bluuebunny/crossref_metadata_2025 huggingface dataset:
Density dependence of clutch size: habitat heterogeneity or individual adjustment?
Authors: BOTH CHRISTIAAN
Published: March 2003

1.   Two hypotheses have been proposed to explain density‐dependent patterns in reproduction. The habitat heterogeneity hypothesis (HHH) explains density‐dependent reproduction at the population level from poorer quality territories in hetero geneous environments only being occupied at high densities. No effect of population density is expected on reproduction within a given territory. If reproduction and territory quality are correlated, there will be a negative correlation between reproduction and density. Alternatively, density‐dependent responses could be caused by variation in reproduction with population density. This will be referred to as the individual adjustment hypothesis (IAH).

2.  The HHH predicts that individuals breeding on the same territory in years with different densities would show no response in reproduction to changes in population density. The response in reproduction of the population to changes in density would therefore be stronger than the within territory response. Under the IAH, the response of individuals on specific territories would be similar to the average response in the whole population.

3.  This paper attempts to discriminate between these two hypotheses using data on clutch sizes of great tits Parus major L. from six different populations. In all populations the mean clutch size was negatively correlated with the population density. In four out of the six study populations individual females breeding on the same territory showed a negative correlation between the change in clutch size between years and the change in density. In none of the populations it was found that the response within territories was significantly smaller than the average response for the population.

4.  These data suggest that density dependence of clutch size in great tits is not due to poorer quality territories being occupied more frequently at higher population densities. Within territories, individual females adjusted their clutch size to changes in density, and this was sufficient to explain the density dependence of clutch size in five of the six populations. In one population some evidence supporting the HHH was found.
Rarity in the tropics: latitudinal gradients in distribution and abundance in Australian mammals
Authors: Johnson C. N.
Published: March 2003

1. In many groups of species, range size and population density are positively correlated (the ‘distribution−abundance relationship’) and range size is positively correlated with latitude (‘Rapoport's Rule’). This study investigated these two relationships among Australian forest mammals, and also tested for the existence of a relationship between population density and latitude in the same fauna.

2.All three relationships were significant, with the distribution–abundance relationship the strongest pattern of the three, and Rapoport's Rule the weakest. Partial correlation analysis showed that: (i) range size and population density remained positively correlated when latitude was held constant; and (ii) population density and latitude remained positively correlated when range size was held constant; but (iii) latitude and range size were not significantly correlated when population density was held constant. For (i) and (ii), partial correlation did not reveal stronger relationships than were revealed by simple correlation. These results suggest that the distribution–abundance relationship is conditioned by latitude to produce small ranges and low population densities in the tropics, and that tropical species tend to have low population densities relative to range size.

3.Body mass accounted for little variation in population density, and for no variation in range size and latitudinal position. The small ranges and low population densities of tropical species were not related to a latitudinal gradient in species richness, which did not occur in this fauna.

4.The latitudinal gradient in population density held within widespread species as well as among species. The effect of Bergmann's Rule (the tendency for organisms in cool environments to be larger bodied than organisms in warm environments) meant that in some species, population energy use rose more steeply with latitude than did population density.

5.I argue that all three relationships might be due to a tendency for the foraging efficiency of animals to be lower in tropical than in temperate ecosystems, as a result of a lower concentration of nutrients in tropical ecosystems. For resource‐limited species, foraging efficiency should influence both range size and population density. Differences in foraging efficiency may therefore produce the distribution−abundance relationship as well as latitudinal gradients in range size and population density.
Synchrony in small mammal populations of montane forest patches in southern India
Authors: Shanker Kartik , Sukumar R.
Published: March 2003

1. Small mammal populations were studied in montane evergreen forests in the Nilgiris, southern India, using live‐trapping from January 1994 to September 1996. Two sites were selected, one with a single large forest patch and adjoining plantations, and the other with several small forest patches separated by grassland.

2. Nine species were recorded, of which eight were trapped in the forest patches, two in the grasslands and five in plantations. Rattus rattus was the most abundant species (2–36 individuals ha−1) followed by Suncus montanus (0–11 individuals ha−1). Densities of seven other species recorded were very low.

3. Synchrony in various population variables – density, biomass, mean weight, density of adults and adult females, and proportion of adults, adult females and sub‐adults – was examined for Rattus rattus populations in the forest patches and plantations. Density and biomass were studied in seven other species recorded at these sites. Spearman's rank order correlation was used as a measure of synchrony between the population variables.

4. Within‐site synchrony was higher than between‐site synchrony in population characters. Synchrony was also higher between plots within the unfragmented site than they were between plots in the fragmented site. Relatively high synchrony in proportions of adults, adult females and juveniles in the forest patches implied that breeding is probably influenced by climate and food availability, which are seasonal in this habitat.

5. Given the small patch sizes (≈1–10 ha) and low population sizes, asynchrony is likely to be an outcome of demographic and environmental stochasticity, and low dispersal rates may impede establishment of synchrony.
Behavioural interactions in stream food webs: the case of drift‐feeding fish, predatory invertebrates and grazing mayflies
Authors: Huhta Arto , Muotka Timo , Juntunen Antti , Yrjönen Mikko
Published: March 2003

1. We examined direct and indirect behavioural responses of grazing mayfly nymphs (Baetis rhodani) to predation risk from a drift‐feeding fish (European minnow; odour manipulated), and two types of invertebrate predators, Diura bicaudata (stonefly) and Rhyacophila nubila (caddis larva). We also assessed the direct responses of the invertebrate predators to fish chemicals.

2. Based on diel gut content periodicity, D. bicaudata nymphs were strongly nocturnal foragers. R. nubila was also nocturnal, but only in a fish stream; in a stream lacking fish, their gut contents did not vary significantly on a diel basis. In the laboratory, Diura was nocturnal even in the absence of fish and almost ceased moving when fish was present. Rhyacophila shifted from aperiodic to nocturnal foraging in the presence of fish.

3. The contrasting behaviours of the two predators may be explained by their respective field distributions: D. bicaudata always co‐occurs with fish, whereas R. nubila is also found in streams without fish. Therefore, a capacity for plastic anti‐fish responses is beneficial for R. nubila, but not for D. bicaudata.

4. Drift of large Baetis nymphs was aperiodic when Rhyacophila was present, but nocturnal in all other treatments. Drift rate was highest when both Diura and fish were present and lowest in treatments with Rhyacophila. Predatory stoneflies, but not the caddis larva, induced a night‐time peak in the drift of both Baetis size‐classes. In the absence of predators, small Baetis drifted aperiodically. Interaction terms in three‐way anovas testing for the indirect effects of fish and invertebrate predators on mayfly drift periodicity and drift rate were all non‐significant, indicating that the response of Baetis to one type of predator was not modified by the other predator.

5. Our results indicate that the drift periodicity of lotic mayflies may be fine‐tuned to variations in the multi‐predator environment and that prey responses are size‐specific. We conclude that the effects of invertebrate predators on prey behaviour vary in relation to predator's foraging strategy and generalizations based on studies with only one type of predator should be avoided.
The distribution and abundance of populations limited at multiple spatial scales
Authors: Bevers Michael , Flather Curtis H.
Published: March 2003

1. We use mathematical programming to explore long‐term equilibrium population distribution and abundance effects arising from a single‐patch reaction–diffusion model when ecological factors limit carrying capacity at two different spatial scales. Both homoogeneous and heterogeneous habitat patches are investigated under passive diffusion conditions. Capacity limits, reproduction and dispersal rates, and habitat preferences are individually varied.

2. When landscape‐scale habitat factors are strongly limiting, and reproduction and dispersal processes are less so, suitable population arrangements can be either clustered or highly fragmented.

3. Population clustering also occurs when strong habitat preferences are introduced, even though few members of the clusters may actually occupy the preferred sites.

4. As reproduction or dispersal rates become increasingly limiting in the system (i.e. as the parameters approach extinction thresholds), population clustering within a habitat patch is required to reach long‐term equilibrium population levels.

5. These results offer a potential explanation for why species–habitat association studies have been characterized by high variability.
Habitat duration, predation risk and phenotypic plasticity in common frog (Rana temporaria) tadpoles
Authors: Laurila Anssi , Kujasalo Jutta
Published: March 2003

1. Common frogs (Rana temporaria) breed readily in small pools and thus expose their offspring to catastrophic mortality by desiccation. Amphibian larvae exhibit considerable phenotypic plasticity in metamorphic traits, and some species respond to environmental uncertainty by metamorphosing earlier and at smaller size. In a factorial laboratory experiment, we studied whether common frog tadpoles possess this ability.

2. We also studied the interaction between pool drying and predation risk, because in a previous study the presence of a predatory dragonfly larva delayed metamorphosis of the tadpoles.

3. We gradually removed water from half the experimental containers, while in the other half the water volume was kept constant. In the laboratory it was possible to remove the effect of increased water temperature in the decreasing volume treatment by using fluorescent lights. Tadpoles responded to decreasing volume by metamorphosing earlier and at smaller size. A greater proportion of the tadpoles metamorphosed in the decreasing volume treatment.

4. Tadpoles were less active at decreasing water level and there were significant positive correlations between activity late in the experiment and metamorphic size. This suggests that the metamorphic response to habitat drying is behaviourally mediated.

6. Early in the experiment, tadpoles developed slower in the presence of predators. At metamorphosis presence of a dragonfly larva had no effect in the whole data set, but when the constant volume treatment was analysed separately, larval period was longer in the presence of a predator.

7. Our results indicate that common frog larvae are able to respond to pond‐drying adaptively by speeding up their development, and that temperature advantage is not needed to induce this adaptive plasticity. Furthermore, pond‐drying seems to be a more important determinant of development rate than the presence of odonate predators.
Interaction between phenology and density effects on mortality from natural enemies
Authors: Hunter Alison F. , Elkinton Joseph S.
Published: March 2003

1. The gypsy moth is one of many early spring‐feeding Lepidoptera that have outbreaks. Previous work has revealed that mortality from natural enemies varied between egg hatch dates separated by only a few weeks. However, this phenology effect was confounded with density differences caused by differential dispersal and early larval survival of the releases.

2. Five different densities of larvae were released at two different times in early spring, and monitored for the causes and amount of mortality in the field. We thus sought to separate the effects of timing and density on mortality.

3. Mortality in the larval stages due to parasitoids, disease and physiological causes was density‐dependent for both release date treatments, but the slope of the mortality vs. density line was much steeper for the early release. Total mortality over the entire generation, which included additionally mortality from predation, was density‐dependent for the early release but not for the late release.

4. The timing of egg hatch thus affects fecundity, mortality from natural enemies and the form of the relationship between density and mortality. These compounding effects may help explain why spring‐feeding insects are liable to have outbreaks.
Influence of connectivity on demography and dispersal in two contrasting habitats: an experimental approach
Authors: Boudjemadi Katia , Lecomte Jane , Clobert Jean
Published: March 2003

1. Habitat fragmentation is involved in the present extinction crisis and is known to influence many aspects of population dynamics. The level of connectivity between populations is one of its components. In an experimental study on the common lizard (Lacerta vivipara Jacquin), we analysed in two contrasting habitats the influence of connection on dispersal patterns, on differences between dispersing and philopatric individuals, on population size, survival rates, and reproduction.

2. The experimental design consisted of eight two‐patch units. Half of them were connected by dispersal. We performed this treatment in a grassland and a wood clearance, two habitats with different resource availability (respectively called ‘rich’ and ‘poor’ habitat).

3. We expected that the loss of connection would modify juvenile dispersal patterns, have a different effect on dispersing and philopatric individuals, negatively affect survival rates and fecundity by modifying social interactions and would have different consequences in ‘rich’ habitats compared to ‘poor’ ones.

4. Connection modified dispersal patterns. Two dispersal periods occurred in unconnected units, but only one occurred in connected ones. Moreover, dispersers of the second dispersal period were morphologically similar to philopatric individuals. The loss of connection probably caused the late dispersal of individuals which would have remained philopatric in a connected context. Dispersers of the second period seemed to have lower winter survival rates than philopatric individuals, as expected if these individuals had been ‘forced’ to disperse.

5. We found that connection modified demography through juvenile winter survival rates and female reproduction depending on habitat type. Connection led to higher juvenile survival rates in ‘rich’ habitats, whereas it was not the case in ‘poor’ habitats. The loss of connection decreased female reproductive rate whatever the type of habitat.

6. This study suggests that connection can directly modify demographic parameters depending on habitat quality and involving behavioural mechanisms. It confirms that conservation management, like installation of dispersal corridors, must take into account habitat characteristics and behavioural features.
Estimating the contributions of population density and climatic fluctuations to interannual variation in survival of Soay sheep
Authors: Milner J. M. , Elston D. A. , Albon S. D.
Published: March 2003

1. The relative importance of density‐dependent and ‐independent factors on interannual variation in over‐winter survival was investigated in the fluctuating population of Soay sheep on St Kilda, Scotland, over the period 1985–96.

2. Population density had a negative effect on survival in lambs and adult males while adult female survival showed no evidence of density dependence over the observed range of population densities.

3. Climatic fluctuations associated with the winter North Atlantic oscillation index (NAO) also affected survival, which decreased in winters that were relatively warm, wet and windy. The effect was most pronounced in lambs.

4. Survival was modelled using logistic regression analysis with and without year fitted as a random effect. The former incorporated stochastic year to year variation in survival. Results from the two modelling approaches were similar in terms of the regression coefficients estimated. However, the standard errors of the year‐dependent covariates, population size and NAO, were underestimated when the random year effect was ignored, leading to incorrect inferences about the relative significance of terms being made.

5. Using both modelling approaches, density dependence was found to have a greater influence on survival than the effect of NAO in lambs and adult males, whereas in adult females NAO was the more important.

6. Once random between‐year effects were taken into account, the individually varying terms such as body weight and faecal egg count were the most significant factors explaining differences in survival.
Breeding habitat selection in cliff swallows: the effect of conspecific reproductive success on colony choice
Authors: Brown Charles R. , Brown Mary Bomberger , Danchin Etienne
Published: March 2003

1. One way that animals may select breeding sites is by assessing the reproductive success of conspecifics in one season and settling the next year in those habitat patches where success collectively had been greatest. This sort of habitat assessment may promote the formation of colonies at high quality sites.

2. We examined whether cliff swallows, Petrochelidon pyrrhonota, in south‐western Nebraska used conspecific breeding performance to choose colony sites.

3. Reproductive success at colony sites varied spatially within seasons and between seasons, and was autocorrelated at a site from one year to the next, but not over longer time intervals. Cliff swallows thus met the conditions for potential use of information on conspecific breeding performance.

4. Among sites re‐used in consecutive years, those with highest collective success in one season showed the greatest rates in colony growth the next season, including the greatest influx of immigrants.

5. The probability of colony‐site re‐use in successive years increased with collective reproductive success and average breeder body mass (a measure of individual condition) the previous season.

6. Cliff swallows probably use conspecific breeding performance in selecting colonies. This mechanism is one component of habitat selection that also includes attraction to conspecifics and assessment of an individual's own success.
Group breeding dramatically increases reproductive success of yearling but not older female scrubwrens: a model for cooperatively breeding birds?
Authors: Magrath Robert D.
Published: March 2003

Many studies of cooperatively breeding birds have found no effect of group size on reproductive success, contrary to predictions of most adaptive hypotheses. A model is proposed for variation in group‐size effects: group size has a reduced effect on success when conditions for breeding are good, such as in good environmental conditions or in groups with older breeders. This hypothesis is tested with a case study of white‐browed scrubwrens Sericornis frontalis and a review of the literature.

 The scrubwren is a cooperatively breeding passerine with male helpers. Previous analyses revealed no effect of group size on reproductive success, but those analyses were restricted to groups with older females (Magrath & Yezerinac 1997). Here 7 years’ data are used to contrast the effect of group size on reproductive success for yearling and older females.

 Yearling females breeding in groups had more than double the seasonal reproductive success than those breeding in pairs, even after controlling for territory quality. However, group size still had no effect on the reproductive success of older females. Yearling females tended to survive better in groups, but older females tended to survive better in pairs, emphasizing this pattern.

 Yearlings breeding in pairs were more likely to be found on poor‐quality territories than those breeding in groups, exaggerating the already‐strong effect of group size on yearling success. Older females were not affected significantly by territory quality.

 Group size, territory quality and female age affected different components of seasonal reproductive success. Group size increased the success of individual nesting attempts, while both territory quality and female age affected the length of the breeding season, and thus the number of breeding attempts.

 A sample of the literature on cooperative breeders shows that group size has a larger effect on reproductive success in poorer conditions, caused either by younger, inexperienced breeders or poorer environmental conditions. Scrubwrens therefore illustrate a widespread pattern, which provides an explanation for much of the variation in group‐size effects among and within species. Clearly single estimates of group‐size effects for species can be inadequate to test ideas about the evolution of cooperative breeding.
The impact of weasel predation on cyclic field‐vole survival: the specialist predator hypothesis contradicted
Authors: Graham Isla M. , Lambin Xavier
Published: March 2003

The delayed density‐dependent predation of specialists such as weasels (Mustela nivalis L.) may result in cycles in the abundance of their prey. We estimated the demographic impact of weasel predation on field‐vole (Microtus agrestis L.) survival using capture–recapture data from a large‐scale, replicated predator‐manipulation experiment conducted in six unfenced populations in Kielder Forest, northern England. The density of weasels was experimentally reduced through continuous live‐trapping in the three removal populations. Field‐vole survival was compared between paired removal and control populations during the increase, peak and decline phases of a vole population cycle. Apparent survival and recapture probabilities were estimated using open population Cormack‐Jolly Seber models.

Field‐vole survival varied extensively both spatially between populations and temporally, being highest in the late autumn and over winter and lowest each year in spring and early summer. Patterns of variation in male and female survival were similar between populations over time, but there was independent spatial and temporal variation between adults and juveniles. Variation in weasel abundance explained 18% of this independent spatial and temporal variation between adult and juvenile field‐vole survival.

The average increase in annual vole survival resulting from weasel removal over the 2‐year period was 27% and 25% for adult male and female field voles, respectively. Decreased weasel abundance increased adult field‐vole survival. Adult‐vole survival in the absence of weasels was predicted to be approximately 8% higher than in the presence of one weasel per 4–5 ha. Surprisingly, weasel removal resulted in lowered juvenile field‐vole survival, possibly reflecting increased emigration or mortality due to infanticide. A simple two age‐class demographic model indicated that the decrease in juvenile survival in response to reduced weasel predation pressure did not fully compensate for the increase in adult survival.

As weasel numbers fluctuated seasonally, with highest weasel densities occurring in late summer and autumn and little spring to spring variation, the impact of weasel removal on field‐vole survival was greatest during the breeding season. However, vole population‐growth rates were unrelated to adult survival at that time and correspondingly no divergence was observed between the trajectories of control and removal vole populations even when weasel numbers were greatest in control sites. In contrast, vole population‐growth rates were closely correlated with survival during the non‐breeding season. Nevertheless all control and removal populations experienced a cyclic decline in winter 1999–2000 in spite of the increased adult survival in experimental treatments.

We conclude that the impact of weasel predation on field‐vole survival was neither sufficient nor necessary to initiate and drive the cyclic decline of field‐vole populations in Kielder Forest.
Landscape complementation and food limitation of large herbivores: habitat‐related constraints on the foraging efficiency of wild pigs
Authors: Choquenot David , Ruscoe Wendy A.
Published: March 2003

The effect that the proximity of habitats containing essential resources has on animal abundance at large spatial scales is called landscape complementation. Landscape complementation can influence interaction between large herbivores and their food resources where the proximity of habitats containing essential resources constrains their foraging or demographic efficiency.

We tested the effect that the proximity of a thermal refuge (riverine woodlands) had on interaction between wild pigs and their food resources (pasture). The numerical response of pigs (Sus scrofa L.) (estimated as r quarter−1) to pasture biomass was contrasted between four sites that were progressively more isolated from a major floodplain containing extensive areas of riverine woodland. To test whether proximity to riverine woodlands affected the numerical response of pigs through a constraint on foraging efficiency, we contrasted the numerical response of pigs between the four areas as pasture biomass declined. To test whether pigs exploited riverine woodlands primarily as a thermal refuge, we contrasted the numerical response of pigs between the four areas in different seasons.

We found that although pasture biomass was similar in the four areas, r was lower than expected for given pasture biomass on the two areas further away from riverine woodlands. We also found that while the effect that proximity to riverine woodlands had on the numerical response of pigs became more pronounced when pasture biomass was low, it was not significantly affected by season.

These results suggest that the need to access riverine woodlands compromises the foraging efficiency of wild pigs when the distance to this habitat is relatively high, but that the need to access this habitat may not be purely related to thermoregulation.

We developed a simple mechanistic model that allows the effects of landscape complementation on herbivore foraging and demographic efficiency to be estimated, and used the model to predict the effect that proximity to riverine woodlands would have on variation in pig density. The model suggests that wild pigs cannot persist in areas more then 10 km from extensive riverine woodlands, unless those areas are periodically re‐colonized. This suggests that at the margin of their range around inland river systems, given locations that can be occupied by wild pigs will vary temporally between being sources, pseudosinks and sinks.
Effects of low‐level jet aircraft noise on the behaviour of nesting osprey
Authors: Trimper Perry G. , Standen Neil M. , Lye Leonard M. , Lemon David , Chubbs Tony E. , Humphries Gary W.
Published: March 2003

1.  Nesting osprey Pandion haliaetus L. were exposed to controlled low‐level CF‐18 jet aircraft overflights along the Naskaupi River, Labrador, Canada, during 1995. Jet aircraft flew near five nests at distances ranging from 2·5 nautical miles (nm) to directly overhead at speeds of 400–440 knots.

2.  Maximum noise levels (L1) and other noise metrics were influenced by many factors including topography, distance, altitude, wind speed and direction.

3.  Based on 240 h of observations from blinds, we recorded osprey nest attendance and egg exposure during 139 individual overflights. Similar observations were completed at two control nests. Overflights as low as 30 m above ground occurred during incubation, nestling and prefledging only when observers were present.

4.  Osprey behaviour did not differ significantly (P = 0·126) between pre‐ and post‐overflight periods. Despite L1 values occasionally exceeding 100 decibels, adult osprey did not appear agitated or startled when overflown.

5.  Osprey were attentive to and occasionally flushed from nests when float planes, other osprey or raptors entered territories, and when observers were entering or exiting blinds.
The effect of local change in habitat quality on populations of migratory species
Authors: Sutherland William J.
Published: March 2003

1. Habitat deterioration is a major problem world‐wide as a result of processes such as change in land use, introduced species, human disturbance and exploitation of food supplies. Many studies have shown that habitat change can have considerable effect on the numbers of individuals using a site. For migratory species, however, the consequences for the total population cannot be deduced from local studies.

2. For a migratory species, the change in total population size ΔN, as a consequence of habitat change in the wintering area, can be calculated from ΔN = LMγd′/(b′ + d′), where γ is the expected proportional change in the number of birds using a site as a result of the habitat change, L is the area affected, M is the density of individuals using the site prior to habitat change, b′ is the strength of the per capita density‐dependent breeding output, and d′ is the strength of the per capita density‐dependent winter mortality. Similarly the consequences of habitat change in the breeding area can be calculated from ΔN = LMγb′/(b′ + d′).

3. The same approach can be used for predicting the consequences of improvements in habitat quality.

4. A worked example is given to illustrate how this approach could be used to predict the consequences for the total population of changes in the food supply of oystercatchers within one estuary.

5. There is a need for more measures of γ, the expected proportional change in the number of birds using a site as a result of various forms of habitat deterioration, and the strengths of density dependence.
The impact of changing the season in which cereals are sown on the diversity of the weed flora in rotational fields in Denmark
Authors: Hald Anna Bodil
Published: March 2003

1. Surveys have shown that there has been a dramatic decrease in the weed flora of fields under rotational cultivation during the last 30 years. This trend has been particularly noticeable in winter cereals, a crop of increasing importance in the landscape.

2. The weed flora of spring and winter cereals was compared in 19 unsprayed fields during a 5‐year study to test the hypothesis that cereal type exerts no effect on the flora or on the absolute and relative abundance of single species.

3. Plant and species densities, and accumulated species richness, were lower in winter than in spring cereals.

4. Floristic similarity was greater among spring cereal fields and between spring and winter cereals within the same fields than among winter cereal fields.

5. Species that occurred with unequal density in spring and winter cereals occurred at higher densities in the spring cereals; these species germinated mainly in the spring. However, for a few species the relative plant abundance was highest in winter cereals; these species were able to germinate both in the spring and autumn.

6. Some species – on the relative scale – occurred indifferently of season of sowing; all but one of these species were able to germinate both in the spring and autumn.

7. Plant species and taxa that are important food resources for arthropod herbivores occurred at greater densities in spring than in winter cereals and, in addition, occurred with the highest relative abundance in spring cereals.

8. Change in land use from spring to winter cereals involves not only an immediate reduction of more than 25% in the density of plants and species, but also a change and increased uncertainty in the composition of the weed flora. These findings may have serious implications for the ecology of wildlife in the agricultural landscape.
Conservation of arthropod diversity in montane wetlands: effect of altitude, habitat quality and habitat fragmentation on butterflies and grasshoppers
Authors: Wettstein Walter , Schmid Bernhard
Published: March 2003

1. The patterns of arthropod diversity were investigated in 24 montane wetlands in Switzerland. These differed in altitude, management regime (cattle‐grazing vs. mowing), vegetation structure (index combining vegetation height and density) and degree of habitat fragmentation.

2. The general arthropod diversity was determined by net sampling at 10 sampling points per site. The diversity of grasshoppers and butterflies was measured by counting species richness at the site and species density (species richness per unit area) on transects. The species richness of grasshoppers and butterflies was found to be more sensitive to the geographical attributes of the site whereas species density was more affected by the habitat quality.

3. Grasshopper diversity decreased within the observed altitudinal range (800–1400 m) and was higher at grazed sites, whereas butterfly diversity was higher at mown sites. Arthropod diversity but not abundance of arthropods was positively related to the vegetation structure.

4. The species richness of butterflies was negatively influenced by the degree of habitat fragmentation: both the size of habitat as well as the area of wetland habitats within 4 km were related positively to the number of specialist wetland butterflies.

5. Late mowing as well as low‐density cattle‐grazing are appropriate management actions to maintain arthropod diversity in montane wetlands. In order to establish site‐specific management plans, the biology of the present target species as well as the historical context should be considered.

6. We suggest that the best protection for the species examined in this study would be a network of wetland sites managed using a variety of traditional, non‐intensive methods. This can only be achieved by coordinated planning of conservation measures among sites.
Virus‐vectored immunocontraception to control feral cats on islands: a mathematical model
Authors: Courchamp Franck , Cornell Stephen J.
Published: March 2003

1. Feral catsFelis catusintroduced onto oceanic islands pose a major ecological threat to endemic vertebrates, but their control is difficult. Immunocontraception has not been considered previously as a method for their control or eradication, and therefore we used a modelling approach to assess whether virus‐vectored immunocontraception (VVIC) might be effective.

2. We compared the relative efficiency of cat control/eradication using immunocontraception and three different disseminating techniques, i.e. baits, genetically modified viral vectors, or both. We accounted for several forms of dynamic compensation likely to arise in a population with artificially reduced fertility.

3. We conclude that, under the assumptions of our model, immunocontraception can control or eradicate feral cats on oceanic islands. VVIC was found to be a more efficient dissemination technique than baits, but an integrated method involving viral‐infected baits was the most likely to lead to eradication.

4. We advocate field trials of this VVIC technique, when available, under island conditions where any risks to non‐target fauna would be minimal.
Sea otter population dynamics and the ExxonValdez oil spill: disentangling the confounding effects
Authors: Garshelis David L. , Johnson Charles B.
Published: March 2003

Oil that spilled after the grounding of the Exxon Valdez in 1989 killed large numbers of sea otters in western Prince William Sound, Alaska, USA. However, our boat‐based counts of sea otters during 1990–96 at oiled sites were as high or higher than boat‐based counts in the same area in the early 1980s.

 Another study reported a significant decline in sea otter numbers after the spill, but our reanalysis of that data produced results very similar to ours. Counts of otters were higher than pre‐spill counts in the oiled area; the only detectable decline was in the northern part of the sound, outside the area of oiling.

 We suggest that otter numbers in the western sound may have been increasing during the late 1980s, masking the loss due to the spill. Direct evidence for such an increase is lacking because no counts were conducted during this period. However, for several years after the spill pup production was higher than normal, which, if characteristic of the period immediately pre‐spill, could have spurred a population increase.

Heightened pup production may have been caused by increased food supplies: after the spill, otters obtained more and larger clams per dive and spent less time feeding per day than in the early 1980s.

 We postulate that in the early 1980s clams were still recovering from the uplift caused by the 1964 earthquake, which resulted in massive clam mortality and habitat change in the western sound. Lingering effects of previous catastrophic events, like the earthquake and even 19th‐century fur harvests, have hampered attempts to assess the impacts of the oil spill on sea otter population dynamics. The effects of uncontrolled and unreplicated environmental incidents, even major disasters, may be difficult to assess because of confounding factors.
The effect of dairy farming on barn swallowHirundo rusticaabundance, distribution and reproduction
Authors: Møller Anders Pape
Published: March 2003

Recent changes in agricultural practice have been hypothesized to affect the abundance and reproductive success of farmland birds. The influence of dairy farming on barn swallowsHirundo rusticawas investigated by comparison of their abundance, phenotype and reproduction on the same farms before and immediately after dairy farming ceased, while a control sample of farms without change in farming practice in the same years was used to check for temporal changes unrelated to farming practice.

The abundance of barn swallows decreased significantly when dairy farming ceased, with an average reduction of 48%, while there was no significant difference in the sample of control farms. This was mainly due to a decrease in the abundance of yearling immigrants. The abundance of insect food measured with sweep nets decreased significantly in the absence of cattle, while there was no significant change in the sample of control farms.

The mean phenotypes of adult barn swallows breeding on the same farms with and without dairy farming did not differ significantly for any of the 16 variables measured (11 morphological variables, body mass, parasite load, haematocrit, leucocyte counts, and arrival date), nor were there any significant differences recorded in the sample of control farms.

The mean phenotypes of barn swallow nestlings on the same farms with and without dairy farming differed for tarsus length, body mass, haematocrit, leucocyte concentration and T‐cell mediated immune response, with nestlings being of poorer quality in the absence of cattle. There was no significant differences recorded in the sample of control farms.

Start of reproduction was not delayed in the absence of cattle, but size of first clutches was reduced by the absence of cattle. The frequency of second clutches decreased in the absence of cattle, and laying of second clutches was also delayed. Hatching, fledging and breeding success did not differ between the two types of farming practice. This led to an overall reduction in annual reproductive success in the absence of cattle. None of these significant differences was recorded in the sample of control farms.

These observations suggest that termination of dairy farming reduces local population size, reproductive success and the quality of offspring produced. There is little evidence of the distribution of phenotypes of adult barn swallows being affected by the presence of cattle.
Simulating the long‐term dynamics of slug populations: a process‐based modelling approach for pest control
Authors: Shirley Mark D.F. , Rushton Stephen P. , Young Andrew G. , Port Gordon R.
Published: March 2003

An individual‐based simulation model was developed to investigate the long‐term dynamics of slug populations using the field slug Deroceras reticulatum as the test species.

The model consisted of two components: a model to provide meteorological data and simulate crop growth; and a model to simulate slug life histories. The patterns of slug population dynamics produced by the model matched the characteristic bivoltine life cycle of D. reticulatum observed in field studies.

A sensitivity analysis showed that the output of the model was significantly influenced by soil moisture, air temperature and leaf area index. These three environmental factors relate directly to important features in the biology of slugs. Of the life‐history variables used as inputs to the model, growth rate, environmental mortality and the mortality of sexually mature slugs had a significant effect on the population dynamics predicted by the model.

The model output was compared with the observed dynamics of slug populations collected in the field. Predicted slug dynamics and observed slug numbers showed a high degree of fidelity.

The simulation model was used to investigate the effects of varying the timing of control methods used to combat slug damage to crops. This result suggested that there is a ‘window of opportunity’, immediately post‐harvest, when slug control is likely to have a significant effect on both the current slug population and the slug numbers in the following spring.

Modelling approaches that link crop growth and population dynamics of pests have potential in assessing their agricultural impacts and in evaluating alternative strategies for their management.
Relationships between biomass and plant species richness in arid‐zone grazing lands
Authors: Oba Gufu , Vetaas Ole R. , Stenseth Nils C.
Published: March 2003

The relationship between biomass and species richness in temperate vegetation has been described as a hump‐back response model. The hump‐back model predicts that herbaceous species richness is highest at an intermediate level of biomass. However, this has not been investigated in arid‐zone grazing lands.

We tested the hump‐back prediction in an arid tropical grazing region in northern Kenya where a seasonal grazing exclosure system is practised. We compared vegetation structure, species richness and composition on an open range and exclosures at five sites to elucidate the potential mechanisms behind variation in species richness.

More biomass was accumulated within seasonal exclosures than in continuously grazed areas. Species richness in exclosure plots varied from 5·3 to 8·3 species m−2, while that in open plots varied from 5·1 to 7·5 species m−2. A pair‐wise test showed no difference in two of the five sites with respect to both total and herbaceous species richness.

The primary floristic gradient illuminated through ordination was related to biomass, while the secondary gradient was related to species richness. The exclosure plots had more abundant species, especially compared with open plots, which had more rare and occasional species. A total of 37 herbaceous species was recorded; 22% were indifferent to grazing, 30% grazing intolerant and 48% promoted by grazing.

The relationship between biomass and herbaceous species richness showed (i) no trend within the exclosures (maximum biomass 800 g m−2); (ii) a positive trend in the open grazing land (maximum of 500 g m−2); and a hump‐back pattern when (i) and (ii) were analysed together. Optimum richness corresponded to a biomass level of 400–500 g m−2. Species richness declined with increase in age of exclosures.

We confirmed that species richness will decline when biomass exceeds 500 g m−2, as predicted by the hump‐back model, even in arid grazing lands. Seasonal grazing exclosures may increase species richness to a certain level, but the decline in species richness with age of exclosures indicates that long‐term exclusion of grazing may not necessarily increase species richness in arid‐zone grazing lands.
Exploring spatial vegetation dynamics using logistic regression and a multinomial logit model
Authors: Augustin Nicole H. , Cummins Roger P. , French Donald D.
Published: March 2003

This study presents statistical methodology that uses spatial explanatory variables to improve simpler estimates of transition probabilities from categorical data, such as vegetation type, that have been recorded as classified cells (pixels) in a grid or lattice at different times.

A specific application is to examine successions in semi‐natural vegetation in north‐east Scotland. Questions related to these data include: Do transition probabilities of a pixel depend on the size of a patch of vegetation (polygon) and pixel location within the polygon? Do stable areas remain stable? Does the proximity of certain vegetation types influence transitions?

We selected spatial variables that were likely to be important in this application, where short‐range vegetative spread was thought to be an important factor.

The multinomial logit model is used to estimate the transition probabilities as a function of explanatory variables, including location, neighbourhood information and other factors recorded at the start of the transition period. This model allowed the testing of different assumptions about the dynamics of underlying processes leading to transitions.

When the number of categories, for example vegetation types, observed is large in comparison to the sample size, estimates of transition probabilities can be unreliable. We show that using change of category within the time period as the response in a logistic regression can still provide insight to the underlying dynamics of change in such a case.

The methods are illustrated with some Scottish vegetation classification data with pixels of size 5 × 5 m covering a square of area 0·25 km2. Two contrasting squares were investigated: the first was upland moorland grazed by sheep and the second was a lowland area with more varied vegetation and low intensity grazing by cattle.

In both squares there are strong spatial trends, and the neighbourhood of a pixel affected its transition. Prediction misclassification rates estimated from different models were compared using K‐fold cross‐validation. The multinomial model, including position in the square and number of neighbouring pixels in the same category as the pixel modelled, reduced the misclassification rate compared with the model without spatial explanatory variables.

The improved estimates of transition probabilities could be incorporated into Markov models used in simulation studies to predict future vegetation changes under different management strategies.