398 research outputs found
Variational Studies of Triangular Heisenberg Antiferromagnet in Magnetic Field
We present a variational study of the Heisenberg antiferromagnet on the
spatially anisotropic triangular lattice in magnetic field. First we construct
a simple yet accurate wavefunction for the 1/3-magnetization plateau uud phase
on the isotropic lattice. Beginning with this state, we obtain natural
extensions to nearby commensurate coplanar phases on either side of the
plateau. The latter occur also for low lattice anisotropy, while the uud state
extends to much larger anisotropy. Far away from the 1/3 plateau and for
significant anisotropy, incommensurate states have better energetics, and we
address competition between coplanar and non-coplanar states in the high field
regime. For very strong anisotropy, our study is dominated by quasi-1d physics.
The variational study is supplemented by exact diagonalization calculations
which provide a reference for testing the energetics of our trial wavefunctions
as well as helping to identify candidate phases.Comment: 15 pages, 11 figure
Among-year and within-population variation in foraging distribution of European shags Phalacrocorax aristotelis over two decades: implications for marine spatial planning
Marine spatial planning aims to deliver sustainable use of marine resources by minimizing environmental
impacts of human activities and designating Marine Protected Areas. This poses a challenge where
speciesâ distributions show spatio-temporal heterogeneity. However, due to logistic constraints and challenging
timescales many studies of distribution are undertaken over few years or on a restricted subset of
the population. Long-term studies can help identify the degree of uncertainty in those less comprehensive
in space and time. We quantify inter-annual and sub-colony variation in the summer foraging distribution
of a population of European shags Phalacrocorax aristotelis, using a tracking data set
comprising 320 individuals and 1106 foraging trips in 15 years from 1987 to 2010. Foraging distribution
over the study period was concentrated in three areas. Data from one and two years captured an average
of 54% and 64% of this distribution, respectively, but it required 8 yearsâ data to capture over 90% of the
distribution. Foraging range increased with population size when breeding success was low, suggesting
interplay between extrinsic and intrinsic effects. Furthermore, females had foraging ranges on average
36% greater than males. Finally, sub-colony segregation occurred in foraging areas up to 4 km from the
colony and in the most distant locations (>10 km), whilst there was considerable overlap at intermediate
distances (6â10 km). Our study highlights important considerations for marine spatial planning in particular,
and species conservation in general, notably the proportion of the population distribution identified,
the prevailing conditions experienced and the need for balanced sampling across the population
Modelling and mapping how common guillemots balance their energy budgets over a full annual cycle
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.
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.
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.
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.
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-driven questions that were previously unanswerable due to logistical complexities in collecting data on wild, mobile animals across full annual cycles
The PRISMS taxonomy of self-management support: derivation of a novel taxonomy and initial testing of its utility
Additive genetic and environmental variation interact to shape the dynamics of seasonal migration in a wild bird population
We thank everyone who contributed to long-term field data collection, particularly Raymond Duncan, Sarah Fenn, Hannah Grist, Calum Scott, Jenny Sturgeon, Moray Souter, John Anderson, and Harry Bell; and thank NatureScot for allowing work on the Isle of May National Nature Reserve, and Isle of May Bird Observatory Trust for supporting the longterm ringing of shags. We thank Stefanie Muff for helpful discussions, and Rita Fortuna and Thomas R. Haaland for useful comments on a manuscript draft. The current study was funded by Natural Environment Research Council (NERC; awards NE/M005186/1, NE/R000859/1, and NE/R016429/1 as part of the UK-SCaPE program delivering National Capability), Norwegian Research Council (SFF-III grant 223257, FRIPRO grant 313570), NTNU and University of Aberdeen. Analyses were performed using the IDUN cluster of NTNUPeer reviewedPublisher PD
Quantifying the impact of offshore wind farms on Gannet populations: a strategic ringing project
The sensitivity of seabird populations to density-dependence, environmental stochasticity and anthropogenic mortality
The balance between economic growth and wildlife conservation is a priority for many governments. Enhancing realism in assessment of populationâlevel impacts of anthropogenic mortality can help achieve this balance. Population Viability Analysis (PVA) is commonly applied to investigate population vulnerability, but outcomes of PVA are sensitive to formulations of densityâdependence, environmental stochasticity and life history. Current practice in marine assessments is to use precautionary models that assume no compensation from densityâdependence or rescueâeffects via âreâseedingâ from other colonies. However, if we could empirically quantify regulatory population processes, the responses of populations to additional anthropogenic mortality may be assessed with more realism in PVA.
Using Bayesian stateâspace models fitted to population time series from three sympatric seabird populations, selected for varied life histories, we inferred the extent to which their dynamics are driven by environmental stochasticity and densityâdependence.
Based on these inferences, we conducted an exhaustive PVA across credible parameterizations for intrinsic and extrinsic population regulation, simulated as a closed and reâseeded system. Scenarios of anthropogenic mortality, along a sliding scale of precaution, were applied both proportionally and as a fixed quota using Potential Biological Removal (PBR).
Baseline results from fitting revealed clear environmental regulation in two of our three species. Crucially, we found that for our empirically derived, realistic model parameterizations there are risks of decline to real populations even under very precautionary mortality scenarios. We find that PBR is dubious in application as a sustainable tool for population assessment when we account for regulation. Closed versus reâseeded models showed a large divergence in outcomes, with sharper declines in closed simulations. Fixedâquota mortality typically induced greater population declines comparative to proportional mortality, subject to regulation and reâseeding.
Synthesis and applications. Practitioners using arbitrary formulations of population regulation risk overâprecaution (economic constraint) or underâprecaution (endangering populations). The demands of increased economic development and preservation of wildlife require that methodologies apply techniques that confer reality and rigour to assessment. The current practice of employing models lacking densityâdependence and empirical environmental information imposes limitations in the efficacy of estimating impacts. Here, we provide a method to quantify the conditions that predominantly regulate a population and exacerbate the risk of decline from anthropogenic mortality. It is in the interests of both developers and conservationists to apply methods in population impact assessments that capture realism in the processes driving population dynamics
Identification of candidate pelagic marine protected areas through a seabird seasonal-, multispecific- and extinction risk-based approach
With increasing pressure on the oceans from environmental change, there has been a global call for improved protection of marine ecosystems through the implementation of marine protected areas (MPAs). Here, we used species distribution modelling (SDM) of tracking data from 14 seabird species to identify key marine areas in the southwest Atlantic Ocean, valuing areas based on seabird species occurrence, seasonality and extinction risk. We also compared overlaps between the outputs generated by the SDM and layers representing important human threats (fishing intensity, ship density, plastic and oil pollution, ocean acidification), and calculated loss in conservation value using fishing and ship density as cost layers. The key marine areas were located on the southern Patagonian Shelf, overlapping extensively with areas of high fishing activity, and did not change seasonally, while seasonal areas were located off south and southeast Brazil and overlapped with areas of high plastic pollution and ocean acidification. Non-seasonal key areas were located off northeast Brazil on an area of high biodiversity, and with relatively low human impacts. We found support for the use of seasonal areas depending on the seabird assemblage used, because there was a loss in conservation value for the seasonal compared to the non-seasonal approach when using âcostâ layers. Our approach, accounting for seasonal changes in seabird assemblages and their risk of extinction, identified additional candidate areas for incorporation in the network of pelagic MPAs
Numbers and distribution of the Great Cormorant in Iceland: Limitation at the regional and metapopulation level
Publisher's version (Ăștgefin grein)We studied a metapopulation of great cormorant (Phalacrocorax carbo) in Iceland, using complete aerial censuses of nests in 25 years during 1975â2015. Age composition was estimated in 1998â2014 by ground surveys in September and February. Brood size was estimated from aerial photographs in 2007â2015. Weather, food, breeding habitat, and density were considered as explanatory variables when examining numerical and distributional changes in the cormorant metapopulation. In 1975â1990 total nest numbers changed little, very low numbers about 1992 were followed by an annual increase of 3.5% in 1994â2015. Total nest numbers were positively correlated with estimates of spawning stocks of cod and saithe and inversely related to the subpolar gyre index (SPG-I). During the increase in 1994â2015, average colony size at first increased and then declined. Habitat use also changed: the proportion of nests on small rocky islets (skerries) at first declined, from 69% to 44% in 1995â2003 and then increased again to about 58% in 2012â2014. Habitat changes were probably a response to changed patterns of human disturbance. Breeding density, as nests per km 2 sea <20 m deep, was rather uniform among five defined regions in 1975â1996. Thereafter, densities became much higher in two sheltered regions with kelp forests and in one mostly exposed region. A second exposed region remained low and in the third nest numbers declined markedly. Thus, carrying capacity was higher in sheltered regions where cormorant breeding had historically been depressed by human disturbance. Brood size varied little among regions but declined with the years from about 2.5 to 1.8. The proportion of juveniles in September (fecundity) declined in 1998â2015 from over 0.4 to 0.3 and was inversely correlated with year and nest numbers, if outlier years were excluded, suggesting resource limitation. Survival of juvenile cormorants in SeptemberâFebruary was estimated at 0.471 ± 0.066 SE. Commercial fish stocks and climate indices were not correlated with the proportion of juveniles. Annual survival of adults (breeding and nonbreeding) was estimated from nest counts and age composition 1999â2014, as 0.850 ± 0.026 SE and showed no trend in 1998â2014. We conclude that the metapopulation of cormorants in Iceland was resource-limited at two levels: fecundity at the regional and winter survival at the total level.This study was supported mainly by the University of Iceland Research Fund by annual grants to AG (1994â2008) and to JEJ (2009â2015). We thank Ălfar Henningsson for piloting the cormorant censuses and brood surveys. For help with local and historical information we are grateful to many colleagues and local naturalists, especially Tryggvi EyjĂłlfsson (1927â2017), Hafsteinn GuĂ°mundsson, Ari GuĂ°mundur Ăvarsson, SteinĂłlfur LĂĄrusson (1928â2012), Ăvar Petersen, Kristinn H. SkarphĂ©Ă°insson, ĂstĂŸĂłr SkĂșlason, GuĂ°brandur Valdimarsson, Ălafur ĂĂłrĂ°arson (1915â2003), and Böðvar ĂĂłrisson. For guidance in environmental and biological matters we are especially indebted to Karl Gunnarsson, Sarah Wanless and Thomas Bregnballe. We thank Sigmundur Helgi Brink for preparing the map in Figure 2 and Erling Ălafsson for photos of cormorant in Figure 1. We thank the editors and 2 anonymous reviewers for comments that improved an earlier version of this manuscript.Peer Reviewe
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