No description available.

The relative importance of habitat and landscape for chalk grassland butterflies was investigated among a network of chalk grassland habitat patches. Local scale habitat selection models were derived from analyses on a single large chalk grassland site (Porton Down). Landscape scale models were . then derived from species-specific local scale variables added to variables measured in the patch network, which included Porton Down as a major source site. An investigation was also carried out as to whether landscapes cale processes were acting within Porton Down. Models showed that landscapes cale processes may be acting within the single, large site for some species. However, lack of patch definition and natural variations in local population densities may give rise to apparent landscapes cale processes within this site which fundamentally differ from true landscape scale processes. At the true landscape scale across the patch network, patch area was the most commonly included landscape scale variable in models, although only for five species. For one species, dark green fritillary Argynnis aglaja, distance from the major source site and average distance from other sites were of over-riding importance. For most species, local scale habitat and environment variables were sufficient to describe their presence or density among the patch network. Incidence Function Models were used to estimate metapopulation dynamic parameters for seven species, Both sensitivity to environmental stochasticity and colonisation ability were correlated with species' scores on canonical community analysis axes (related to local habitat, patch area, distance from source site and inter-patch'distance measures). This suggests that species conforming to conventional metapopulation models form an ecological continuum with those responding to local scale habitat and landscape scale processes in a less prescriptive fashion. Species groups with either restricted or widespread distributions appear to conform to a landscape connectivity model. Five ecologically similar species appear to be moving from the former to latter group across a 'connectivity threshold', possibly due to recent increases in larval food-plant availability in the wider landscape.

No description available.

Parasites are vital aspects of an ecosystem, and yet have only recently begun to be included in theoretical studies of food webs. There are numerous reasons for this neglect, but recent interest has arisen in remedying the situation. Key to the addition of parasites are various structural features that ecological networks display, and the mechanisms behind them. We look at two of these features, nestedness and downward asymmetry, and describe mathematically the forces that create them. We discover from the basic reproductive ratio that population dynamics are insufficient to drive nested and anti-nested patterns in host-parasite networks, and instead we demonstrate the manner in which adaptive dynamics may be used to explain patterns through the coevolution of species interaction traits. We use the same technique in a mutualistic network in order to compare results. Following this, we use the basic reproductive ratio to demonstrate how the population dynamics of infectious systems promote the presence of trophic parasites in particular interaction motifs, and discuss the implications of the addition of parasites to the stability of ecological networks as a whole. In summary, we demonstrate that the optimal use of resources by species in order to promote population growth results in two important structural patterns of host-parasite networks. Insights gained from this motivate us to investigate the importance of parasites on models of food web dynamics, and in particular, their stability.

No description available.

Oceanic islands contribute significantly to global biodiversity due to the high levels of endemism they exhibit. Many island species have, however, become extinct following the arrival of humans, and many others remain threatened. This thesis focuses on one of these threatened island species; the St. Helena Plover, Charadrius sanctaehelenae. The objectives of my PhD were threefold: to understand the current status of the St. Helena Plover, to investigate aspects of its environment that may threaten its persistence, and to compare conservation options. Observational data were used to understand how variation in the environment influences the plover’s distribution and demography. This information was used to inform a controlled trial investigating the use of nest exclosures as a way to improve productivity. New understanding of the species’ population ecology was brought together to create a stochastic meta-population model. This model was used to understand the influence of demographic and life history parameters on the population growth rate and to compare alternative conservation options. Indicators of the future status of the St. Helena Plover population appear mixed; with numbers predicted to decline slowly, but with a high chance of species persistence over the timeframe simulated, 50 years. Nest predation by introduced species, predominately cats, was predicted to suppress nest survival and bring productivity at some sites below that required for population stability. The use of nest exclosures did not increase nest survival and led to a decrease in adult survival. Habitat characteristics were more important in determining the species distribution than influencing its demography. The plovers appeared to select breeding habitat to maximise the visibility from the nest and the accessibility of invertebrate prey. Simulations suggested that predator control would have a large beneficial effect on the population growth rate and that concurrent habitat improvement would have greater than additive benefits. Targets for conservation management include Deadwood Plain; predator control as this key site was predicted to lead to the largest increase in the overall population growth rate, and Man and Horse; there may be potential at this site to increase adult survival, which was found to be the most influential demographic parameter.

1. Although flounders are present throughout the year in the estuary and the Bristol Channel, it was observed that sexually mature animals occurred in late summer/autumn and only in small numbers during the winter. Young animals (0+ and 1+), however, were present at all times but in varying numbers. The absence of sexually mature animals in late spring/early summer is occasioned by breeding factors, while in the case of the younger fish the variation in numbers is probably due to feeding migration, Sea snails and five bearded rocklings, on the other hand, are winter residents in the study area (except in the case of five bearded rocklings at Minehead), arriving in small numbers in autumn which build up to peak numbers by mid-winter, only to diminish by late winter/early spring. From year to year fish populations vary in this region, but the patterns of behaviour within species remains reasonably constant. 2. The age structure of the Severn fish population has been studied by otolith readings combined with length-frequency distributions and it was observed that the majority of the fish population in this area belong to the first and second year classes, indicating that this region is a nursery ground for a number of different species of fish. 3. Despite the fact that the Severn region is heavily industrialised, no disparity in growth in length and weight was observed for animals obtained from this region when compared with those from areas where little industrial activity exists and our results show rapid growth for young fish during their period of residency in the estuary and the Bristol Channel. 4. We note that there is generally a preponderance of females (in some cases, vary marked) over males in the three main species of teleosts investigated, but so far we can offer no explanation for this observation. 5. Flounders, sea snails and five bearded rocklings spawn from spring to early summer, but there is some evidence for species differences. Further, it is concluded that the rate of growth of the gonads is similar at all the sites within the estuary and the channel. Immediately prior to the onset of the spawning migration, the gonads of female sea snails, five bearded rocklings and flounders grew rapidly and eventually accounted for at least 10-13% of the total body weight, but the increase in weight of the testis was less dramatic and at the time of departure from the study area formed about 1,0-1.5% of the total body weight of the fish. 6. The spawning migratory pattern is the same for male and female flounders and five bearded rocklings, but in the case of sea snails there was a tendency for the males to leave for the spawning grounds a few weeks earlier than the females. 7. The utilisation of food organisms by teleosts has not always been closely correlated with the relative abundance of such organisms, but where many dietary organisms are present, food preferences may be observed. Thus, Gammarus spp. were found to be an important and relatively constant food organism for all the teleosts studied, except for mature flounders taken in the Barnstaple area, where the bivalve, Macoma baltica was found to be widely ingested. Despite the abundance of shrimps, Crangon erangon, throughout the year, there was an actual decrease in the uptake of this decapod in all the three species investigated since 1973 in the Severn Estuary and the Bristol Channel. 8. Although there was variation in the percentage of nematode infestation from species to species, it was seen however, that in any one species the percentage of infection did not vary from site to site. 9. The rate of uptake of heavy metals in the vertebrate and invertebrate populations of the study area depend largely on such factors as salinity, water temperature, type of food uptake, and age, as well as differing from species to species. As a result of continuous monitoring a considerable amount of data has been collected concerning the uptake of zinc, lead and cadmium by fish and their prey. Concentrations of cadmium and lead in fish tissues are lower than in their food organisms. Thus, the ratios of concentrations in predator and prey varied from 1 r 25,0 for cadmium and 1 : 2-3 for lead. On the other hand, zinc concentrations in predator and prey are similar. Present studies suggest that while lead and zinc are to some extent present in high concentrations in the natural run off of the region, cadmium is an industrial by-product, since fish from the less industrialised areas contain much lower concentrations of this element compared with those from sampling sites closer to industrial activity.

The studies described in this thesis sought, to establish the importance of Rattus exulans and Rattus rattus as pests of two crops, coconuts and cocoa, in Fiji. Movement patterns of R. exulans were investigated. Males moved greater distances in both crops with distances increasing with population density. The eye-lens technique proved to be useful for ageing rats up to about 100 days. Population levels fluctuated considerably at all sites but there was no marked seasonal trend. No simple relationship existed between rat densities and damage to the two crops. Damage only increased when marked population peaks coincided with a possible shortage of other sources of food. Both species attacked developing coconuts but R. rattus caused most damage because it foraged more extensively in the crowns of taller palms. R. rattus also caused most damage to cocoa despite both species being in constant contact with the crop. Attack of cocoa appeared to be a learned pattern of behaviour. Coconut damage and production was surveyed at 16 sites over three years. Damage varied considerably between palms, sites and years. Rats favoured coconuts aged three to seven months, stages of development during which sugar concentrations were highest. Because coconuts on particular palms were favoured a possible basis for this palm selection was sought. Despite concentration of attack, favoured palms did not produce fewer mature coconuts. A trial simulating rat damage established that the coconut palm was capable of compensating for the premature removal of nuts. Compensation was conservatively estimated to replace 50 percent of the nuts lost. Control of rat damage to coconuts in Fiji was not economical because of the low level of loss after compensation and the inefficiency of trunk banding, the most convenient method of reducing damage. Damage to cocoa was serious at some localities. No compensation occurred but control, using warfarin based baits, proved highly economical.

The perennial herbaceous understorey found in well-maintained hedgerows and grassy field margins provides resources or refuge for a highly diverse fauna. Invertebrates are especially important as they underpin food chains, with many species being valuable polyphagous predators that feed on arable crop pests. However, agricultural intensification has caused many of these non-cropped areas to become degraded or lost, resulting in very high field area to edge ratios. Farmland biodiversity has declined markedly as a consequence. Predator assemblages dependent on margins within their lifecycles are often less abundant and diverse, with slower colonisation of feeding sites. The temperature-buffered conditions found within dense tussocky grasses in margins are particularly crucial for predator overwinter survival. 'Beetle banks' are grass-sown ridges designed to replicate and increase the availability of this kind of habitat, in a simple and inexpensive form. The original design was for single island strips extending across large arable fields, where predators are most impoverished, to effectively shrink the field in terms of spring dispersal into the crop. Early experimental banks still exist, and many more have been sown over the decade since the concept was first put into practice through the recommendation of advisory organisations. However, long-term management strategies for their successful upkeep are not clearly identified. With a known establishment date, beetle banks provide a means of exploring potential successional change or biodiversity development within new habitat. I assessed age-related differences in the botanical composition of beetle banks, considering potential degradation as overwintering sites, and resource provision for other wildlife. A dense structure was retained, despite age, thus banks continue to be functional for predators for over a decade at least. Increasing summer floral diversity with age may also benefit other insects. Weed presence was no worse than conventional margins, and so control should be relatively straightforward. Associated with this work was an evaluation of whether levels of polyphagous invertebrates, previously reported as high in new sites, were sustained in older beetle banks. I discovered little difference in densities per m' between beetle banks and conventional field margins, regardless of age, while boundary-overwintering carabid densities increased though time. Predator diversity was also similar between habitat types through the year. A large-scale trapping experiment was undertaken to evaluate the spatial-temporal patterns of predatory Carabidae in fields adjacent to beetle banks through the crop season. A novel spatial analysis was used to explore the distributions of different species groups. I found evidence to support the description of a 'wave' of boundary-species emerging from refuges and dispersing across the field. In contrast, field-inhabiting species were slow to develop from field centres and may be of less value for pest control. Beetle banks appear as valuable as conventional boundaries for aiding carabid dispersal into crops. Subtle microclimate, prey distribution or edaphic factors probably accounted for the spatially and temporally fluctuating activity-densities of beetles observed through the season. In addition, the abundance of predatory Empididae was similar within a beetle bank and a hedgerow, with a low, homogeneous presence at increasing distances from them, coinciding in time with serious cereal pests. Thus beetle banks may also contribute useful habitat resource for these little studied insects. 1 hypothesised that simple sown grass strips would contain a lower diversity of other invertebrates when compared to older, botanically complex habitats, although this difference might lessen with age. There was evidence to support this view. Beetle banks were found to contain useful, albeit lower, densities of game bird chick-food, when compared with conventional field margins; additionally furnishing nesting cover for adults birds. Severe declines in wild game are attributed to losses of these invertebrates vital for chick survival, as well as inadequate provision of nesting and brood-rearing habitat. Orthopteran species richness was similar between beetle banks and conventional margins, although there were compositional differences in capture; and older banks were increasingly speciose. Grasshoppers favoured mid-field banks, whereas bushcrickets tended to prefer hedgebottoms. Additionally, Lepidopteran species composition was investigated. As expected fewer butterflies were observed in beetle banks than hedge banks on conventionally managed farmland, but the grass swards clearly were of value, with butterfly presence related to floristic species richness and diversity. Better management, such as incorporating conservation headlands alongside beetle banks, and protection &om agronomic activities in the field, may be a means of further enhancing the resources that beetle banks provide for these invertebrates of increasing conservation concern. A supplementary chapter of this thesis describes the findings of a questionnaire survey sent to a cohort of farmers in southern England, to elucidate their current perceptions, opinions and use of the beetle bank design.

No description available.