bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spatial patterns of population regulation in sage grouse (Centrocercusspp.) population viability analysis
Authors: LaMontagne Jalene M. , Irvine Robyn L. , Crone Elizabeth E. 
Published: March 2003

SummaryPopulation viability analyses (PVAs) are commonly used to identify species of concern. Many PVA techniques assume that all populations are regulated by a single mechanism.We compared population viability predictions for three subspecies of sage grouse (Centrocercusspp.) based on the assumptions that: (i) population regulation was density‐independent vs. dependent on more complex feedback mechanisms; (ii) the mechanism of population regulation was homogeneous within a region vs. heterogeneous among leks; (iii) environmental variation was spatially correlated within regions vs. uncorrelated among leks.We used sage grouse as a model species for this analysis because counts of lekking male grouse are available in some areas since the 1950s, these counts are known to fluctuate widely, and sage grouse appear to be declining throughout their range.We fit population regulation models to data including density‐independence, density‐dependence, delayed density‐dependence and a simplified version of Turchin & Taylor's (1992) response surface model.We show that the best‐fit models typically include spatial heterogeneity in mechanisms of population regulation. Inclusion of spatial heterogeneity increased expected time for population persistence, and changed the rank order of risk of extinction for different regions.We suggest that it is important to consider multiple models of population regulation when applying population viability analysis techniques because viability projections are influenced strongly by model structure.
Role of forage, habitat and predation in the behavioural plasticity of a small African antelope
Authors: Brashares Justin S. , Arcese Peter 
Published: March 2003

SummaryEcological influences on social organization can best be measured by comparing populations of a socially variable species living in different environments. We examined causes of variability in social behaviour in a socially plastic small antelope, the oribi,Ourebia ourebiZimmermann, in Ghana, West Africa.In total, 161 individually identified oribi were observed at five sites along an ecological gradient through dry and rainy seasons. Behavioural observations and ecological data were used to test the relative influence of forage abundance and quality, habitat structure and predation pressure on female dispersion and male social behaviour.Of nine ecological variables that were quantified, forage abundance and quality accounted best for variation in female dispersion among and within study populations. Specifically, female oribi formed larger groups and had smaller ranges where dry season forage was relatively abundant and low in fibre.Male territorial behaviour differed among sites and was related to female home range size. Males were most active in territory maintenance where females had small home ranges, and males defended a female rather than a territory where females ranged widely.These results show that variation in social organization among oribi subpopulations in Ghana reflects female responses to the availability and quality of dry season food resources and male responses to the variable distribution and ranging behaviour of females.
The importance of host spatial distribution for parasite specialization and speciation: a comparative study of bird fleas (Siphonaptera: Ceratophyllidae)
Authors: Tripet Frédéric , Christe Philippe , Møller Anders Pape 
Published: March 2003

Summary

The environment of parasites is determined largely by their hosts. Variation in host quality, abundance and spatial distribution affects the balance between selection within hosts and gene flow between hosts, and this should determine the evolution of a parasite's host‐range and its propensity to locally adapt and speciate.
We investigated the relationship between host spatial distribution and (1) parasite host range, (2) parasite mobility and (3) parasite geographical range, in a comparative study of a major group of avian ectoparasites, the birds fleas belonging to the Ceratophyllidae (Siphonaptera).
Flea species parasitizing colonial birds had narrower host ranges than those infesting territorial nesters or birds with an intermediate level of nest aggregation.
The potential mobility and geographical ranges of fleas decreased with increasing level of aggregation of their hosts and increased with the fleas’ host ranges.
Birds with aggregated nest distribution harboured more flea species mainly due to a larger number of specialists than solitarily nesting hosts.
These results emphasize the importance of host spatial distribution for the evolution of specialization, and for local adaptation and speciation in Ceratophyllid bird fleas.
Strategy and constraint in the winter fattening response to temperature in the great tit Parus major
Authors: Gosler Andrew G. 
Published: March 2003

Summary

For many small birds at temperate latitudes, the low temperatures and long nights of winter are associated with an increased risk of starvation. Birds such as the great tit Parus major L. face this by increasing their fat reserves. Theoretical, correlative and experimental evidence indicates that the amount of fat carried is not maximized for a given set of circumstances, but optimized, at least partly because of a need to trade off starvation risk against predation risk, part of which may be mass‐dependent, and therefore increased by carrying increased reserves. Evidence for such a trade‐off strategy includes the observed negative correlation between fat reserves and ambient winter temperature operating as a proximate factor.
However, the relationship between fat and temperature requires further investigation for two reasons. First, field evidence that temperature acts as a proximate factor is equivocal. Secondly, measures of temperature used (e.g. mean temperature over the previous 24 h) have given little resolution as to the time over which temperature effects operate, and whether low temperatures might constrain the bird's ability to fatten, in addition to signalling the need to fatten further.
This paper uses observations of fat from a wild great tit population, studied over 17 years, to address these two areas. First, it demonstrates that temperature acts as a proximate factor influencing fat reserve levels. Secondly, by relating the time of observation more precisely than previously to temperatures prevailing at particular times during the previous 24 h, it is shown that while past temperature (e.g. 5 + h previously) is used to optimize fat reserves (strategic fattening), more immediate temperature may constrain the bird's ability to achieve the optimum.
This constraint is demonstrated by a positive correlation between fat reserves and current temperature; probably the first demonstration of such a relationship. However, this constraining effect of temperature has diminished over the last 17 years, presumably because of climate amelioration.
Parasite transmission: reconciling theory and reality
Authors: Fenton A. , Fairbairn J. P. , Norman R. , Hudson P. J. 
Published: March 2003

Summary

Arguably the most important and elusive component of host–parasite models is the transmission function. Considerable empirical and theoretical work has focused on determining the correct formulation of this function although, to date, there has been little attempt to combine these studies to develop general insights into how observed transmission rates affect host–parasite dynamics.
Here, estimates of transmission rates from a range of host–parasite systems in the literature are described using a phenomenological function which takes into account how transmission varies with host and parasite densities. This function is placed in the appropriate model framework to determine the consequences of the observed transmission rates for each system.
All of the parasites had decreasing per capita transmission rates with increasing parasite densities suggesting that parasites tend to saturate at high densities, either as hosts become limiting or due to heterogeneities amongst the host population. In terms of the responses to host density, the parasites fell into two groups: those with increasing or decreasing transmission rates. This dichotomy was due to the biology of the organisms; the former group infect through cannibalism, which increased at high densities as the individuals became stressed, whereas the latter group infected through free‐living stages, resulting in a form of spatial structuring reducing the number of hosts available for infection.
A metapopulation model was developed where hosts and parasites interacted in discrete patches according to the appropriate transmission function, with neighbouring patches linked by dispersal. The model suggested that small‐scale, localized transmission events can drive large‐scale epizootics at the metapopulation level. This emphasizes the importance of correctly describing and quantifying the transmission function at the individual level.
Traditionally, the formulation of the transmission function has depended on the scale of observation. This work shows that transmission should be considered from the viewpoint of the organisms concerned. Observed transmission rates are a consequence of the biology of the individuals meaning it should be possible to develop a priori hypotheses concerning the nature of the transmission function from a basic understanding of the life history of the organisms concerned.
Group living and investment in immune defence: an interspecific analysis
Authors: Wilson Kenneth , Knell Robert , Boots Michael , Koch‐Osborne Jane 
Published: March 2003

SummarySince parasite transmission is often density‐dependent, group living is normally thought to lead to an increased exposure to parasitism. As a consequence, it is predicted that animals living in groups will invest more resources (energy, time, risk, etc.) in parasite defence than those living solitarily.We tested this prediction by measuring basal immune parameters in the larvae of 12 species of Lepidoptera, grouped into six phylogenetically matched species‐pairs, each comprising one solitary feeding and one gregariously feeding species.Contrary to expectation, the solitary species in all six species‐pairs had higher total haemocyte counts than the gregarious species, and in five out of six species‐pairs the solitary species also exhibited higher phenoloxidase activity. Both measurements were positively correlated with each other and with the magnitude of the cellular encapsulation response.The relationship between infection risk and group living was investigated with a dynamic, spatially explicit, host–pathogen model. This shows that when individuals aggregate in groups, theper capitarisk of infection can be reduced if the lower between‐group transmission more than compensates for the higher within‐group transmission.We conclude that the expectation that group living always leads to increased exposure to pathogens and parasites is overly simplistic, and that the specific details of the social system in question will determine if there is increased or decreased exposure to infection.
Size‐ and density‐dependent habitat use in predators: consequences for habitat shifts in young fish
Authors: Byström Pär , Persson Lennart , Wahlström Eva , Westman Erika 
Published: March 2003

Summary

When different size classes of predators use different habitats, the habitat‐specific predation risk for prey may change as prey increase in size due to the size‐dependency in predator–prey interactions.
To study the effects of predation risk on habitat shifts in growing prey, we used a combination of whole‐lake manipulations and field data, covering 5 years from three lakes on (1) body sizes at habitat shifts in young‐of‐the‐year perch (YOY) perch (Perca fluviatilis) and (2) habitat use and densities of two size classes of cannibals (1 year old and ≥ 2 years old).
YOY perch shifted habitat to the pelagic immediately after hatching and exhibited a second shift back to the littoral zone at a size range of 11–30 mm. Cannibalism from 1‐year‐old perch decreased over time, whereas cannibalism from perch ≥ 2 years old increased over time but did not include early larval stages. The habitat use of 1‐year‐old perch was restricted to the littoral habitat, whereas perch ≥ 2 years old used both the pelagic and the littoral habitats.
Habitat shifts in YOY perch were unrelated to differences in resource levels between the two habitats as YOY perch were either yolk sac feeding (first habitat shift) or had similar foraging gains in both habitats (second habitat shift).
The size when YOY perch shifted back to the littoral zone was related positively to the densities of 1‐year‐old perch, whereas no clear relationship was found between shift size and the density of perch 2 ≥ years old. The latter was suggested to be due to the higher density and the larger variation in densities of 1‐year‐old perch and that the predation risk from perch ≥ 2 years old was less habitat‐specific.
Our results suggest that size‐dependent habitat use and size specific predation risk evoked by cannibals affect the size when YOY perch shift back to the littoral zone.
Effects of phosphate on the reproductive symbiosis between bitterling and freshwater mussels: implications for conservation
Authors: REYNOLDS JOHN D. , GUILLAUME HENRI P. 
Published: March 2003

SummaryWe examined the effects of phosphorus, an important pollutant in fresh waters, on musselsAnodonta anatinaand embryos of European bitterlingRhodeus sericeus amarus. These species are involved in an unusual symbiosis, with the fish depositing eggs inside the mussels' gills, where the embryos develop for 3–6 weeks until the larvae leave the mussel.High concentrations of phosphate (500 μg L−1and higher) caused a significant increase in the rate of premature expulsion and emergence of bitterling embryos from the mussels, with a concomitant increase in fish mortality.At concentrations of 750 μg L−1and higher, mussels reduced their frequency of ventilation and movements through the substratum.These results have implications for understanding the nature of the symbiosis between the fish and their mussel hosts, showing that mussels have control over bitterling developing in their gills. The premature expulsion of bitterling embryos at high phosphate levels is detrimental to the fish that they harbour.Recommendations for the conservation of bitterling and species of mussels, which are threatened or endangered over parts of their ranges, include reductions in levels of phosphorus in natural habitats and the need for low levels in reintroduction sites for the fish and their mussel hosts.
The effect of predation by wintering cormorants Phalacrocorax carbo on grayling Thymallus thymallus and trout (Salmonidae) populations: two case studies from Swiss rivers. Reply
Authors: SUTER WERNER 
Published: March 2003

SummaryData relating to possible predatory effects of cormorants Phalacrocorax carbo (L.) on grayling Thymallus thymallus (L.) in the river Rhine did not support hypotheses of overfishing, but were consistent with changes in the fish population resulting from variable recruitment and density‐dependent effects (Suter 1995). In a government report, fisheries managers using the same data claimed to have found a causal relationship between cormorant predation and grayling decline (Staub et al. 1992). In their comment, these authors argue that certain data transformations and statistical procedures in my paper were responsible for the different outcome (Staub et al. 1998). However, the authors are unable to quantify any effects of procedures they question, and resort to vague reasoning. Their technical objections are either irrelevant or misleading, since they repeatedly make wrong assumptions or misinterpret parts of my analysis. Different interpretations of predation effects are rather a consequence of the shortcomings of the original report: Staub et al. (1992) had not applied any statistical treatment to the data not tested any hypotheses, but had arrived at their ‘conclusions’ simply by looking at a number of raw graphs depicting grayling catches grouped by fish‐length classes. Moreover, their approach (both in the original report and in the present comment) suffers from a lack of biological understanding, particularly of dynamic processes in fish populations and density‐related effects.
Attempts to control tuberculosis in cattle by removing infected badgers: constraints imposed by live test sensitivity
Authors: Woodroffe Rosie , Frost Simon D.W. , Clifton‐Hadley Richard S. 
Published: March 2003

Summary 1. Bovine tuberculosis is a serious disease of cattle caused by the bacillus Mycobacterium bovis. In south‐west England, badgers Meles meles sustain endemic M. bovis infection and almost certainly transmit the disease to cattle. When tuberculosis outbreaks have occurred in cattle, the Ministry of Agriculture, Fisheries and Food (MAFF) therefore culled badgers to try to avert further outbreaks. 2. To limit the number of badgers killed, MAFF has assessed a possible new strategy (the ‘live test strategy’) that used a serological test to identify and remove infected badgers. However, because the test correctly identified only 41% of truly infected badgers, individuals were pooled according to the setts at which they were sampled. All badgers were culled at setts where one or more seropositive animals were caught. 3. On average, 1·9 ± 1·4 (SD) badgers were sampled at each sett. Using a simple model, we show that this level of sampling still gives a low (24–37%) probability of detecting infection at a given sett. 4. Badger social groups typically occupy more than one sett. We allocated setts to social groups by using Dirichlet tessellations and field signs to predict territory borders. On average, 3·3 ± 2·8 badgers were sampled in each group. Our model shows that this increase in sample size gives probabilities of detecting M. bovis in truly infected groups of 43–62%, which is still likely to be unacceptably low. 5. Culling badgers according to the setts where they were trapped led to incomplete removal of social groups; some seronegative badgers were released in 61% of groups containing seropositive animals. As infection is clustered within groups, it is likely that some infected animals were released even though they tested seronegative. Incomplete removal might also cause social disruption that could accelerate the transmission of M. bovis between social groups. 6. We conclude that the live test strategy, as implemented, would be unlikely to reduce the overall prevalence of M. bovis infection in badgers, and thus the risk to cattle. Furthermore, the poor sensitivity of the serological test makes it unlikely that modifications to the live test protocol could increase its cost‐effectiveness.
Variation in standing stock of the mackerel icefish Champsocephalus gunnari at South Georgia
Authors: Everson Inigo , Parkes Graeme , Kock Karl‐Hermann , Boyd Ian L. 
Published: March 2003

Summary 1. Bottom trawl surveys on the South Georgia shelf indicate episodic declines in the abundance of the mackerel icefish Champsocephalus gunnari that, since 1990, are not directly attributable to commercial fishing. 2. The greatest effect has been observed in years when krill are known to have been scarce on the South Georgia shelf. 3. It is thought that C. gunnari survivorship is closely related to, but indirectly influenced by, krill availability. 4. Examination of food chain relationships indicates that predation by fur seals is likely to play a major role in the observed changes in C. gunnari abundance. 5. Periodic shifts in food chain relationships, involving krill, C. gunnari and fur seals, are postulated that could explain the episodic declines in the abundance of C. gunnari in the absence of fishing. 6. The study demonstrates how a key predator can have an over‐riding impact on a commercial fish species, highlighting the importance of the ecosystem approach to fisheries management.
The impact of American mink Mustela vison on water birds in the upper Thames
Authors: Ferreras P. , Macdonald D.W. 
Published: March 2003

Summary 1. The effect of mink predation on water birds during the breeding season was studied between March and September 1996 in a 33‐km long stretch of the upper Thames river, England. 2. Mink presence significantly affected the density of breeding coots and the number of chicks hatched per pair of coots, as well as the average number of nests per pair of moorhens and the percentage of moorhen clutches hatched. 3. Mink diet during the birds’ breeding season (March–September) was studied through scat analysis. Ralliformes (coots or moorhens) represented 10% of the ingested biomass and were the fourth prey in importance after rabbits (45%), fish (25%) and small mammals (14%). Mink obtained 11% of their energy requirements from coots and moorhens. 4. Impact of predation by mink during the bird breeding season was moderate to high for moorhens (16–27% of adults and 46–79% of broods) and high for coots (30–51% of adults and 50–86% of broods). 5. Although moorhens seem well adapted to withstand predation by mink, nesting behaviour by coots make them very vulnerable to mink predation. We hypothesize that the persistence of coot populations in areas with high mink density requires immigration from surrounding populations with lower mink impact.
Alate production by aphids on sugar beet: how likely is the evolution of sugar beet‐specific biotypes?
Authors: Williams I.S. , Dewar A.M. , Dixon A.F.G. , Thornhill W.A. 
Published: March 2003

Summary 1. Aphid clones that feed on the same plant species for many generations often develop biotypes specifically adapted to the particular host‐plant. Such biotypes can be particularly damaging to crops. In the case of sugar beet Beta vulgaris, which is not available for colonization for parts of the year, aphids must migrate into and out of the crop each year if host‐specific biotypes are to evolve. The likelihood of biotypes evolving to be specifically adapted to sugar beet was examined and the results are discussed in relation to the control of aphids on this crop. 2. The frequency and causes of alate production by aphids on sugar beet were investigated by examining natural field populations, manipulated field populations and laboratory populations. Data from these studies were used in a model that estimated the probability of an aphid clone on sugar beet successfully migrating from the crop, surviving the winter and migrating back on to the crop the following year. 3. In 2 years out of 4, no alatoid nymphs of the aphids Myzus persicae and Macrosiphum euphorbiae were produced on naturally infested crops of sugar beet, and negligible numbers were produced in the other 2 years. 4. In spring, a much greater proportion of the My. persicae population was alatoid on oilseed rape Brassica napus than on sugar beet. Similarly in laboratory studies, a greater proportion of My. persicae was alatoid on Chinese cabbage Brassica pekinensis than on sugar beet at the same levels of crowding. The colour of the alatoid nymphs reared on Chinese cabbage indicated that it was a more suitable host than sugar beet and this might account for the greater alate production. Infection of sugar beet with beet yellows virus did not increase alate production by My. persicae. 5. A deterministic model showed that the probability of an alate My. persicae migrating from sugar beet and recolonizing sugar beet the following year would, at best, be only 1 in 40 000. 6. As there is negligible transfer of genetic material from year to year, we suggest that biotypes of My. persicae specifically adapted to sugar beet are unlikely to evolve. As a result, if particular aphicides were only used on sugar beet, resistance to those aphicides would be unlikely to evolve because any new resistance would not be transmitted from year to year.