Michael Schaub - Integrated Population Models Theory and Ecological Applications with R and JAGS

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Population biology is a substantial discipline composed of two important subdisciplines, population genetics and population ecology. Population genetics and closely related fields ultimately provide a bridge to molecular and organismal processes by revealing the mechanisms responsible for genetic differences among individuals within and between populations. Population ecology, on the other hand, is the study of the mechanisms responsible for changes in the distribution and abundance of individuals over space and time. These evolutionary and ecological topics are in fact interrelated. To paraphrase an eloquent statement by G. Evelyn Hutchinson, the individuals of a population acting within an ecological theatre can affect the forces of natural selection for individuals and the evolution of the population (the company of actors). These eco-evolutionary dynamics affect lower organizational levels of biology and are essential ingredients that influence higher levels of organization such as biological communities and ecosystems.

At first glance, the primary parameters affecting the distribution and abundance of individuals over space and time are so simple that a troglodyte could be a population ecologist. In fact, when I attempt to explain my work as a wildlife population ecologist to family and friends, their responses are usually something along the lines of Oh, you count animals for a living. Sigha stark reminder to work on my elevator talk! I say this because when expressed as rates, the abundance of individuals at a particular location x and point in time t ( N x , t ) is simply a function of the preceding local b irth rate, the rate at which individuals i mmigrate into the location from elsewhere, the local d eath rate, and the rate at which individuals e migrate out of the location to other locales, which collectively act on local abundance at the previous point in time t 1: N x , t = N x , t 1 + ( b x,t 1 + i x,t 1 d x,t 1 e x,t 1) N x , t 1. When this simple discrete-time equation is rearranged, the balance of these bide demographic parameters defines the geometric rate of population growth from one time step to the next: N x , t / N x , t 1 = = 1 + b x,t 1 + i x,t 1 d x,t 1 e x,t 1. However, the dynamics of some (e.g., birth-flow) populations are better described in continuous time, and the demographic parameters and abundance state variable might importantly be structured by age, developmental stage, size, phenotype, or even genotype. These states of individuals might be measured continuously or with discrete categories, and in the real world, all demographic parameters are influenced by a large suite of biotic, abiotic, and anthropogenic forces. Too many, in fact, to estimate everything, and we must always deal with multiple uncertainties when making inferences about population dynamics. Alas, the job of a population ecologist is extremely challenging!

Population ecology is a mature subdiscipline of biology with a rich theory set and empirical findings to back it up. Nevertheless, there are many unknowns in population ecology and perhaps an equally great number of misunderstandings. Given that community and even ecosystem-level processes hinge on population-level processes, this should be concerning. Can we pretend to seek truths about these higher levels of biological organization by throwing demographic mechanisms under the rug and instead chase patterns in the outcomes at these higher levels? I think not. Can we work toward decreasing uncertainty and enhancing our understanding of demography to achieve a more mechanistic understanding of how multiple levels of biological organization are connected? I think so.