Publications (6)25.17 Total impact
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Article: A primer on the application of Markov chains to the study of wildlife disease dynamics
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ABSTRACT: Summary1. For wildlife researchers, disease specialists and policy analysts unfamiliar with the mathematical/statistical language of disease models, translation of probability statements into meaningful terms for disease research and control may be challenging. Markov chain models are powerful tools, applicable to the study of disease dynamics that allow straightforward calculations of easily interpretable metrics of interest including probabilities of infection/recovery, expected times to initial infection, duration of illness and life expectancies for susceptible and infected individuals.2. We present the basic principles and assumptions behind Markov chain modelling with an intuitive interpretation of parameter estimates and a step-by-step guide (including software code) for implementing this approach in the study of wildlife diseases. We also include an explanation of the estimation process necessary to implement Markov chain modelling (i.e. estimating the probability of state transitions between consecutive time steps) from typical survey data.3. We demonstrate the usefulness and ease of calculation of Markov chains through an example using a house finch Carpodacus mexicanus–Mycoplasma gallisepticum (MG) system. Our results show how semi-weekly transition estimates of susceptible and infected individuals can be used to estimate a wide array of seasonal disease-associated metrics.4. Markov chain modelling can provide a basic understanding of parameters estimated from wildlife disease studies, and can aid in understanding the implications of disease on wildlife populations and in evaluation of control measures. We envision this paper serving as an entry point into the extensive literature and potential applications of Markov chains in epidemiological modelling.Methods in Ecology and Evolution 05/2010; 1(2):192 - 198. · 5.09 Impact Factor -
Article: When can efforts to control nuisance and invasive species backfire?
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ABSTRACT: Population control through harvest has the potential to reduce the abundance of nuisance and invasive species. However, demographic structure and density-dependent processes can confound removal efforts and lead to undesirable consequences, such as overcompensation (an increase in abundance in response to harvest) and instability (population cycling or chaos). Recent empirical studies have demonstrated the potential for increased mortality (such as that caused by harvest) to lead to overcompensation and instability in plant, insect, and fish populations. We developed a general population model with juvenile and adult stages to help determine the conditions under which control harvest efforts can produce unintended outcomes. Analytical and simulation analyses of the model demonstrated that the potential for overcompensation as a result of harvest was significant for species with high fecundity, even when annual stage-specific survivorship values were fairly low. Population instability as a result of harvest occurred less frequently and was only possible with harvest strategies that targeted adults when both fecundity and adult survivorship were high. We considered these results in conjunction with current literature on nuisance and invasive species to propose general guidelines for assessing the risks associated with control harvest based on life history characteristics of target populations. Our results suggest that species with high per capita fecundity (over discrete breeding periods), short juvenile stages, and fairly constant survivorship rates are most likely to respond undesirably to harvest. It is difficult to determine the extent to which overcompensation and instability could occur during real-world removal efforts, and more empirical removal studies should be undertaken to evaluate population-level responses to control harvests. Nevertheless, our results identify key issues that have been seldom acknowledged and are potentially generic across taxa.Ecological Applications 09/2009; 19(6):1585-95. · 5.10 Impact Factor -
Article: Overcompensatory response of a smallmouth bass (Micropterus dolomieu) population to harvest: release from competition?
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ABSTRACT: An intensive seven-year removal of adult, juvenile, and young-of-the-year smallmouth bass (Micropterus dolomieu) from a north temperate lake (Little Moose Lake, New York, USA) resulted in an increase in overall population abundance, primarily due to increased abundance of immature individuals. We developed a density-dependent, stage-structured model to examine conditions under which population control through harvest could result in the increase of a targeted species. Parameter values were derived from a 54-year data set collected from another north temperate lake (Lake Opeongo, Ontario, Canada) smallmouth bass population. Sensitivity analyses identified the demographic conditions that could lead to increased abundance in response to harvest. An increase in population abundance with harvest was most likely to occur when either (i) per capita recruitment at low levels of spawner abundance was large, juvenile survivorship was high, and maturation of age-4 and older juveniles was moderately high or (ii) per capita recruitment at low levels of spawner abundance was slightly lower, yet the maturation rate of age-3 juveniles and adult survivorship were high. Our modeling results together with empirical evidence further demonstrate the importance of overcompensation as a substantial factor to consider in efforts to regulate population abundance through harvest.Des captures intensives pendant sept ans des adultes, des juvéniles et des jeunes de l'année d'achigans à petite bouche (Micropterus dolomieu) dans un lac de la région tempérée nord (lac Little Moose, New York, É.-U.) ont eu pour effet un accroissement dans l'abondance globale de la population, principalement à cause d'une augmentation de l'abondance des individus immatures. Nous avons élaboré un modèle dépendant de la densité et structuré en fonction des stades afin d'évaluer les conditions sous lesquelles un contrôle de la population par la récolte peut entraîner une augmentation de l'espèce ciblée. Les valeurs des paramètres ont été tirées d'une banque de données couvrant 54 années et provenant d'une population d'achigans à petite bouche d'un autre lac de la région tempérée nord (lac Opeongo, Ontario, Canada). Des analyses de sensibilité ont permis d'identifier les conditions démographiques qui pourraient mener à une abondance accrue en réaction à la récolte. Une augmentation de l'abondance de la population en réaction à la récolte va plus vraisemblablement se produire quand ou bien (i) le recrutement par individu aux faibles densités de reproducteurs est important, la survie des juvéniles est élevée et la maturation des juvéniles d'âge 4 ou plus est modérément élevée ou alors (ii) le recrutement par individu aux faibles densités de reproducteurs est un peu plus faible, mais malgré tout le taux de maturation des juvéniles d'âge 3 et la survie des adultes sont élevés. Les résultats de notre modélisation combinés à des données empiriques démontrent de plus l'importance de la surcompensation comme facteur important à considérer lorsqu'on tente de contrôler l'abondance d'une population par des récoltes.Canadian Journal of Fisheries and Aquatic Sciences 09/2008; 65(10):2279-2292. · 2.21 Impact Factor -
Article: State-specific detection probabilities and disease prevalence.
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ABSTRACT: Investigations of disease dynamics in wild animal populations often use estimated prevalence or incidence as a measure of true disease frequency. Such indices, almost always based solely on raw counts of infected and uninfected individuals, are often used as the basis for analysis of temporal and spatial dynamics of diseases. Generally, such studies do not account for potential differences in observer detection probabilities of host individuals stratified by biotic and/or abiotic factors. We demonstrate the potential effects of heterogeneity in state-specific detection probabilities on estimated disease prevalence using mark-recapture data from previous work in a House Finch (Carpodacus mexicanus) and Mycoplasma gallisepticum system. In this system, detection probabilities of uninfected finches were generally higher than infected individuals. We show that the magnitude and seasonal pattern of variation in estimated prevalence, corrected for differences in detection probabilities, differed markedly from uncorrected (apparent) prevalence. When the detection probability of uninfected individuals is higher than infected individuals (as in our study), apparent prevalence is negatively biased, and vice versa. In situations where state-specific detection probabilities strongly interact over time, we show that the magnitude and pattern of apparent prevalence can change dramatically; in such cases, observed variations in prevalence may be completely spurious artifacts of variation in detection probability, rather than changes in underlying disease dynamics. Accounting for differential detection probabilities in estimates of disease frequency removes a potentially confounding factor in studies seeking to identify biotic and/or abiotic drivers of disease dynamics. Given that detection probabilities of different groups of individuals are likely to change temporally and spatially in most field studies, our results underscore the importance of estimating and incorporating detection probabilities in estimated disease prevalence (specifically), and more generally, any ecological index used to estimate some parameter of interest. While a mark-recapture approach makes it possible to estimate detection probabilities, it is not always practical, especially at large scales. We discuss several alternative approaches and categorize the assumptions under which analysis of uncorrected prevalence may be acceptable.Ecological Applications 02/2007; 17(1):154-67. · 5.10 Impact Factor -
Article: Dynamics of a novel pathogen in an avian host: Mycoplasmal conjunctivitis in house finches.
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ABSTRACT: In early 1994, a novel strain of Mycoplasma gallisepticum (MG)--a poultry pathogen with a world-wide distribution--emerged in wild house finches and within 3 years had reached epidemic proportions across their eastern North American range. The ensuing epizootic resulted in a rapid decline of the host population coupled with considerable seasonal fluctuations in prevalence. To understand the dynamics of this disease system, a multi-disciplinary team composed of biologists, veterinarians, microbiologists and mathematical modelers set forth to determine factors driving and influenced by this host-pathogen system. On a broad geographic scale, volunteer observers ("citizen scientists") collected and reported data used for calculating both host abundance and disease prevalence. The scale at which this monitoring initiative was conducted is unprecedented and it has been an invaluable source of data for researchers at the Cornell Laboratory of Ornithology to track the spread and magnitude of disease both spatially and temporally. At a finer scale, localized and intensive field studies provided data used to quantify the effects of disease on host demographic parameters via capture-mark-recapture modeling, effects of host behavior on disease and vice-versa, and the biological and genetic profiles of birds with known phenotypic characteristics. To balance the field-based component of the study, experiments were conducted with finches held in captivity to describe and quantify the effects of experimental infections on hosts in both individual and social settings. The confluence of these various elements of the investigation provided the foundation for construction of a general compartmentalized epidemiological model of the dynamics of the house finch-MG system. This paper serves several purposes including (i) a basic review of the pathogen, host, and epidemic cycle; (ii) an explanation of our research strategy; (iii) a basic review of results from the diverse multi-disciplinary approaches employed; and (iv) pertinent questions relevant to this and other wildlife disease studies that require further investigation.Acta Tropica 05/2005; 94(1):77-93. · 2.72 Impact Factor -
Article: Mycoplasma gallisepticum infection dynamics in a house finch population: seasonal variation in survival, encounter and transmission rate
Journal of Animal Ecology 06/2004; 73(4):651 - 669. · 4.94 Impact Factor
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Institutions
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2004–2010
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Cornell University
- Department of Natural Resources
Ithaca, NY, USA
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