Abstract
In most ecological studies, within-group variation is a nuisance that obscures patterns of interest and reduces statistical power. However, patterns of within-group variability often contain information about ecological processes. In particular, such patterns can be used to detect positive growth autocorrelation (consistent variation in growth rates among individuals in a cohort across time), even in samples of unmarked individuals. Previous methods for detecting autocorrelated growth required data from marked individuals. We propose a method that requires only estimates of within-cohort variance through time, using maximum likelihood methods to obtain point estimates and confidence intervals of the correlation parameter. We test our method on simulated data sets and determine the loss in statistical power due to the inability to identify individuals. We show how to accommodate nonlinear growth trajectories and test the effects of size-dependent mortality on our method's accuracy. The method can detect significant growth autocorrelation at moderate levels of autocorrelation with moderate-sized cohorts (for example, statistical power of 80% to detect growth autocorrelation ρ 2 = 0.5 in a cohort of 100 individuals measured on 16 occasions). We present a case study of growth in the red-eyed tree frog. Better quantification of the processes driving size variation will help ecologists improve predictions of population dynamics. This work will help researchers to detect growth autocorrelation in cases where marking is logistically infeasible or causes unacceptable decreases in the fitness of marked individuals.
Highlights
Ecologists and evolutionary biologists have long been interested in growth in body size
Studies of growth typically focus on differences among means of populations or treatment groups, striving for low variability around the mean to increase statistical power; variation within groups is often treated as noise obscuring the phenomena of interest
Ecological studies are increasingly considering among-individual variation as either a treatment or a response variable [1,2,3,4,5,6]. These studies have shown that variation among individuals is itself the result of important biological processes and that population dynamics are sensitive to among-individual variation [7,8,9,10,11,12]
Summary
Ecologists and evolutionary biologists have long been interested in growth in body size. Ecological studies are increasingly considering among-individual variation as either a treatment or a response variable [1,2,3,4,5,6]. These studies have shown that variation among individuals is itself the result of important biological processes and that population dynamics are sensitive to among-individual variation [7,8,9,10,11,12]. By expanding the range of organisms and experimental designs where among-individual variation can be estimated, this method will enable researchers to better understand sources of variation in body size data
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