bluuebunny/crossref_metadata_2025_split huggingface dataset:
Does Holling’s disc equation explain the functional response of a kleptoparasite?
Authors: Caldow R. W. G. , Furness R. W. 
Published: March 2003

Summary

Type II functional responses, which can be described by Holling’s disc equation, have been found in many studies of predator/prey and host/parasite interactions. However, an increasing number of studies have shown that the assumptions on which the disc equation is based do not necessarily hold. We examine the functional response of kleptoparasitically feeding Arctic skuas (Stercorarius parasiticus L.) to the abundance of fish‐carrying auks and, by examination of the assumptions of the disc equation, test whether it can explain the function.
The rate at which individual skuas make successful chases is a decelerating function of the abundance of auks. However, it would appear that this is not determined by factors that influence their probability of success, but by the rate at which they initiate chases. This too is a decelerating function of the abundance of auks. Arctic skuas have a Type II functional response.
Although Arctic skuas exhibited a direct numerical response there was no evidence that components of predation connected to the density of predators (direct prey stealing, or increased host avoidance) had any effect on the rate at which individual skuas made chases or were successful in their chases. We conclude that the observed functional response is free from any effects of interference.
We suggest that abnormally high levels of foraging effort expended by breeding skuas and their poor breeding success in the years of observation argue against the limit to the observed functional response being determined by skuas’ energetic requirements.
Several of the assumptions underlying the disc equation do not hold. The duration of chases (handling time) was not a constant; it decreased with increasing host abundance. Moreover, the chase duration predicted by the disc equation, if handling time limited the functional response, was far in excess of that observed. Furthermore, the observed rate of decline in the searching time per victim with increasing host abundance suggested that skuas’ instantaneous rate of discovery was also not constant. Possible reasons for these observations are discussed. The basic disc equation may describe Arctic skuas’ functional response, but it cannot explain it.
Condition‐dependent dispersal and ontogeny of the dispersal behaviour: an experimental approach
Authors: Massot M. , Clobert J. , Lorenzon P. , Rossi J.‐M. 
Published: March 2003

SummaryAlthough little investigated, developmental processes that generate dispersal condition evolution of this behaviour. We have shown previously prenatal and postnatal influences on dispersal in the common lizard (Lacerta vivipara). The observation of these developmental processes was based on independent experiments; our primary goal in this paper is to test their interactions. Interactions could indeed be a source of inconsistencies in studies because they can mask, or even reverse, effects of factors treated additively.We studied dispersal of juveniles releasedin naturafrom 416 pregnant females captured in the field. We used a factorial design to test interactions among the maternal habitat (dry vs. humid), prenatal conditions (temperature, humidity), and postnatal environments (dry vs. humid).We found that juvenile dispersal was dependent on the humidity level at different developmental stages, but with varying and sometimes opposite effects. Dispersal was also influenced by the temperature during gestation and by populational differences not related to humidity (differences between replicated populations for the maternal and postnatal habitats).These results confirm our previous findings that dispersal of the common lizard is condition‐dependent and has multiple causation. In addition, most of the effects exhibited interactions, and the ontogeny of dispersal appeared as a sequential process where the maternal habitat conditioned prenatal influences, and the prenatal environment modulated postnatal influences.The robustness of our results is supported by the finding of the same interactions in independent tests on both juvenile males and females. This militates in favour of future studies on the multiple causation of dispersal because the same dispersal status might originate from different causes, and different dispersal outcomes might be due to the same factor.
Density‐dependent populations require density‐dependent elasticity analysis: an illustration using the LPA model of Tribolium
Authors: Grant Alastair , Benton Tim G. 
Published: March 2003

Summary

Elasticity and sensitivity analyses are used widely in evolutionary biology, ecology and population management. However, almost all applications ignore density dependence, despite the widespread assumption that density dependence is ubiquitous. We assess whether this matters by comparing density‐dependent and density‐independent elasticity analyses for the LPA model of Tribolium.
Density‐independent elasticities of λ are a poor indicator of the effects of changes in demographic parameters on population size, even for populations at stable equilibrium. With non‐equilibrium dynamics, the divergence can be particularly large. In the extreme, a change in a demographic parameter with a positive effect on individual fitness can reduce mean population size, so even the sign of a density‐independent elasticity may be wrong. Elasticities of larval, pupal and adult numbers are not proportional to each other, neither are they proportional to elasticities of total population size.
A full density‐dependent analysis is therefore vital when concerned with effects on population numbers, as in population management, pest control and prediction of population effects of toxins.
When examining the consequences for individual fitness of changes in demographic parameters, density‐independent elasticities provide a more useful approximation to the density‐dependent values. However, they fail to detect cases where non‐equilibrium dynamics means that particular life histories gain an advantage by exploiting predictable periods when density dependence is relaxed.
This phenomenon can produce a marked change in the pattern of elasticities as a bifurcation is crossed. The corresponding changes in selection pressures may act to stabilize dynamics in some circumstances and destabilize them in others. There is no single answer to the question of whether selection should favour equilibrium or non‐equilibrium dynamics.
Demography of the vegetable ivory palm Phytelephas seemannii in Colombia, and the impact of seed harvesting
Authors: Bernal Rodrigo 
Published: March 2003

1.  The demography of the vegetable ivory palm Phytelephas seemannii was studied on the Pacific coast of Colombia and a female‐based matrix model was used to determine the proportion of seed that can be sustainably harvested from the population. 2.  The density of the palm stands ranged from 240 to 420 adult palms ha−1. The sex ratio was 1:1 and palms of both sexes produced leaves at the same rate. Seedlings produced 1·2 leaves per year on average, juveniles 1·8, and adults 6·1–7·4. 3.  Adult females had fewer leaves than males (18·5 vs. 21·3 on average). Leaves of females lasted about 2·7 years in the crown, those of males about 3·2 years. 4.  Stems creep on the ground, growing at the apex and often dying behind, the extant portion not always reflecting the palm’s total age. The longest stem of a female palm in the study plots was 2·5 m, corresponding to an ‘extant’ age of 85 years; the longest stem recorded was that of a male (outside the plots) 10·5 m long, corresponding to an ‘extant’ age of 184 years. These figures reflect differences in growth habit, not in longevity. 5.  Reproduction began at about 24 years, when the palm still lacked an above‐ground stem. 6.  The population growth rate λ was 1·059, and was most sensitive to changes in survival of juveniles and adults, and relatively insensitive to changes in fecundity and growth. 7.  River channel migration is the most important cause of adult mortality. Phytelephas seemannii is apparently an efficient colonizer of the understorey in the late phases of riverine forest succession. 8.  The population can tolerate a harvest intensity of 86% of all seeds before λ decreases to the equilibrium level of 1·00. Monitoring of the populations under intensive harvesting is required.
The effect of predation by wintering cormorants Phalacrocorax carbo on grayling Thymallus thymallus and trout (Salmonidae) populations: two case studies from Swiss rivers. Comment
Authors: STAUB E. , EGLOFF K. , KRÄMER A. , WALTER J. 
Published: March 2003

Summary

Using the same data on the predation effect of cormorants Phalacrocorax carbo on the grayling Thymallus thymallus population located in the river Rhine below Lake Constance, Staub et al. (1992) and Suter (1995) reached contradictory conclusions (additional and compensatory mortality, respectively). This comment clarifies the reasons for the differences.
The biomass of older fish (age classes 4 and 5–7) was not judged to be decreasing by Suter because of the use of inappropriate (parametric) inferential statistics and no use of power analysis. More powerful (non‐parametric) inferential tests show a significant decrease in biomass.
Staub et al. compared fish numbers per length class in the angling catch (autumn) and in the spawning catch (spring). Differences were thought to be the effect of cormorant predation during winter. Suter converted fish numbers per length class into biomass per age class for the same comparisons. However, this conversion considered neither the length increases of grayling during winter (c. 1 cm) nor the dominance of faster growing males (> 1.5 cm longer than females of the same age) in the spawning catch. Thus, changes of the age pyramid during winter have been masked.
Do host‐plant requirements and mortality from soil cultivation determine the distribution of graminivorous sawflies on farmland?
Authors: Barker A. M. , Brown N. J. , Reynolds C. J. M. 
Published: March 2003

1.There are many arthropod species on farmland that are not economic pests but which have an important role in the wider ecology of agricultural ecosystems. Few of these have been studied in detail. This study investigated the factors underlying the distribution of one such group, graminivorous sawflies (Hymenoptera: Tenthredinidae).2.In a farm survey, sawfly larvae of the generaDolerusandPachynematuswere over six times more abundant in fields of sown perennial rye‐grassLolium perenneL. than in fields of spring barley, winter wheat or long‐established grazing pastures.3.In rearing trials the growth rates and survival of larvae of the two genera were high onL. perenne andlow on two cereals, spring barley and winter wheat. Differences were sufficiently marked to have had a strong influence on the distribution of larvae between crops in the field.4.In an experimental study, nearly twice as many adults emerged from uncultivated as from cultivated ground, suggesting that disturbance of the overwintering prepupae and pupae in the soil caused about 50% extra mortality. This mortality will have reinforced the effects of host‐plant requirements, suppressing sawfly numbers in annually cultivated cereal fields whilst numbers built up in less‐frequently cultivated grass fields.5.Sawfly abundance is known to have declined on farmland. Our results suggest that changes in patterns of crop rotation have been the main cause of this decline. Farming practice in Britain has moved away from mixed rotations alternating cereals with undersown grass fields. Modern intensive farms grow monocultures of cereal crops or of heavily grazed grass, both poor habitats for sawflies.6.Increasing insecticide use in cereals has probably been only a minor influence on population abundance of sawflies as wheat and barley fields are not high‐quality habitats.7.Provision of undisturbed grassy habitats would promote the conservation of graminivorous sawflies on farmland.
Extinction risk assessment and optimal harvesting of anchovy and sprat in the Azov Sea
Authors: Senina Inna , Tyutyunov Yuri , Arditi Roger 
Published: March 2003

1. We explored harvested fish populations in which an extinction risk exists as a consequence of a notable drop in abundance. An adequate formulation of the optimal harvesting problem must account for several criteria. Economic maximization should be balanced against minimization of extinction risk. In addition, environmental stochasticity should be taken into consideration to obtain a realistic description of the population dynamics. 2. The approach to multicriteria optimization of the harvesting policy is illustrated by models of two commercially harvested planktophagous fish found in the Azov Sea in Russia. A stochastic simulation model for the community of competing anchovy Engraulis encrasicholus and sprat Clupeonella delicatula was developed and investigated. The parameters of the model were estimated on the basis of time series of population abundance and environmental factors that influence reproduction. 3. The model was used to assess numerically the extinction risk of this exploited community. A Pareto approach was used to treat the optimization problem with two criteria: maximizing the total catch and minimizing the extinction probability. No single solution exists but a set of Pareto‐optimal fishing strategies was defined, i.e. a set of trade‐off solutions. These various solutions can easily be compared by fishery managers and experts before making final decisions. 4. It was found that the harvesting strategy that is currently applied is quite efficient since it is very close to the Pareto‐optimal set of solutions. Our recommendation is that these two populations should be considered as a community. Specifically, by increasing the harvest of endemic sprat and decreasing the harvest of the less productive migratory anchovy, fishermen will obtain a higher total harvest with only a slightly higher extinction risk.
Breeding success of the hen harrier Circus cyaneus in relation to the distribution of grouse moors and the red fox Vulpes vulpes
Authors: Green R.E. , Etheridge B. 
Published: March 2003

Summary 1. A recent analysis showed that the survival and breeding success of female hen harriers were much lower on grouse moors than on other upland land management classes in Scotland, excluding the Orkney islands: a difference attributed to persecution of hen harriers by humans on grouse moors. However, this study did not look for possible beneficial effects on hen harrier breeding success of the control by moorland gamekeepers of other predators, particularly red foxes. 2. These and other recently published data were re‐examined to see whether there was regional variation in hen harrier nest success associated with the presence or absence of foxes and the control of foxes and other predators by moorland gamekeepers. 3. The main finding of the previous analysis, that nest success was much lower on grouse moors than on moorland that was not managed for grouse or in young conifer forests, was even more striking when comparisons were made among land management classes within the same region. 4. The mean number of broods fledged per female hen harrier per year on two grouse moors at which gamekeepers prevented human interference with harrier nests and continued to control other predators was compared with the average for moorland in Scotland. Harrier productivity on these moors was much higher than the average for grouse moors, but similar to the average for moorland not managed for grouse shooting. 5. Nest success of hen harriers breeding on moorland not managed for grouse averaged 19% higher in regions where there were, on average, more grouse moors in the vicinity, but in young conifer forests success was 34% lower in regions with more grouse moors nearby. Neither of these differences was statistically significant. 6. Nest success within a particular land management class was not significantly different inside and outside the range of the red fox. 7. Even a generous assessment of the magnitude of a supposed beneficial effect of the control of foxes and other predators by moorland gamekeepers on hen harrier nest success indicated that its effect on the population trend of hen harriers would be small relative to the large negative effect of persecution of harriers on grouse moors.
Does culling predatory gulls enhance the productivity of breeding common terns?
Authors: Magella Guillemette , Brousseau Pierre 
Published: March 2003

Summary

Large gulls Larus spp. are voracious predators of eggs and chicks of other colonial birds and may threaten rare or endangered species. In this study we tested the effectiveness of removing individual predatory gulls as a management technique for enhancing the productivity of common terns Sterna hirundo nesting in Carleton, Québec, Canada.
The productivity and fate of common tern chicks were assessed by following ringed individuals from hatching to fledging during three breeding seasons (1993–95). Concurrently, predation and consumption rates of all predatory gulls were measured before and after the culling started. The culling programme was conducted serially in 1994 by removing the most important predator first until all predators were removed.
The rate of chick disappearance was lower and the life span of tern broods was higher in 1994 when the culling was conducted, compared with 1993 and 1995. As a result, the productivity of the tern colony was zero in 1993 and 1995, but positive in 1994 (0·33 chicks pair−1). Measurements of chick mass in 1993 and 1994 showed that growth was normal, indicating that poor feeding conditions or disease were not the cause of chick disappearance.
Average predation rates for 1993 (23·3 chicks day−1) and 1995 (14·8 chicks day−1) equated to 61% and 66% of available chicks being taken by gulls, respectively. The predation rate before the culling started in 1994 was similar to 1993 and 1995, with 15·9 chicks day−1, but dropped to 5·1 chicks day−1 after the first gull was shot, and decreased to zero once all predatory gulls were removed. Only five individual predatory gulls were identified during the cull.
Predation rates differed markedly amongst specialist predatory gulls, with one individual accounting for 85% of all successful attempts made during the baseline period. Once that gull was removed, the remaining predators increased their predation rate in a manner suggestive of a despotic system. Observations conducted in 1995 showed that the predation rate was almost zero at the beginning of the season but increased dramatically later in the summer, with two gulls together making about 60% of the captures.
It is concluded that culling predatory gulls can be an effective management tool to enhance productivity in sensitive or endangered species. However, our data suggest that such culling would need to be repeated each year in order to protect a sensitive species over consecutive years.
Species accumulation curves and the measure of sampling effort
Authors: Willott S.J. 
Published: March 2003

Summary

Moreno & Halffter (2000) described the problems associated with comparing species richness among communities that have inventories compiled using different methods or with different sampling effort. They used species accumulation curves to standardize samples among sites, to predict the species richness of sites and to estimate the minimum effort required for adequate completeness of inventories.
I argue that their measure of sampling effort, number of nights, is inappropriate because it does not distinguish between genuine differences in species richness among sites and differences in trap efficiency. The number of individuals is the best measure of sampling effort to avoid this problem, as illustrated by data on moths collected from a Bornean rainforest. Furthermore, the approach of Moreno & Halffter requires the species accumulation curves to be approaching an asymptote for accurate estimation, and so for practical reasons is probably limited to less diverse taxa.
Grasslands, grazing and biodiversity: editors’ introduction
Authors: A.R. Watkinson , S.J. Ormerod 
Published: March 2003

Summary

Natural, semi‐natural and artificial grasslands occur extensively around the globe, but successful management for production and biodiversity poses several dilemmas for conservationists and farmland managers. Deriving from three continents (Africa, Australia and Europe), papers in this Special Profile interface three specific issues: plant responses to grazing, plant invasions and the responses to management of valued grassland biota.
Although pivotal in grassland management, plant responses to grazing are sometimes difficult to predict. Two alternative approaches are presented here. The first uses natural variations in sheep grazing around a water hole to model the dynamic population response of a chenopod shrub. The second analyses a long‐term grazing experiment to investigate the links between plant traits and grazing response.
Linked often crucially with grazing, but also driven sometimes by extrinsic factors, invasions are often cause for concern in grassland management. The invasions of grasslands by woody plants threatens grassland habitats while the invasions of pastures by alien weeds reduces pasture productivity. The papers in this section highlight how a complementary range of management activities can reduce the abundance of invaders. A final paper highlights how global environmental change is presenting new circumstances in which grassland invasion can occur.
The impact of grassland management on biodiversity is explored in this Special Profile with specific reference to invertebrates, increasingly recognized both for the intrinsic conservation value of many groups and for their role in ecosystem processes. The potential for manipulating flooding in wet grasslands to increase the soil invertebrate prey of wading birds is illustrated, together with the roles of management and landscape structure in enhancing insect diversity.
In the face of climate change and growing demands for agricultural productivity, future pressures on grassland ecosystems will intensify. In this system in which productivity and conservation are so closely bound, there is a need both to raise the profile of the issues involved, and to improve our understanding of the applied ecology required for successful management.