bluuebunny/crossref_metadata_2025 huggingface dataset:
Size‐related life history traits: insights from a study of snow petrels (Pagodroma nivea)
Authors: Barbraud Christophe , Weimerskirch Henri , Robertson Graham G. , Jouventin Pierre
Published: March 2003

1. Body size is commonly tied to major life history traits in many animals. The main problem with studies on this topic is that the range of body sizes within species is generally too small to produce enough variation in traits for quantitative analysis. The snow petrel shows considerable differences in body size with structural body size varying by ≈ 1·6‐fold. This high variability in size provides a good opportunity for examining how life history traits are related to body size intraspecifically.

2. We studied the breeding phenology, foraging parameters and chick growth in relation to body size in two populations of snow petrel in Antarctica. We also investigated the relationship between body size and distance separating colony from the edge of the pack ice using morphometric data from 16 breeding stations.

3. The largest snow petrels laid eggs ≈ 2‐fold larger and hatched hatchlings ≈ 1·9‐fold heavier than smallest ones. During the chick rearing period the smallest adults made longer foraging trips (≈ 70 h) and fed their chick less frequently than the largest ones (≈ 40 h). Meal size was positively related to adult body size.

4. Chicks raised by large parents grew more rapidly, reached higher peak mass and asymptotic size than chicks raised by small parents. Chick body size at fledging was correlated to the size of their biological parents. A sample of 20 chicks from parents that differed in body size were swapped. Body size of swapped chicks at fledging was not correlated to their foster parent size. Swapped chicks tended to resemble their biological parents in size but no significant relationship was found.

5. These results provide evidence of intraspecific variability in several ecological variables and life history traits linked to body size. The difference in foraging trip duration between large and small birds was not only because of a lower flight speed or a higher metabolic rate per mass unit of smaller birds and suggests that small birds had lower feeding efficiency or fed on more distant areas. This was reinforced at the population level by a negative correlation obtained between body size and distance between colony and the edge of the pack ice for 16 breeding stations.

6. The results suggest that body size is probably in the main genetically determined but that there is a significant environmental component in fledgling body size.
Spatial dynamics in a host–multiparasitoid community
Authors: Amarasekare Priyanga
Published: March 2003

1. The harlequin bug, a herbivore on bladderpod, is attacked by two specialist egg parasitoids Trissolcus murgantiae and Ooencyrtus johnsonii. Ooencyrtus can out‐compete Trissolcus in the laboratory, but coexistence is the norm in field populations. Despite the heavy mortality inflicted by the two parasitoids, the host–parasitoid interaction is persistent in all sites that have been studied in southern California.

2. I manipulated inter‐patch distances in a field experiment to determine whether spatial processes drive parasitoid coexistence and/or host–parasitoid dynamics. I first tested the hypothesis that the parasitoids coexist via a dispersal–competition trade‐off. Both parasitoid species took significantly longer to colonize isolated patches than well‐connected patches, suggesting that they have comparable dispersal abilities. Ooencyrtus did not exclude Trissolcus even when inter‐patch distances were reduced to 25–30% of those observed in natural populations. These data suggest that parasitoid coexistence can occur in the absence of a dispersal advantage to the inferior competitor.

3. Since the treatments with isolated vs. well‐connected patches did not differ in parasitoid composition, I next asked whether isolation would destabilize, or drive extinct, the host–multiparasitoid interaction. No local extinctions of bugs or parasitoids were observed during the 18‐month study period. Bug populations in the isolated patches were no more variable than those in the well‐connected patches. In fact, temporal variability in the experimentally isolated patches was comparable to that observed in highly isolated natural populations.

4. These data argue against a strong effect of spatial processes on host–parasitoid dynamics. Local processes may mediate both parasitoid coexistence as well as the host–parasitoid interaction.
Role of the host plant in the decline of populations of a specialist herbivore, the spruce bud moth
Authors: Ostaff Don P. , Quiring Dan T.
Published: March 2003

1. The following three hypotheses explaining why spruce bud moth (Zeiraphera canadensis Mut. and Free.) populations decline at crown closure were evaluated: (i) first‐instar larval mortality increases as a result of asynchrony between budburst and egg hatch (phenology hypothesis); (ii) larval mortality increases and fecundity of females decreases due to decreased nutritional quality of shaded foliage (nutrition hypothesis); and (iii) egg densities decrease because females lay fewer eggs in closed stands (oviposition preference hypothesis).

2. Field experiments and surveys supported the phenology and oviposition preference hypotheses but not the nutrition hypothesis.

3. Increases in the degree of asynchrony between the time of egg hatch and budburst following increased shading of tree crowns resulted in high mortality of first instars in closed stands.

4. In closed stands budburst was delayed because snow cover reduced soil and root temperatures when air temperatures were above the minimum threshold for egg development. Consequently, many eggs (which are located in the crown) hatched before buds burst.

5. Decreases in larval survival following crown closure were not attributable to changes in the nutritional quality of shaded foliage for larval development and did not influence adult sex ratios in 2 of 3 years.

6. Females that developed on closed or open trees were of similar size, indicating that declines in population density following crown closure were not due to reduced fecundity.

7. When trees in a closed stand were opened by removing nearby trees, they were subjected to higher levels of herbivory. Egg hatch was more closely synchronized with budburst on opened than on closed trees.

8. We conclude that declines in bud moth populations following crown closure are caused by increased first‐instar larval mortality, resulting from temporal asynchrony between budburst and egg hatch, and reduced oviposition caused by female preference for open trees.
Species diversity in a mycophagous insect community: the case of spatial aggregation vs. resource partitioning
Authors: Wertheim Bregje , Sevenster Jan G. , Eijs Irene E. M. , Van Alphen Jacques J. M.
Published: March 2003

1. Previous work has suggested that species diversity in resource‐limited insect communities on patchy resources is maintained by spatial aggregation, not by resource partitioning. The most comprehensive test of this claim to date was by Shorrocks & Sevenster (1995), but some of their datasets included only a few resource types, which reduces the likelihood of finding a strong effect of resource partitioning. Also, methods of analysis have since been refined, e.g. to account for patch size.

2. We collected 733 mushroom samples belonging to 66 taxa in a Dutch woodland area. From these mushrooms, 38,891 insects were reared, belonging to 60 taxa of Diptera and Hymenoptera. Drosophilid species and their parasitoids were identified to the species level; other taxa to the family level. We argue that the community is resource limited.

3. Generally, the insects have fairly narrow diets, including only a few of the available mushroom species. The degree of niche overlap varies widely in this community.

4. Within single resource types, co‐existence can be explained by intra‐specific aggregation over patches alone, in accordance with previous studies.

5. This conclusion remains unchanged for the mycophagous community as a whole: intra‐specific aggregation of competitors is a sufficient and necessary mechanism for co‐existence in this diverse community, while resource partitioning does not contribute detectably to species diversity. This is the first time that this pattern has been demonstrated in a dataset involving such a large number of resource types.

6. Our conclusions are strongly supported by data manipulations in which we removed or intensified the effect of resource partitioning and spatial aggregation.

7. We argue that this community may be close to saturation, but we emphasize that saturation is a gradual phenomenon in patchy systems.

8. Since differential use of resource types does not reduce competitive interactions among the insects, it seems unlikely that inter‐specific competition constitutes the selective pressure favouring specialization.
Dispersal of organisms in a patchy stream environment under different settlement scenarios
Authors: Bond Nick R. , Perry George L. W. , Downes Barbara J.
Published: March 2003

1. Previous work in streams, where many organisms disperse downstream by drifting in the water column, has suggested dispersal distances could be related to the overall proportion of the stream within dead water zones (DWZ), places such as backwaters, pools or behind large obstacles, where the water is out of the main flow and is stored temporarily. However, dispersal distances might also be influenced by the spatial organization of DWZ within a reach; this would require us to distinguish between reaches containing many small DWZ compared with those having a few large ones in order to understand fully differences in dispersal patterns among reaches differing in this regard.

2. We constructed a spatial model of flow in stream reaches, in which we varied the density and size distribution of DWZ distributed through the reach. We defined flow conditions under which stream organisms, modelled as passive particles, would settle or continue to drift downstream. We then examined the horizontal dispersal profiles of such particles through reaches differing in arrays of DWZ.

3. Our results support those of previous studies, but suggest that the spatial arrangement of DWZ may be just as important as the overall proportion of DWZ within a reach. On a more general note, the importance of patch configuration observed in this study reflects the growing view that a thorough understanding of population and ecosystem processes will require the explicit consideration of spatial pattern in the environment.
Phenotypic correlates and consequences of dispersal in a metapopulation of house sparrows Passer domesticus
Authors: Altwegg Res , Ringsby Thor Harald , SÆther Bernt‐Erik
Published: March 2003

1. We examine causes and consequences of natal dispersal within a metapopulation of house sparrows Passer domesticus in an archipelago in Northern Norway where a large proportion of the individuals is colour‐ringed.

2. Less than 10% of the fledglings dispersed, i.e. left their natal island.

3. Dispersal was female biased and almost exclusively performed by juveniles.

4. The probability of natal dispersal was not related either to the body condition or the body mass of the juvenile. Similarly, neither clutch size nor hatching date explained a significant proportion of the variance in the probability of dispersal.

5. The probability of male natal dispersal was related to the rank of the fledgling in the size‐hierarchy within the brood. Low ranking individuals that hatched early in the season were more likely to disperse.

6. In both sexes, the survival of dispersers at the island of establishment was higher than among the residents on that island. Similarly, dispersers survived better than adults that remained on their island of birth.

7. These results suggest that dispersal may be an adaptive strategy to avoid poor conditions in the natal area.
North Atlantic Oscillation (NAO) effects of climate on the relative importance of first and second clutches in a migratory passerine bird
Authors: Møller Anders Pape
Published: March 2003

summary

Climatic variables are known to affect reproduction and survival in birds.

Barn swallowsHirundo rusticaL. are long‐distance migratory passerines that produce two broods in much of the European range. The relative importance of first and second broods for annual reproductive success in relation to a measure of general climatic conditions – the North Atlantic Oscillation (NAO) – was assessed during a long‐term study from 1970 to 2000.

The size of first clutches increased with increasing NAO values, and clutch size increased during the study period. Brood size and hatching, fledging or breeding success did not show a significant temporal change, nor were they related significantly to NAO. However, in years with high NAO index values first broods were relatively larger than second broods compared to years with low NAO values. Similarly, the breeding success of first relative to second broods was larger in years with high NAO index values compared to years with low NAO values.

The quality of offspring measured in terms of T cell‐mediated immune response at age 12 days was related to NAO, with offspring of first broods having stronger immune responses than offspring from second broods during years with high NAO values.

Individual females showed patterns of change in brood size in response to change in NAO that paralleled the patterns observed at the population level. This provides evidence of phenotypic plasticity.

Immigration to the study population increased following a year when annual productivity was caused mainly by fledglings from the first brood. This implies that increases in NAO increases the size of the barn swallow population through effects on the relative productivity of first broods.
Control of bracken and restoration of heathland. VI. The response of bracken fronds to 18 years of continued bracken control or 6 years of control followed by recovery
Authors: MARRS R. H. , JOHNSON S. W. , LE DUC M. G.
Published: March 2003

This paper examines the effectiveness of a range of bracken control and heathland restoration treatments on bracken performance over an 18‐year period on a Calluna health in Breckland, UK. Initial treatments were a combination of cutting and spraying with asulam, with and without sowing Calluna seeds. After 6 years a further treatment was added, with the bracken treatments being either continued/reapplied or discontinued.

Seeding with Calluna had no long‐term effect on bracken levels recorded under any bracken control treatment.

No treatment eradicated bracken completely, although all achieved some control. Asulam was the most effective treatment in the 2 years after application, but bracken recovered rapidly thereafter. Bracken frond biomass was reduced to c. 6% of untreated levels after 18 years by cutting once yearly, irrespective of whether combined with an initial asulam application or not, but to 3% by cutting twice yearly. Re‐applications of asulam produced similar responses to the initial application, with a rapid decrease followed by recovery.

Where bracken control was stopped after 6 years there was an increase in bracken in all treatments compared to where control was continued/reapplied. However, even after 12 years of remission, frond biomass was lower than that of untreated bracken in most treatments. Where cutting was applied twice yearly for 6 years and then allowed to recover for 12 years, the frond biomass was still less than 40% of untreated values.

The response of bracken to asulam was compared to predictions from published models and a reasonable fit was obtained, although the sensitivity of one such model to the efficacy of the initial bracken reduction was noted.

The implications of these results for both long‐term bracken control and the monitoring of bracken stocks are discussed.
The influence of forest fragmentation and landscape pattern on American martens
Authors: Hargis Christina D. , Bissonette John. A. , Turner David L.
Published: March 2003

1. We investigated the effects of forest fragmentation on American martens (Martes americana Rhoads) by evaluating differences in marten capture rates (excluding recaptures) in 18 study sites with different levels of fragmentation resulting from timber harvest clearcuts and natural openings. We focused on low levels of fragmentation, where forest connectivity was maintained and non‐forest cover ranged from 2% to 42%.

2. Martens appeared to respond negatively to low levels of habitat fragmentation, based on the significant decrease in capture rates within the series of increasingly fragmented landscapes. Martens were nearly absent from landscapes having > 25% non‐forest cover, even though forest connectivity was still present.

3. Marten capture rates were negatively correlated with increasing proximity of open areas and increasing extent of high‐contrast edges. Forested landscapes appeared unsuitable for martens when the average nearest‐neighbour distance between open (non‐forested) patches was <100 m. In these landscapes, the proximity of open areas created strips of forest edge and eliminated nearly all forest interior.

4. Small mammal densities were significantly higher in clearcuts than in forests, but marten captures were not correlated with prey abundance or biomass associated with clearcuts.

5. Conservation efforts for the marten must consider not only the structural aspects of mature forests, but the landscape pattern in which the forest occurs. We recommend that the combination of timber harvests and natural openings comprise <25% of landscapes ≥9 km2 in size.

6. The spatial pattern of open areas is important as well, because small, dispersed openings result in less forest interior habitat than one large opening at the same percentage of fragmentation. Progressive cutting from a single patch would retain the largest amount of interior forest habitat.
The importance of variation in the breeding performance of seed‐eating birds in determining their population trends on farmland
Authors: Siriwardena Gavin M. , Baillie Stephen R. , Crick Humphrey Q.P. , Wilson Jeremy D.
Published: March 2003

1. Changes in agriculture have been linked to widespread declines in farmland bird populations. One approach to the identification of the causes of observed population changes is to investigate historical changes in national demographic rates.

2. We analysed the British Trust for Ornithology’s nest records database to investigate whether long‐term farmland population trends could have been driven by changes in several components of the annual breeding performance of 12 granivorous bird species. Clutch size, brood size, chick : egg ratio and daily nest failure rates were analysed with respect to blocks of years during which abundance (as measured by the Common Birds Census) was increasing, stable or declining. The individual components of breeding performance were combined to provide estimates of the production of fledglings per breeding attempt.

3. Most species’ population declines were not associated with poor breeding performance per attempt. Effects of environmental change on this parameter therefore cannot be a general mechanism behind the major population declines seen.

4. A fall in linnet Carduelis cannabina fledgling production per attempt, driven primarily by increased nest failure rates during the egg period, represented the strongest evidence for an important effect of breeding performance on abundance. This change could have driven the principal population decline (1975–86) for this species.

5. Conversely, at least for the declining turtle dove Streptopelia turtur, skylark Alauda arvensis, tree sparrow Passer montanus, yellowhammer Emberiza citrinella and corn bunting Miliaria calandra, breeding performance per attempt was higher while populations declined.

6. Variation in annual survival and fledgling production per breeding attempt alone could not explain changes in abundance for at least seven species. This may suggest that changes in post‐fledging survival rates and/or the number of breeding attempts per year could have been important.

7. Management to improve over‐winter survival may be critical in reversing the population trends of most declining species, but such management might still best be directed at the breeding season. Post‐fledging survival rates and the number of breeding attempts made within a season are little‐studied demographic variables that are high priorities for future research and long‐term monitoring.
Concentrations of nitrogen and phosphorus in streams draining the mixed land‐use Dee Catchment, north‐east Scotland
Authors: Edwards A.C. , Cook Y. , Smart R. , Wade A.J.
Published: March 2003

1. There is still some uncertainty about large‐scale influences on nutrient budgets in rivers. In particular, reduced forms of nitrogen (N) in organic forms might represent a significant fraction of the soluble N present in headwater streams, but this is not well quantified. Nitrate increases in relative importance downstream within agriculturally dominated areas. Here we appraise variations in N dynamics for a representative temperate but upland river, the Dee.

2. In the Dee catchment, the source of organic N appears to vary seasonally. During summer under low flow conditions it originates primarily from in‐stream biological production, while during the winter–spring period leaching from the plant–soil system would be the major contributor.

3. On any individual sampling day, a wide range of N : P ratios can occur in the catchment area. Generally the narrowest N : P ratios occur during the summer and early autumn, particularly for upland catchments dominated by semi‐natural vegetation. It is possible that some of the tributaries and upper region of the main river may be limited by N during the summer. The interpretation of the N : P ratios depends greatly upon the potential biological availability of the organic N, which remains unknown.

4. Together, these data further illustrate that simple ideas about the relative limiting effects of N and P in temperate freshwaters may be misleading.
Nesting habitat selection by booted eagles Hieraaetus pennatus and implications for management
Authors: Suárez Susana , Balbontín Javier , Ferrer Miguel
Published: March 2003

1.
 The booted eagle Hieraaetus pennatus is a poorly known and scarce raptor that breeds in Spain. In Doñana National Park (south‐west Spain) its population has increased from only six breeding pairs in the early 1980s to about 150 today.

2.  In order to guide habitat management for this raptor in Doñana National Park, we related nesting habitat selection to breeding success.

3.
 Birds withstood some human disturbance when nesting, choosing sites closer to pastures besides marshes, footpaths and crops than would occur in a random distribution. Birds also selected areas near to marsh and stands of cork oak Quercus suber.

4.
 Trees used for nesting were wider and taller than would occur at random. They were usually in small groups or were large isolated trees, typically eucalyptus (Eucaliptus spp.).

5.
 The most productive nests were close to marshland and stone pine trees Pinus pinea.

6.  Habitat management to improve the breeding success of booted eagles in Doñana should include: (i) retaining small groups of trees or large isolated trees, especially eucalyptus and cork oaks close to marshland, isolated buildings and crops; (ii) creating clearings in stone pine plantations; (iii) burying potentially dangerous power lines to reduce collision risks; (iv) clearing some areas of scrubland to increase the rabbit population; and (v) controlling forest activities, especially in the breeding season.

7.  The increase in booted eagle populations in western Europe during recent decades may be a consequence of the species’ capacity to adapt to environmental change. Deforestation policies designed to favour agricultural use implemented during the second half of the 20th century have not had a detrimental effect on this raptor.

8.  Our work demonstrates how scarce and important organisms can be favoured by sensitive management in forestry and agricultural habitats.
Spatially explicit, individual‐based, behavioural models of the annual cycle of two migratory goose populations
Authors: Pettifor R.A. , Caldow R.W.G. , Rowcliffe J.M. , Goss‐Custard J.D. , Black J.M. , Hodder K.H. , Houston A.I. , Lang A. , Webb J.
Published: March 2003

1. Behaviour‐based models of animal population dynamics provide ecologists with a powerful tool for predicting the response of such populations to both natural and human‐induced environmental changes.

2. We developed this approach by addressing two outstanding issues in the application of such models: the need to adopt a large‐scale spatially explicit approach, and the need to consider the year‐round dynamics of animal populations.

3. Spatially explicit, year‐round, behaviour‐based models of two populations of arctic‐breeding geese, the Svalbard population of the barnacle gooseBranta leucopsisand the dark‐bellied race of the brent gooseBranta bernicla, were developed. Both populations have been the subject of serious conservation concern and are currently a source of increasing conflict with agricultural interests.

4. There was generally good agreement between empirically derived and model‐generated density‐dependent functions, and of seasonal patterns of the distribution and movement of populations within and between sites, and of energy reserve levels within a population.

5. Sensitivity analyses, however, highlighted the importance of accurate parameter estimation with respect to the predictions of such models, and the potential flaws in the predictions of existing models that have not adopted a spatially explicit approach when dealing with wide‐ranging migratory populations.

6. The effect of the removal of a given area of habitat on both populations was predicted to vary depending upon the spatial configuration of the change. This further emphasizes the need for a spatially explicit approach.

7. Both barnacle goose and brent goose populations were predicted to decline following habitat loss in their winter or spring‐staging sites. Simulations suggested that barnacle geese might be less vulnerable to winter habitat loss than brent geese. This reflected the relative strengths of the density‐dependence of productivity and winter mortality in the two models and provided a clear illustration of the need for a year‐round approach to animal population dynamics.

8. We believe that these models, and this approach to understanding the population dynamics of long‐distance migrants, will be beneficial in attempting to answer the increasingly urgent and frequent requests to predict the response of such populations to environmental change.
The influence of the rice bug Leptocorisa oratorius on rice yield
Authors: Van Den Berg H. ,   SOEHARDI
Published: March 2003

1.
 
Leptocorisa oratorius, which feeds on the panicle of rice, is a major target for insecticide applications by farmers in Indonesia. The management of this insect has been insufficiently addressed because of unknown damage relationships in modern rice varieties. Hence, a study was conducted to assess the impact of L. oratorius on the yield of variety IR64.

2.
 Sources of bias typically associated with studies on yield loss (spray effects, caging, plant compensation for damage) were avoided by studying the effect of differences in natural infestation levels of L. oratorius on rice yield at a large number of field sites. Several parameters, including the number of spikelets m−2 prior to panicle emergence (i.e. prior to attack by L. oratorius), were measured at each site to account for local conditions. The effect of these variables on yield was analysed in a statistical model. After removing non‐significant variables from the model, the effect unambiguously attributable to L. oratorius density was determined.

3.
 The median density of L. oratorius in farmers' fields (pooled from panicle emergence until the milky stage of rice) was 0·10 adults hill−1 (i.e. 2·6 adults m−2) in the dry season (75 sites) and 0·14 adults hill−1 (3·5 adults m−2) in the wet season (94 sites).

4.
 The analysis showed that L. oratorius populations caused no measurable reduction in yield of rice. If a linear relationship in the range of 2–10% yield loss for every added adult per hill was assumed, under the measured field populations of L. oratorius in East Java the median loss would have been in the range of 0·2–1·0% in the dry season of 1997 and 0·2–1·3% in the wet season of 1997/98.

5.
 Additional field cage experiments were conducted to compare the susceptibility of different stages of rice development to L. oratorius attack and to study yield loss components. In field cages with a bug/panicle ratio substantially higher than observed in the field, L. oratorius reduced yield when feeding at peak flowering of rice, indicating that rice is most susceptible to L. oratorius attack at flowering. The presence of L. oratorius at peak flowering and at the peak milky stage, moreover, caused empty seed. There was, however, no indication that L. oratorius caused partially filled seed. Further study is needed of the possible mechanism by which the plant, under moderate attack from L. oratorius, redirects assimilates from damaged spikelets to those that would have gone unfilled.

6.
 In conclusion, L. oratorius did not cause an important reduction in rice yield in this study, and we conclude that the application of insecticides targeted at L. oratorius is mostly unwarranted. Farmers need practical training on the biology of L. oratorius and its feeding on rice to appreciate why chemical control is generally not required.
Responses of male brushtail possums to sterile females: implications for biological control
Authors: Ji Weihong , Clout Mick N. , Sarre Stephen D.
Published: March 2003

1.  Increasingly immunocontraception is being considered as a potential method for the biological control of mammalian pests. Although several studies have investigated or modelled its demographic consequences, there have been few studies of the possible effects of the presence of sterile females on local males.

2.
 In New Zealand, the brushtail possum Trichosurus vulpecula is an important mammalian pest, for which immunocontraception has been proposed as a control method.

3.  We surgically sterilized female brushtail possums in two populations to study the potential effects of immunocontraception on body condition and local sex ratios.

4.  Large numbers of leucocytes (indicating oestrus) were present in the reproductive tracts of sterile female possums several weeks after most control (non‐sterile) females were pregnant and had ceased mating activity. This indicates that the sterilization of females prolonged the mating season of local possums.

5.  The body condition of males was significantly poorer in the presence of sterilized females in the winter post‐mating period following the treatment. In contrast, the body condition of females showed no change following the sterilization treatment, either during the autumn mating period or in the winter post‐mating period.

6.  More adult male possums were recorded at both study sites after the sterilization treatment, resulting in a change to the originally female‐biased sex ratio. This may have been caused by the prolonged presence of oestrous females attracting males from surrounding areas.

7.  The implications of our results for the control of possums by sexually transmitted immunocontraception are twofold. First, the reduced body condition of males in the presence of sterilized females might result in increased male mortality. Secondly, the attraction of males from surrounding areas to those containing sterilized females might facilitate the local spread of an immunocontraceptive agent.

8.  We advocate further assessments of the effects of sterilization and/or immunocontraception on the population demography, individual condition and survival of pest animals.
Habitat associations and breeding success of yellowhammers on lowland farmland
Authors: Bradbury Richard B. , Kyrkos Antonios , Morris Antony J. , Clark Stéphanie C. , Perkins Allan J. , Wilson Jeremy D.
Published: March 2003

1. Yellowhammers began to decline on British lowland farmland in the late 1980s and losses are presently 10% per year. This study examined variation in the habitat selection and breeding success of yellowhammers, allowing an evaluation of whether Britain's yellowhammer decline might have been caused by recent changes in agriculture.

2. Yellowhammer territories were associated with hedgerows, vegetated ditches and wide uncultivated grassy margins around fields. Pasture and silage leys were avoided. Nests were built among herbaceous vegetation in ditches or in the shrubby vegetation of hedgerows.

3. Breeding started slightly earlier on organic farms than on intensively managed farms, but no measure of breeding success differed between farm types. Predation was the cause of most (64%) nest failures. A maximum of three breeding attempts (two successful) was observed per pair, with a mean clutch size of 3·3, a Mayfield nest success rate of 0·46, and 2·6 nestlings fledged per successful brood. These data, together with published estimates of adult yellowhammer survival and of post‐fledging survival among other passerines, suggest that breeding productivity is too low to maintain a stable population.

4. The removal of hedgerows or abandonment of hedge management, filling or clearing of ditches, intensification of grassland management and cropping or grazing right up to the field edge, are all likely to have adversely affected yellowhammers on lowland farmland in southern England. Policy reforms that redirect subsidy support to environmentally beneficial management of field margin habitats and retention of winter‐feeding sites such as stubbles should assist in restoring populations of breeding yellowhammers on lowland farmland.

5. Our data expand further the array of farmland bird species for which interactions between agricultural change and population change are increasingly understood.
The ecological challenge of immunocontraception: editor's introduction
Authors: Barlow N.D.
Published: March 2003

1.  The problems of vertebrate pests are greater now than ever before, with vertebrate control constrained by problems of humaneness, scale and environmental impact. However, immunocontraception involves a conceptually ideal solution. Although not intrinsically novel, its delivery in baits or by a self‐spreading vector and its effectiveness in pest control, are now the focus of growing international interest.

2.  Major ecological questions correspond to the two forms of delivery: baits and vectors. First, given an effective immunocontraceptive, inserted into a bait and eaten by a pest, would the resulting level of sterilization in the population effectively suppress densities? Secondly, given that the immunocontraceptive agent can be inserted into a microparasitic or macroparasitic infective vector, would the modified vector persist at sufficient prevalence in the host population, and hence suppress densities to the required extent?

3.  The papers published in this Special Profile focus on behaviour following sterilization or they model the likely impact of viral‐vectored immunocontraception. They highlight advantages and disadvantages of immunocontraception and some general, novel and specific issues. These include the possibility of behaviourally mediated population responses to fertility control; the possible advantages of a mixed baiting and vector strategy; the competitiveness of a modified vector; the appropriateness of immunocontraception for controlling invasive vertebrates on islands; and the need for a ‘pay‐off’ methodology for assessing genetic modifications against alternatives.

4.  The findings offer significant benefits for management and policy: they will inform decisions on whether to pursue immunocontraception as a control option, and they provide evidence about efficacy and risk in applications to release genetically modified vectors.

5.  Although many of the problems in developing immunocontraception technology are biotechnological, questions about the effectiveness of immunocontraceptive pest control are ultimately in the domain of ecologists.
The seasonal abundance of blowflies infesting drying fish in south‐west India
Authors: Wall R. , Howard J.J. , Bindu J.
Published: March 2003

Blowfly infestation of sun‐drying fish is a major economic problem in many developing countries of Asia, Africa and the Pacific. To consider the ecology of infestation, adult and larval blowfly populations were monitored between 27 October 1997 and 27 April 1999 at a fish landing and drying site, approximately 5 km north of Calicut, in Kerala state on the coast of south‐west India.

During the 548‐day sampling period, a total of 96 953 adult Diptera was collected from 16 sticky targets, placed inside and outside eight fish‐storage sheds. Of these, 91 912 (95%) were Chrysomya megacephala, 3719 (4%) were other Calliphoridae and 1322 (1%) were other species, largely Sarcophagidae.

The population of C. megacephala showed pronounced seasonal fluctuations in response to climate, particularly relative humidity. Significantly shorter‐frequency fluctuations within fish‐processing sheds were also evident, the periodicity of which corresponded approximately to C. megacephala generation cycles. Spatial variation in C. megacephala abundance was evident within the site, higher populations occurring closest to the beach and numbers declining with distance inland.

The pattern of drying fish infestation by C. megacephala broadly followed changes in the density of adult flies and the seasonal change in weather, with peaks during the monsoon and troughs in the dry hot periods. High relative humidity played a significant but secondary role in increasing infestation.

Quantification of the relationship between larval infestation and percentage fish loss suggests that, given the infestation levels observed, between 10% and 60% post‐harvest wet weight losses would be expected in the monsoon period, depending on the species of fish landed.

The study emphasizes the importance of developing a clear understanding of the basic ecology and spatial and temporal dynamics of an insect pest, prior to the design or implementation of any pest management programme.
Can grazing response of herbaceous plants be predicted from simple vegetative traits?
Authors: Díaz Sandra , Noy‐Meir Imanuel , Cabido Marcelo
Published: March 2003

Range management is based on the response of plant species and communities to grazing intensity. The identification of easily measured plant functional traits that consistently predict grazing response in a wide spectrum of rangelands would be a major advance.

 Sets of species from temperate subhumid upland grasslands of Argentina and Israel, grazed by cattle, were analysed to find out whether: (i) plants with contrasting grazing responses differed in terms of easily measured vegetative and life‐history traits; (ii) their grazing response could be predicted from those traits; (iii) these patterns differed between the two countries. Leaf mass, area, specific area (SLA) and toughness were measured on 83 Argentine and 19 Israeli species. Species were classified by grazing response (grazing‐susceptible or grazing‐resistant) and plant height (< or > 40 cm) as well as by life history (annual or perennial) and taxonomy (monocotyledon or dicotyledon).

 Similar plant traits were associated with a specific response to grazing in both Argentina and Israel. Grazing‐resistant species were shorter in height, and had smaller, more tender, leaves, with higher SLA than grazing‐susceptible species. Grazing resistance was associated with both avoidance traits (small height and leaf size) and tolerance traits (high SLA). Leaf toughness did not contribute to grazing resistance and may be related to selection for canopy dominance.

 Plant height was the best single predictor of grazing response, followed by leaf mass. The best prediction of species grazing response was achieved by combining plant height, life history and leaf mass. SLA was a comparatively poor predictor of grazing response.

 The ranges of plant traits, and some correlation patterns between them, differed markedly between species sets from Argentina and Israel. However, the significant relationships between plant traits and grazing response were maintained.

 The results of this exploratory study suggest that prediction of grazing responses on the basis of easily measured plant traits is feasible and consistent between similar grazing systems in different regions. The results challenge the precept that intense cattle grazing necessarily favours species with tough, unpalatable, leaves.
Recovery of bird populations after clearfelling of tall open eucalypt forest in Western Australia
Authors: Williams M.R. , Abbott I. , Liddelow G.L. , Vellios C. , Wheeler I.B. , Mellican A.E.
Published: March 2003

Increasing concern over the sustainable management of forested landscapes and the extent of forest clearance world‐wide has led to a growing interest in the impacts of logging and associated habitat disturbance on biodiversity.

We conducted an experimental study of the impact of clearfelling on birds of the karri Eucalyptus diversicolor forest in south‐west Western Australia over a 17‐year period, and a retrospective study of both clearfelled and naturally regenerated karri stands aged from 0 to 146 years.

One‐third of species still had significantly reduced abundance 14 years after disturbance ceased, although all affected species made limited use of regenerating forest.

Multivariate analysis of changes in bird community structure showed that the effects of disturbance were still evident 14 years after clearfelling. Clearfelling may also have produced some temporary changes in community structure in adjacent unlogged forest.

Species richness and total abundance of birds declined by 58% and 96%, respectively, in the first year after clearfelling, and 14 years after logging were still 17% and 55% below levels in adjacent undisturbed forest. During this early successional phase both measures increased as a simple function of stand age. Species richness of regrowth reached that of old‐growth at 30–50 years and total abundance of all bird species in regrowth was similar to that of old‐growth after approximately 70 years.

Several bird species offer potential as indicators of the ecological sustainability of karri forest management. These species nest in large hollows in standing live trees (two cockatoo species) or have been slow to recolonize immature regrowth karri forests (six species).

Post‐hoc power analysis showed that even the long‐term and intensive sampling employed in this study failed to detect declines in abundance of less than 80–90% for most bird species. For many uncommon species, trying to estimate changes in abundance is problematic and likely to require replication which is difficult to achieve in field situations where logged and unlogged forests are compared. Despite this, the present study identified some key impacts of forest clearfelling on bird communities, with implications both for the consequences of clearfelling of forests and the criteria for sustainable forest management.
Evaluating presence–absence models in ecology: the need to account for prevalence
Authors: Manel Stéphanie , Williams H. Ceri , Ormerod S.J.
Published: March 2003

Models for predicting the distribution of organisms from environmental data are widespread in ecology and conservation biology. Their performance is invariably evaluated from the percentage success at predicting occurrence at test locations.

Using logistic regression with real data from 34 families of aquatic invertebrates in 180 Himalayan streams, we illustrate how this widespread measure of predictive accuracy is affected systematically by the prevalence (i.e. the frequency of occurrence) of the target organism. Many evaluations of presence–absence models by ecologists are inherently misleading.

With the same invertebrate models, we examined alternative performance measures used in remote sensing and medical diagnostics. We particularly explored receiver‐operating characteristic (ROC) plots, from which were derived (i) the area under each curve (AUC), considered an effective indicator of model performance independent of the threshold probability at which the presence of the target organism is accepted, and (ii) optimized probability thresholds that maximize the percentage of true absences and presences that are correctly identified. We also evaluated Cohen's kappa, a measure of the proportion of all possible cases of presence or absence that are predicted correctly after accounting for chance effects.

AUC measures from ROC plots were independent of prevalence, but highly significantly correlated with the much more easily computed kappa. Moreover, when applied in predictive mode to test data, models with thresholds optimized by ROC erroneously overestimated true occurrence among scarcer organisms, often those of greatest conservation interest. We advocate caution in using ROC methods to optimize thresholds required for real prediction.

Our strongest recommendation is that ecologists reduce their reliance on prediction success as a performance measure in presence–absence modelling. Cohen's kappa provides a simple, effective, standardized and appropriate statistic for evaluating or comparing presence–absence models, even those based on different statistical algorithms. None of the performance measures we examined tests the statistical significance of predictive accuracy, and we identify this as a priority area for research and development.
Using satellite telemetry to define spatial population structure in polar bears in the Norwegian and western Russian Arctic
Authors: Mauritzen Mette , Derocher Andrew E. , Wiig Øystein , Belikov Stanislav E. , Boltunov Andrei N. , Hansen Edmond , Garner Gerald W.
Published: March 2003

Animal populations, defined by geographical areas within a species’ distribution where population dynamics are largely regulated by births and deaths rather than by migration from surrounding areas, may be the correct unit for wildlife management. However, in heterogeneous landscapes varying habitat quality may yield subpopulations with distinct patterns in resource use and demography significant to the dynamics of populations.

To define the spatial population structure of polar bears Ursus maritimus in the Norwegian and western Russian Arctic, and to assess the existence of a shared population between the two countries, we analysed satellite telemetry data obtained from 105 female polar bears over 12 years.

Using both cluster analyses and home‐range estimation methods, we identified five population units inhabiting areas with different sea‐ice characteristics and prey availability.

The continuous distribution of polar bear positions indicated that the different subpopulations formed one continuous polar bear population in the Norwegian and western Russian Arctic. Hence, Norway and Russia have a shared management responsibility.

The spatial population structure identified will provide a guide for evaluating geographical patterns in polar bear ecology, the dynamics of polar bear–seal relationships and the effects of habitat alteration due to climate change. The work illustrates the importance of defining population borders and subpopulation structure in understanding the dynamics and management of larger animals.