bluuebunny/crossref_metadata_2025_split huggingface dataset:
The effect of dispersal on the population dynamics of a gall‐forming midge and its parasitoids
Authors: Briggs Cheryl J. , Latto John 
Published: March 2003

Summary 1. Theoretical studies predict that limited amounts of dispersal of individuals in host‐parasitoid systems can both enhance the stability of the subpopulations and promote the co‐existence of competing parasitoid species. We investigated the effects of dispersal on the population dynamics and parasitoid community structure of a natural host‐multi‐parasitoid system consisting of the midge Rhopalomyia californica that forms galls on the shrub Baccharis pilularis and the parasitoids that attack the midge. 2. An experiment involving the release of midges into a field with a low background density of galls demonstrated that the midges, on average, travelled approximately 1·7m in their lifetime. This suggests that the appropriate spatial scale at which to look at the effects of dispersal is relatively small. 3. Dispersal of midges and parasitoids between individual bushes was experimentally eliminated in a caging experiment. The midge populations in all of the uncaged replicates displayed dynamics that were similar to each other, while the dynamics of the midge populations in the caged replicates diverged. The midge dynamics on the uncaged bushes were not significantly more stable than those on the caged bushes. 4. Dispersal among bushes was found to play a major role in co‐existence of the competing parasitoid species. There was a dramatic drop in the parasitoid species diversity on the caged bushes, with only a single parasitoid species, Platygaster californica, persisting at high numbers in the caged populations. In accordance with theoretical models, P. californica is the parasitoid species in the community that has the highest attack rate and is most effective at searching for hosts in a restricted area. Alternative explanations for this pattern are discussed.
State‐dependent behaviour and energy expenditure: an experimental study of European robins on winter territories
Authors: Godfrey J. D. , Bryant D. M. 
Published: March 2003

Summary 1. We sought evidence for state‐dependent behaviour and energy expenditure by manipulating the body‐state of birds. We reviewed three hypotheses that could describe responses to a change in body‐state: the Increased Effort; Reduced Effort; and Balanced Effort Hypotheses. 2. Territorial European robins Erithacus rubecula L. were captured at dusk during winter, and chilled or warmed overnight to manipulate body‐state. 3. The robins were released onto their territories before dawn. Their subsequent behaviour was monitored by radio‐tracking, and energy expenditure was measured using the doubly labelled water technique. 4. Robins with high overnight energy expenditures (= ‘negative’ body‐state; chilled) were less likely to initiate vocal behaviour, and less likely to respond to the song or calls of conspecifics than those with low overnight expenditures (= ‘positive’ body‐state; warmed). 5. Chilled birds moved around their territories at almost twice the rate of warmed birds, and had significantly higher daytime energy expenditures. 6. We concluded that behaviour, including vocalizations and activity levels, and energy expenditures of robins, were sensitive to manipulations of body‐state. 7. These behaviours are interpreted as adaptive responses to restore condition, while minimizing the risk of mortality from predation and starvation. Our results were consistent with the Balanced Effort Hypothesis, entailing an increase in energy expenditure on foraging and a reduction in expenditure on non‐maintenance activities. 8. We suggest that inter‐individual differences in body‐state may be a key cause of previously unexplained variation in daily energy expenditure.
Apparent inferiority of first‐time breeders in the kittiwake: the role of heterogeneity among age classes
Authors: Cam Emmanuelle , Monnat Jean‐Yves 
Published: March 2003

Summary 1. Many studies have provided evidence that first‐time breeders have a lower survival, a lower probability of success, or of breeding, in the following year. Hypotheses based on reproductive costs have often been proposed to explain this. However, because of the intrinsic relationship between age and experience, the apparent inferiority of first‐time breeders at the population level may result from selection, and experience may not influence performance within each individual. In this paper we address the question of phenotypic correlations between fitness components. This addresses differences in individual quality, a prerequisite for a selection process to occur. We also test the hypothesis of an influence of experience on these components while taking age and reproductive success into account: two factors likely to play a key role in a selection process. 2. Using data from a long‐term study on the kittiwake, we found that first‐time breeders have a lower probability of success, a lower survival and a lower probability of breeding in the next year than experienced breeders. However, neither experienced nor inexperienced breeders have a lower survival or a lower probability of breeding in the following year than birds that skipped a breeding opportunity. This suggests heterogeneity in quality among individuals. 3. Failed birds have a lower survival and a lower probability of breeding in the following year regardless of experience. This can be interpreted in the light of the selection hypothesis. The inferiority of inexperienced breeders may be linked to a higher proportion of lower‐quality individuals in younger age classes. When age and breeding success are controlled for, there is no evidence of an influence of experience on survival or future breeding probability. 4. Using data from individuals whose reproductive life lasted the same number of years, we investigated the influence of experience on reproductive performance within individuals. There is no strong evidence that a process operating within individuals explains the improvement in performance observed at the population level.
Non‐linear transmission and simple models for bovine tuberculosis
Authors: Barlow N. D. 
Published: March 2003

1. A new model is presented for a possum–tuberculosis (TB) system (Trichosurus vulpecula–Mycobacterium bovis) that is both realistic and parsimonious. The model includes a phenomenological treatment of heterogeneity of risk for susceptible hosts, similar to that used in insect host–parasitoid systems. 2. Parameter values for the model reflect current knowledge and differ significantly from those in other recent models of this system. Associated with these structural and parametric changes are substantially different predictions for the dynamics and control of TB in possums. 3. The model predictions include (i) only limited host suppression due to the disease (< 10%, cf. several earlier simple models for TB in both possums and badgers); (ii) asymptotically stable disease dynamics (cf. homogeneous‐mixing models that predict either extremely weak stability such that disease fails to recover when host density is temporarily reduced, or oscillatory behaviour and potential elimination of disease following such a perturbation); (iii) TB that is harder to control than in the homogeneous‐mixing model equivalents, in line with practical experience; and (iv) a threshold host density for disease elimination that differs substantially from the host equilibrium density in the presence of disease. 4. Homogeneous‐mixing models are unable to reproduce this behaviour, whatever parameter values are chosen. Heterogeneous‐mixing models with non‐linear transmission may therefore be worth consideration in other endemic wildlife disease systems, as is now commonplace for insect–parasitoid and insect–pathogen ones.
Spatial perturbation caused by a badger (Meles meles) culling operation: implications for the function of territoriality and the control of bovine tuberculosis (Mycobacterium bovis)
Authors: Tuyttens F. A. M. , Delahay R. J. , Macdonald D. W. , Cheeseman C. L. , Long B. , Donnelly C. A. 
Published: March 2003

Summary 1. The spatial organization of a badger population (North Nibley) is described before and after it was subjected to a UK Ministry of Agriculture, Fisheries and Food badger removal operation (BRO) intended to control bovine tuberculosis. Comparison is made with an undisturbed badger population (Woodchester Park). 2. The Woodchester Park population was organized in group territories with clearly defined boundaries that remained stable during the 3 years of study (1995–97). In North Nibley, however, the badgers' spatial organization was severely perturbed in the first year and, to a lesser extent, also in the second year after the BRO, with badgers using latrines further away from their setts. This resulted in enlarged social group ranges that were difficult to define and overlapped considerably. 3. The disturbance was observed in the removal groups, those immediately adjacent, as well as those at a distance of one or two social groups from the removal area, with an unexpected indication that the latter groups may have been the most affected. 4. The apparent increase in the size of the group ranges in North Nibley was likely to have been caused by an increased proportion of badgers making extra‐group excursions in the aftermath of the BRO. 5. Initial recolonization was almost exclusively by females. 6. Although such perturbation might be expected to facilitate disease transmission between badger social groups, there was no evidence that any infectious animals had survived the BRO. However, there were further cattle breakdowns in the area. 7. The behaviour of badgers after the BRO also provided an opportunity to test predictions made by competing hypotheses about the main determinants of the badger's socio‐spatial behaviour.
The role of shared parasites in the exclusion of wildlife hosts:Heterakis gallinarumin the ring‐necked pheasant and the grey partridge
Authors: Tompkins D. M. , Greenman J. V. , Robertson P. A. , Hudson P. J. 
Published: March 2003

Summary1. A two‐host shared‐macroparasite model was parameterized from the results of infection and transmission experiments, to investigate whether apparent competition between the ring‐necked pheasant (Phasianus colchicus) and the grey partridge (Perdix perdix), mediated via the shared nematodeHeterakis gallinarum,could theoretically cause partridge exclusion.2. Both the model created and the experiments conducted show that the bulk ofH. gallinaruminfection to partridges, when they occur in the same locations as pheasants, will be from the pheasants and not from the partridges themselves. This is due toR0for the parasite being 1·23 when infecting pheasants, but only 0·0057 when infecting partridges. Thus, when the pheasant is present in the model the partridge population is impacted by the shared parasite but, when the pheasant is absent, the parasite is lost from the system.3. Based on best available parameter estimates, the observed impact ofH. gallinarumon the grey partridge may be sufficient to cause exclusion when the pheasant is present in the model. This supports the hypothesis that the UK grey partridge decline observed over the past 50 years may be partly due to apparent competition with pheasants.4. Habitat separation between the two host species, where it decreases the rate ofH. gallinarumtransmission from the pheasant to the partridge, may allow them to co‐exist in the field in the presence of the parasite. We predict, however, that grey partridge exclusion would still occur if separation was less than 43%.
Quantitative webs as a means of assessing the impact of alien insects
Authors: Schönrogge K. , Crawley M. J. 
Published: March 2003

Summary1.We use quantitative linkage webs to investigate the impact of alien gall wasps on community structure. Britain has been invaded by four alien species of cynipid gallwasp,Andricus corruptrix, A. lignicola, A. kollari and A. quercuscalicis, over the last 150 years. To date, Britain can be divided into four zones from the north to the south with one, two, three and four invading species established in each zone.2.The four species are naturalized in their new ranges and are locally the most abundant cynipid species, especially in their spring (sexual) generations. Like the native cynipid species they showed dramatic changes (up to three orders of magnitude) in density between generations, and the dominance structure of alien and native host species changed radically from generation to generation.3.All four invading cynipid species were attacked by native parasitoid species. Using quantified linkage webs, we assess the contribution made by individual host gall species to each parasitoids population size. Although the parasitoid species have been described as broadly polyphagous, suggesting that the aliens should be richly linked with the native cynipid communities, we found that the galls of the invading species have become the main, and in a few cases the sole, contributors to local parasitoid populations, indicating major host shifts by the parasitoid species.4.Within generations we found very little overlap among the parasitoid and inquiline communities associated with native and alien galls within generations. Similarly, the quantification of indirect interactions among cynipids between generations suggests that parasitoids and inquilines are not main factors in the dynamics of local cynipid communities. The recruitment of parasitoids and inquilines by the invading cynipid species is therefore unlikely to have a strong affect on native cynipid species.
Size distribution dynamics for a marine sessile organism with space‐limitation in growth and recruitment: application to a coral population
Authors: Muko Soyoka , Sakai Kazuhiko , Iwasa Yoh 
Published: March 2003

Summary

Corals are clonal organisms and show a plastic growth. We study a partial differential equation model for the dynamics of size distribution of corals and predict the trajectory of recovery after a catastrophic disturbance, such as the recent bleaching that killed most corals in southern Japan.
We assume that the mean growth rate of colony size, measured in projected area, is a linear function of colony size, and that the variance in growth rate is proportional to the size, which is consistent with the growth data of a coral Acropora hyacinthus Dana 1846.
The model incorporates the space‐limitation in colony growth and recruitment. The growth rate and recruitment rate are proportional to the fraction of free space within the local habitat.
In many corals, including A. hyacinthus, recruitment occurs in a short period once a year. However, our model illustrates that the colony size distribution does not show distinguishable cohorts since the observed variance in growth rate is large. The model with discrete settlement and a large variance in growth rate results in size distributions that are very well approximated by an explicitly soluble model with constant recruitment and no growth variance.
When mortality is low, the dynamics of size distribution show two different phases in the recovery process. In the first phase, size distribution is determined by recruitment and growth, and can be predicted well by the case without mortality. After free space is depleted, recruitment and growth slow down and become balanced with mortality. The equilibrium size distribution is controlled by all the three processes. Both for the transient and the equilibrium size distribution, the average colony size increases with growth rate but decreases with recruitment rate.
Strongly skewed colony‐size distributions in which small size classes have the largest numbers are generated for a wide range of parameters by the space‐limitation in growth, even without partial death of colonies.
Numerical fluctuations in the northern short‐tailed shrew: evidence of non‐linear feedback signatures on population dynamics and demography
Authors: Lima Mauricio , Merritt Joseph F. , Bozinovic Francisco 
Published: March 2003

Summary We studied a fluctuating population of the northern short‐tailed shrew (Blarina brevicauda) in the Appalachian Plateau Province of Pennsylvania, USA, spanning 21 years of monitoring. We analysed the pattern of annual temporal variation fitting both time‐series models and capture–mark–recapture (CMR) statistical models for survival and recruitment rates. We determined that non‐linear first‐order models explain almost 80% of the variation in annual per capita population growth rates. In particular, a non‐linear self‐excited threshold autoregressive (SETAR) model describes the time‐series data well. Average snowfall showed positive and non‐linear effects on population dynamics. The CMR statistical models showed that a non‐linear threshold model with strong effects of population density was the best one to describe temporal variation in survival rates. On the other hand, population density or climatic variables did not explain temporal variation in recruitment rates. Survival rates were high during the study period. Weekly changes in population size attributable to new recruits entering in the population fluctuate between 21% and 0%, while the changes in population size related to survival fluctuate between 79% and 100%. Two important results arise from this study. First, non‐linear models with first‐order feedback appear to capture the essential features of northern short‐tailed shrew dynamics and demography. Secondly, climate effects represented by snowfall appear to be small and non‐linear on this insectivore. The population dynamics of this shrew in the Appalachian Plateau are determined apparently by a strong non‐linear first‐order feedback process, which is related to survival rates. This study links population dynamics and demography by detecting the underlying demographic mechanisms driving population dynamics. The feedback structure of this shrew suggests the existence of population dynamics dominated by intraspecific competitive interactions, such as aggression, solitary nesting, non‐overlapping home ranges and territoriality.
The effects of cowpox virus on survival in natural rodent populations: increases and decreases
Authors: Telfer Sandra , Bennett Malcolm , Bown Kevin , Cavanagh Rachel , Crespin Laurent , Hazel Sarah , Jones Trevor , Begon Michael 
Published: March 2003

SummaryThe effect of cowpox virus on survival in two rodent hosts was investigated using nearly 4 years of longitudinal data from two sites.We investigated whether an individual's probability of infection influenced the probability of surviving the next month. We also investigated the effect at the population level, examining whether, in addition to seasonal effects, changes in cowpox prevalence explained further temporal variation in survival rates.In bank voles, but not wood mice, individuals with high probabilities of infection survived better than uninfected animals.At the level of the population, the effect of infection on survival varied through the year in both species. Survival rates in late summer increased with cowpox prevalence, whilst survival rates in winter decreased with cowpox prevalence.We discuss why parasites such as cowpox virus may increase or decrease host survival and why the effect may depend on the time of year.
A comparison of the power of categorical and correlational tests applied to community ecology data from gradient studies
Authors: Somerfield P. J. , Clarke K. R. , Olsgard F. 
Published: March 2003

SummaryWhere gradients exist it is possible for a global test of community change to fail to achieve significance even though pairwise tests between groups of samples from opposite ends of gradients reveal significant differences. This study examines the power of alternative tests in situations where spatial (or temporal) gradients exist.Data from an oilfield survey in the Norwegian sector of the North Sea are used to examine the power of two types of test. There is no theoretical statistical framework with which to examine the formal power of the non‐parametric multivariate tests. A univariate analogue is used to demonstrate the behaviour and relative power of linear regression andanovato detect different strengths of gradients.Two multivariate tests are then contrasted using simulations.anosim(Analysis of Similarities) is a categorical analysis and, for an ecological matrix of pairwise dissimilarities among all samples, tests null hypotheses of theanovatype: H0: There are no differences between groups of samples. This test is compared withrelate, a non‐parametric Mantel test which is used here to test the null hypothesis: H0: The dissimilarities among samples in the ecological matrix are not (monotonically) correlated with corresponding ‘model’ distances between samples along the gradient.It is demonstrated that, where a gradient is present in the data, an appropriate univariate test based on linear regression always has greater power thananovato detect it. This is especially true where the gradient is weak and/or replication is low.It is shown that the multivariate tests behave in a similar way to their univariate analogues. Where there is a detectable gradient in the data the correlative test (relate) has greater potential to detect it than does the categorical testanosim. The differences in power are especially apparent at low to intermediate strengths of gradient. Distributing replicates among more groups decreases the power of both tests, but especiallyanosim, to detect a constant gradient effect.Although demonstrated using a practical application, the findings presented here are general in nature and applicable to any ecological investigation in which a gradient in response may be hypothesized.