title: A simple approximation of moments of the quasi-equilibrium distribution of an extended stochastic theta-logistic model with non-integer powers.
abstract: The stochastic versions of the logistic and extended logistic growth models are applied successfully to explain many real-life population dynamics and share a central body of literature in stochastic modeling of ecological systems. To understand the randomness in the population dynamics of the underlying processes completely, it is important to have a clear idea about the quasi-equilibrium distribution and its moments. Bartlett et al. (1960) took a pioneering attempt for estimating the moments of the quasi-equilibrium distribution of the stochastic logistic model. Matis and Kiffe (1996) obtain a set of more accurate and elegant approximations for the mean, variance and skewness of the quasi-equilibrium distribution of the same model using cumulant truncation method. The method is extended for stochastic power law logistic family by the same and several other authors (Nasell, 2003; Singh and Hespanha, 2007). Cumulant truncation and some alternative methods e.g. saddle point approximation, derivative matching approach can be applied if the powers involved in the extended logistic set up are integers, although plenty of evidence is available for non-integer powers in many practical situations (Sibly et al., 2005). In this paper, we develop a set of new approximations for mean, variance and skewness of the quasi-equilibrium distribution under more general family of growth curves, which is applicable for both integer and non-integer powers. The deterministic counterpart of this family of models captures both monotonic and non-monotonic behavior of the per capita growth rate, of which theta-logistic is a special case. The approximations accurately estimate the first three order moments of the quasi-equilibrium distribution. The proposed method is illustrated with simulated data and real data from global population dynamics database.
authors.affiliation: Department of Mathematics, Institute of Chemical Technology, Mumbai, Nathalal Parekh Marg, Mumbai-400019, India. Electronic address: amiyaiitb@gmail.com.
authors.forename: Amiya Ranjan
authors.initials: AR
authors.lastname: Bhowmick
authors.affiliation: Applied Statistics Unit, Indian Statistical Institute, Kolkata, India.
authors.forename: Subhadip
authors.initials: S
authors.lastname: Bandyopadhyay
authors.affiliation: Department of Statistics, Visva Bharati University, Santiniketan, West Bengal, India. Electronic address: souravmath07@gmail.com.
authors.forename: Sourav
authors.initials: S
authors.lastname: Rana
authors.affiliation: Agricultural and Ecological Research Unit, Indian Statistical Institute, 203, B. T. Road, Kolkata 700108, India. Electronic address: sabyasachi@isical.ac.in.
authors.forename: Sabyasachi
authors.initials: S
authors.lastname: Bhattacharya
mesh_terms: D000818:Animals; D008962:Models, Theoretical; D011157:Population Dynamics; D013269:Stochastic Processes
keywords: Approximations of moments; Birth–death process; Cumulant truncation; Extended logistic growth family; Global population dynamics database; Quasi-equilibrium distribution