hantavirus pulmonary syndrome (HPS) in North America. To improve our understanding of factors that contribute to the occurrence of HPS, we conducted an extensive field study of the characteristics of newly infected (as determined by recent acquisition of antibody) deer mice and the temporal pattern of antibody acquisition (seroconversion) from 1994 through 2004 in Montana, USA. We sampled 6,584 individual deer mice, of which 2,747 were captured over multiple trapping periods. Among these 2,747 deer mice, we detected 171 instances of seroconversion. There was no relationship between seroconversion and the acquisition of scars. However, recently infected Montana deer mice were more likely to be older, more likely to be males, and more likely to be in breeding condition. In addition, recently infected male deer mice gained less weight over the 1-mo period following seroconversion than did those that did not acquire antibody, suggesting that SNV infection may have negatively impacted the health of infected rodents. Incidence was highly variable among years, and timing of infections was primarily associated with the breeding season (generally early spring through late fall).     

Strong density-dependent competition and acquired immunity constrain parasite establishment: implications for parasite aggregation

The vast majority of parasites exhibit an aggregated frequency distribution within their host population, such that most hosts have few or no parasites while only a minority of hosts are heavily infected. One exception to this rule is the trophically transmitted parasite Pterygodermatites peromysci of the white-footed mouse (Peromyscus leucopus), which is randomly distributed within its host population. Here, we ask: what are the factors generating the random distribution of parasites in this system when the majority of macroparasites exhibit non-random patterns? We hypothesise that tight density-dependent processes constrain parasite establishment and survival, preventing the build-up of parasites within individual hosts, and preclude aggregation within the host population. We first conducted primary infections in a laboratory experiment using white-footed mice to test for density-dependent parasite establishment and survival of adult worms. Secondary or challenge infection experiments were then conducted to investigate underlying mechanisms, including intra-specific competition and host-mediated restrictions (i.e. acquired immunity). The results of our experimental infections show a dose-dependent constraint on within-host-parasite establishment, such that the proportion of mice infected rose initially with exposure, and then dropped off at the highest dose. Additional evidence of density-dependent competition comes from the decrease in worm length with increasing levels of exposure. In the challenge infection experiment, previous exposure to parasites resulted in a lower prevalence and intensity of infection compared with primary infection of naïve mice; the magnitude of this effect was also density-dependent. Host immune response (IgG levels) increased with the level of exposure, but decreased with the number of worms established. Our results suggest that strong intra-specific competition and acquired host immunity operate in a density-dependent manner to constrain parasite establishment, driving down aggregation and ultimately accounting for the observed random distribution of parasites.     

Toxoplasma infection and response to novelty in mice

Three groups of mice were infected with Toxoplasma and used for behavioral testing using a Y-maze. One group was infected when adult and two groups congenitally, one of these born to dams infected during gestation, the other to dams chronically infected prior to mating. In an initial habituation period each mouse was exposed to a black arm and stem of the maze, entrance to a white arm being blocked by a transparent door. In a subsequent free-choice trial both arms were black and the mouse was free to explore all parts of the maze. During both periods infected mice were more active than controls. Infected mice engaged in less grooming behavior indicative of less approach-avoidance conflict than controls prior to entry into a choice arm at the beginning of the free-choice trial. Infected mice spent more time in the familiar than in the novel (previously blocked) arm during the free-choice trial; conversely, uninfected mice spent more time in the novel than in the familiar arm. It is suggested that the reported behavioural changes would lead to dissemination of the infection in the environment by ultimately making infected mouse intermediate hosts more susceptible to predation by domestic cats, the definitive hosts of Toxoplasma.     

Host genetics as a cause of overdispersion of parasites among hosts: how general a phenomenon?

In the white-footed mouse, Peromyscus leucopus, the tapeworm Hymenolepis citelli occurs at low (2-3%) prevalence in the field. We found that mature infections (i.e., with egg production) developed in up to 100% of hosts. In the laboratory, a majority of hosts lost their infection by 28 days postintubation. In wild mice infected in the laboratory and returned to the field, infections were more prolonged, with half of the mice still infected at 100 days postintubation. A majority of previously infected hosts resisted challenge infection. Our introduction of laboratory-infected mice into a natural population of hosts appeared to cause infections among previously uninfected mice, leading to an increase in the prevalence of tapeworm infection among mice not intubated. Although genetically based expulsion of tapeworms before maturity is important in causing low prevalence in a similar host-parasite system, such resistance cannot explain low prevalence in the present system. It appears that both heterogeneous distribution and rarity of intermediate hosts as well as short parasite lifespan contribute to low prevalence and overdispersion. Host-parasite dynamics of 2 very similar systems appear to differ markedly.     

Haemosporidian parasitism in the blackcap Sylvia atricapilla in relation to spring arrival and body condition

Parasites can exert strong selection on hosts. Throughout the year migrants are exposed to different sets of parasites, which may affect life history traits such as migratory schedules. Here, we studied the relationship between parasite infection and arrival date of blackcaps Sylvia atricapilla to their breeding grounds in Germany throughout a period of six years (2007–2012). We used two data sets, one that included all blackcaps and one that included only recaptured birds. We assesed whether parasites influence spring arrival to breeding grounds, and for the recaptured data set, we analysed temporal variation in parasitism (i.e. infection status and parasitaemia) throughout the breeding season. We used both microscopy and PCR (a fragment of ? 479 bp of the mtDNA cyt b) to determine haemosporidian infection. Blackcaps were mostly infected with Haemoproteus parabelopolskyi (lineages SYAT01 and SYAT02). Infection status, but not parasitaemia, was constant through time for individual birds; meaning that once a bird is infected, it most likely will retain the infection for life. We found that infection by haemosporidian parasites has no relationship to arrival date in this blackcap population; however, infection by H. parabelopolskyi has a marginally significant effect on arrival date of recaptured blackcaps, somewhat delaying their arrival to breeding grounds. Birds captured later in the season were more likely to be infected than those from early spring, and parasitaemia was frequently lower in birds captured earlier in the season compared to those captured later (summer).     

Effect of additional food and water on house mice in a semi‐arid agricultural environment in Australia

Abstract We tested the hypothesis that providing high‐quality food and water would increase reproduction, survival and population size in house mice living in a semi‐arid cropping environment in the mallee region of western Victoria, Australia, where outbreaks of house mice (Mus domesticus) occur irregularly. We employed a factorial design and applied treatments along internal fencelines (16 sites), adjacent to cropping areas, from November 2003 to July 2004. Population abundance was low during the experiment (0–26 mice per site), and the summer rainfall was below average. We confirmed that mice used the supplementary food and water through a reduction in weight of food containers over time and for water through the presence of Rhodamine B in blood samples and positive bands in whiskers. Abundant food was also available through grain spilt on the ground after harvest of the wheat and barley crops. There was some evidence of increased breeding on sites where water was added, but no effect of food or food and water in combination. Sites where free water was available had marginally higher populations over summer (～2 more mice on average; P = 0.07). This difference was well below any biologically meaningful effect. Mice were 0.9 g heavier on sites where water was added (P = 0.04), and were in better condition (P = 0.03). The addition of high‐quality food did not affect mouse population dynamics, and the addition of water resulted in only marginal responses for some demographic characteristics. We conclude that other factors appear to be important for limiting mouse population growth in summer and autumn.