14 results on '"Horswill, Catharine"'
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2. The relative importance of opposing drivers in determining population change in macaroni penguins Eudyptes chrysolophus
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Horswill, Catharine
- Subjects
598.47 ,QH301 Biology ,QL Zoology - Abstract
It is widely recognised that both nutrient-driven processes acting from the “bottom-up” and predator-driven processes acting from the “top-down” are important drivers of population change. However, studies that examine how these joint forces influence the population dynamics of oceanic species are lacking. In the Antarctic and Sub-Antarctic ecosystem, human-mediated changes have driven biological change at both ends of the food chain; rapid regional warming at the bottom and heavy exploitation of apex predator populations at the top. Consequently, many populations of marine predators have rapidly changed in size over the last 50-years. Unravelling the effects of bottom-up and top-down forcing on these open ocean ecosystems, has thus been highlighted as an immediate priority for polar scientists. The overall aims of this study were to use demographic, environmental and diet data to unravel the processes that contributed to a population of macaroni penguins at South Georgia declining rapidly between 1985 and 2012. I use mark–recapture modelling to examine the survival rates of macaroni penguins. Over 10 years, birds were marked with subcutaneous electronic transponder tags and re-encountered using an automated gateway system fitted at the entrance to the colony. These findings were combined with a 28-year time series of population counts and productivity measurements in an age-structured state-space population model to disentangle the processes underlying the observed population decline. Finally, I combined stable isotope analysis and tracking data to investigate the individual strategies macaroni penguins might employ to mitigate the effects of density-dependence during the breeding season. Macaroni penguins at South Georgia declined at 6% per year between 1985 and 2000, stabilising thereafter. This study indicates that the population declined in response to recruitment rates being lower than adult mortality. This trend was potentially accelerated by three large mortality events that were possibly associated with top-down predation pressure from giant petrels. Survival rates were low and variable during the fledging year, increasing to much higher levels from age 1 onwards. Year-to-year variability in demographic rates was induced by a combination of individual quality, top-down predation pressure and bottom-up environmental forces. The relative importance of these covariates on survival rates was age-specific, whereby predation pressure had a considerably greater effect during the fledgling year compared with birds older than 1-year. The population trajectory stabilised after 2000 in response to an increase in survival, as well as density-dependent feedbacks upon productivity. In order to minimise the effects of density-dependence during the breeding season and optimise daily energy expenditure, macaroni penguins appeared to make distinct dietary choices that remained highly consistent from year-to-year. Individually specialised foraging strategies occurred in response to seasonal variations in foraging range, conspecific density and prey availability at the foraging sites. The future stability of this population will depend on the carrying capacity of the environment supporting productivity rates at their present level, and the population size and breeding success of giant petrels not increasing so that adult survival rates can remain stable. More broadly, this study highlights the importance of considering multiple causal effects across different life-stages when examining the demography of seabirds, and demonstrates the additional insights that can be gained by using models with increased precision.
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- 2015
3. Optimizing the automated recognition of individual animals to support population monitoring
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de Lorm, Tijmen A., primary, Horswill, Catharine, additional, Rabaiotti, Daniella, additional, Ewers, Robert M., additional, Groom, Rosemary J., additional, Watermeyer, Jessica, additional, and Woodroffe, Rosie, additional
- Published
- 2023
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4. Relative brain size is associated with natal dispersal rate and species' vulnerability to climate change in seabirds
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Constanti Crosby, Laurel, primary, Sayol, Ferran, additional, and Horswill, Catharine, additional
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- 2023
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5. Optimising the automated recognition of individual animals to support population monitoring
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Lorm, Tijmen de, primary, Horswill, Catharine, additional, Rabaiotti, Daniella, additional, Ewers, Robert, additional, Groom, Rosemary, additional, Watermeyer, Jessica, additional, and Woodroffe, Rosie, additional
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- 2023
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6. Do krill fisheries compete with macaroni penguins? Spatial overlap in prey consumption and catches during winter
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Ratcliffe, Norman, Hill, Simeon L., Staniland, Iain J., Brown, Ruth, Adlard, Stacey, Horswill, Catharine, and Trathan, Philip N.
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- 2015
7. Cryptic population decrease due to invasive species predation in a long‐lived seabird supports need for eradication
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Oppel, Steffen, primary, Clark, Bethany L., additional, Risi, Michelle M., additional, Horswill, Catharine, additional, Converse, Sarah J., additional, Jones, Christopher W., additional, Osborne, Alexis M., additional, Stevens, Kim, additional, Perold, Vonica, additional, Bond, Alexander L., additional, Wanless, Ross M., additional, Cuthbert, Richard, additional, Cooper, John, additional, and Ryan, Peter G., additional
- Published
- 2022
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8. Modelling and mapping how common guillemots balance their energy budgets over a full annual cycle
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Dunn, Ruth E., primary, Green, Jonathan A., additional, Wanless, Sarah, additional, Harris, Mike P., additional, Newell, Mark A., additional, Bogdanova, Maria I., additional, Horswill, Catharine, additional, Daunt, Francis, additional, and Matthiopoulos, Jason, additional
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- 2022
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9. Modelling and mapping how common guillemots balance their energy budgets over a full annual cycle
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Dunn, Ruth E., Green, Jonathan A., Wanless, Sarah, Harris, Mike P., Newell, Mark A., Bogdanova, Maria I., Horswill, Catharine, Daunt, Francis, Matthiopoulos, Jason, Dunn, Ruth E., Green, Jonathan A., Wanless, Sarah, Harris, Mike P., Newell, Mark A., Bogdanova, Maria I., Horswill, Catharine, Daunt, Francis, and Matthiopoulos, Jason
- Abstract
1. The ability of individual animals to balance their energy budgets throughout the annual cycle is important for their survival, reproduction and population dynamics. However, the annual cycles of many wild, mobile animals are difficult to observe and our understanding of how individuals balance their energy budgets throughout the year therefore remains poor. 2. We developed a hierarchical Bayesian state-space model to investigate how key components of animal energy budgets (namely individual energy gain and storage) varied in space and time. Our model used biologger-derived estimates of time-activity budgets, locations and energy expenditure to infer year-round time series of energy income and reserves. The model accounted for seasonality in environmental drivers such as sea surface temperature and daylength, allowing us to identify times and locations of high energy gain. 3. Our study system was a population of common guillemots Uria aalge breeding at a western North Sea colony. These seabirds manage their energy budgets by adjusting their behaviour and accumulating fat reserves. However, typically during severe weather conditions, birds can experience an energy deficit over a sustained period, leading to starvation and large-scale mortality events. 4. We show that guillemot energy gain varied in both time and space. Estimates of guillemot body mass varied throughout the annual cycle and birds periodically experienced losses in mass. Mass losses were likely to have either been adaptive, or due to energetic bottlenecks, the latter leading to increased susceptibility to mortality. Guillemots tended to be lighter towards the edge of their spatial distribution. 5. We describe a framework that combines biologging data, time-activity budget analysis and Bayesian state-space modelling to identify times and locations of high energetic reward or potential energetic bottlenecks in a wild animal population. Our approach can be extended to address ecological and conservation
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- 2022
10. Cryptic population decrease due to invasive species predation in a long‐lived seabird supports need for eradication
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Oppel, Steffen, Clark, Bethany L, Risi, Michelle M, Horswill, Catharine, Converse, Sarah J, Jones, Christopher W, Osborne, Alexis M, Stevens, Kim, Perold, Vonica, Bond, AL, Wanless, Ross M, Cuthbert, Richard, Cooper, John, Ryan, Peter G, Oppel, Steffen, Clark, Bethany L, Risi, Michelle M, Horswill, Catharine, Converse, Sarah J, Jones, Christopher W, Osborne, Alexis M, Stevens, Kim, Perold, Vonica, Bond, AL, Wanless, Ross M, Cuthbert, Richard, Cooper, John, and Ryan, Peter G
- Abstract
SUMMARY 1. Invasive species are one of the greatest drivers of biodiversity loss worldwide, but the eradication of invasive species from islands is a highly efficient management strategy. Because eradication operations require large financial investments, uncertainty over the magnitude of impacts of both invasive species and their removal can impede the willingness of decision makers to invest in eradication. Such uncertainty is prevalent for long-lived species that display an inherent lag between life stages affected by invasive species and those used for population status assessments. 2. Albatrosses are among the longest-living bird species and are threatened on land by invasive species and at sea by industrial fisheries. As in many seabird species, usually only a segment of the population (breeding adults) is used for status assessments, making it difficult to assess their population trends and the potential benefit of conservation action, such as the management of predatory invasive species. 3. We used population monitoring and mark-recapture data to estimate the past population trajectory of the Critically Endangered Tristan Albatross (Diomedea dabbenena) by accounting for unobservable birds at sea in an integrated population model. We then projected the future population trajectory for scenarios with or without predation by invasive house mice (Mus musculus) on their main site, Gough Island. 4. The adult breeding population remained stable between 2004 and 2021, but breeding success was low (31%) and our model indicated that the total population (including unobservable immature birds) decreased from a median estimate of 9795 to 7752 birds. Eradicating invasive mice leading to a two-fold increase in breeding success would result in a 1.8–7.6 times higher albatross population by 2050 (median estimate 10 352 individuals) than without this intervention. 5. Low reproductive output for long-lived species may lead to a cryptic population decrease, which can be obscur
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- 2022
11. Impact assessments of wind farms on seabird populations that overlook existing drivers of demographic change should be treated with caution
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Horswill, Catharine, primary, Miller, Julie A. O., additional, and Wood, Matt J., additional
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- 2022
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12. Minimising the limitations of using dietary analysis to assess foodweb changes by combining multiple techniques
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Horswill, Catharine, Jackson, Jennifer A., Medeiros, R., Nowell, R.W., Trathan, Phil N., O'Connell, T.C., Horswill, Catharine, Jackson, Jennifer A., Medeiros, R., Nowell, R.W., Trathan, Phil N., and O'Connell, T.C.
- Abstract
Dietary studies of marine predators offer an immediate signal of foodweb changes occurring at lower trophic levels, and therefore are often used to assess the ecosystem status of marine systems. Conventionally, these studies are based on morphological analysis of prey remains in stomach contents, involving invasive and destructive techniques to collect samples. More recently, the number of dietary studies based on less invasive biochemical and molecular approaches has dramatically increased. However, all three methods, morphological, biochemical and molecular, have well-documented limitations for resolving taxonomy, temporal variation or biomass composition. In this study, we minimise these limitations by considering multiple techniques in combination. As a case study, we report the target prey species and diet composition of a marine predator that has been used to assess annual change in managed fishing areas for several decades, the macaroni penguin Eudyptes chrysolophus. We use biochemical (stable isotope) and molecular (DNA) analysis of faecal samples collected across the different phases of a single breeding season, and compare the resolved diet to a 26-year dataset of stomach contents collected from a closely located colony (0.25 km apart) that exploits identical foraging grounds. Molecular analysis increased the known target prey species for this highly monitored population by 31%, including a fish species of commercial importance. Biochemical analysis detected subtle changes in the proportion of fish and krill in the diet, demonstrating promising opportunities for using a combined molecular and biochemical method to assess inter-annual foodweb changes at lower trophic levels. The combined approach offers a less invasive sampling methodology, compared to morphological analysis, and provides more information regarding prey species diversity and the overall trophic signature of the diet. Further studies are required to examine the feasibility of using this appr
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- 2018
13. Review of Seabird Demographic Rates and Density Dependence. JNCC Report no. 552
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Horswill, Catharine and Robinson, Robert A.
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Introduction\ud \ud This report presents individual species accounts for a selection of British seabirds, sea ducks, divers and grebes. Each account gathers the most up to date published estimates on the following demographic parameters: age-specific survival, age-specific productivity, age of recruitment, incidence of missed breeding, and natal and adult breeding dispersal. Particular attention has been given to regional variation in demographic rates, indicating the extent to which estimates may be applied to other less-well studied colonies. Where possible, the intrinsic and extrinsic factors that influence demographic rates are also detailed.\ud \ud The reported rates should enable population models that assess the impacts of offshore wind farms to be developed as reliably and realistically as possible. Where sufficient data could not be gathered using UK examples, data from colonies outside of the UK have been presented, or a proxy species has been identified. The evidence for density-dependent regulation of seabird demographic rates is also reviewed using examples from the UK, as well as non-UK studies on similar species.
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- 2015
14. Incorporating density‐dependent regulation into impact assessments for seabirds.
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Merrall, Eve, Green, Jonathan A., Robinson, Leonie A., Butler, Adam, Wood, Matt J., Newell, Mark A., Black, Julie, Daunt, Francis, and Horswill, Catharine
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POPULATION viability analysis , *ENVIRONMENTAL impact analysis , *POPULATION dynamics , *PRECAUTIONARY principle , *WEIBULL distribution - Abstract
Many industries are required to perform population viability analysis (PVA) during the consenting process for new developments to establish potential impacts on protected populations. However, these assessments rarely account for density‐dependent regulation of demographic rates. Excluding density‐dependent regulation from PVA‐based impact assessments is often assumed to provide a maximum estimate of impact and therefore offer a precautionary approach to assessment. However, there is also concern that this practice may unnecessarily impede the development of important industries, such as offshore renewable energy. In this study, we assess density‐dependent regulation of breeding success in 31 populations of seabird. We then quantify the strength and form of this regulation using eight different formulations. Finally, we use PVA to examine how each formulation influences the recreation of observed dynamics (i.e. model validation), as well as the predicted absolute and relative population response to an extrinsic threat (i.e. model projection). We found evidence of both negative (n = 3) and positive (n = 5) regulation of seabird breeding success. In populations exhibiting negative regulation, excluding density‐dependent regulation from PVA‐based impact assessment allowed uncontrolled population growth, such that model outcomes became biologically implausible. By contrast, in populations exhibiting positive regulation, excluding density‐dependent regulation provided an appropriate reconstruction of observed dynamics, but population decline was underestimated in some populations. We find that multiple formulations of density dependence perform comparably at the detection, validation and projection stages of analysis. However, we tentatively recommend using a log‐linear or Weibull distribution to describe density‐dependent regulation of seabird breeding success in impact assessments to balance accuracy with caution. Finally, we show that relative PVA metrics of impact assessment cannot necessarily be used to overcome PVA misspecification by assuming density independence in positively regulated populations. Synthesis and applications: We suggest that a density‐dependent approach when performing PVA‐based assessments for seabird populations will prevent biologically unrealistic, unconstrained population growth and therefore ensure meaningful PVA metrics in populations experiencing negative regulation. It will also maintain a precautionary approach for populations experiencing positive regulation, crucial when estimating impacts for these more vulnerable populations. These conclusions have immediate international application within the consenting processes for marine industries. [ABSTRACT FROM AUTHOR]
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- 2024
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