Iles David T.

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Iles
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David T.
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Quantifying the causes and consequences of variation in satellite-derived population indices: a case study of emperor penguins

2021-08-11 , Labrousse, Sara , Iles, David T. , Viollat, Lise , Fretwell, Peter T. , Trathan, Phil N. , Zitterbart, Daniel , Jenouvrier, Stephanie , LaRue, Michelle

Very high-resolution satellite (VHR) imagery is a promising tool for estimating the abundance of wildlife populations, especially in remote regions where traditional surveys are limited by logistical challenges. Emperor penguins Aptenodytes forsteri were the first species to have a circumpolar population estimate derived via VHR imagery. Here we address an untested assumption from Fretwell et al. (2012) that a single image of an emperor penguin colony is a reasonable representation of the colony for the year the image was taken. We evaluated satellite-related and environmental variables that might influence the calculated area of penguin pixels to reduce uncertainties in satellite-based estimates of emperor penguin populations in the future. We focused our analysis on multiple VHR images from three representative colonies: Atka Bay, Stancomb-Wills (Weddell Sea sector) and Coulman Island (Ross Sea sector) between September and December during 2011. We replicated methods in Fretwell et al. (2012), which included using supervised classification tools in ArcGIS 10.7 software to calculate area occupied by penguins (hereafter referred to as ‘population indices’) in each image. We found that population indices varied from 2 to nearly 6-fold, suggesting that penguin pixel areas calculated from a single image may not provide a complete understanding of colony size for that year. Thus, we further highlight the important roles of: (i) sun azimuth and elevation through image resolution and (ii) penguin patchiness (aggregated vs. distributed) on the calculated areas. We found an effect of wind and temperature on penguin patchiness. Despite intra-seasonal variability in population indices, simulations indicate that reliable, robust population trends are possible by including satellite-related and environmental covariates and aggregating indices across time and space. Our work provides additional parameters that should be included in future models of population size for emperor penguins.

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Shifting vital rate correlations alter predicted population responses to increasingly variable environments

2019-01-07 , Iles, David T. , Rockwell, Robert F. , Koons, David N.

Time series of vital rates are often used to construct “environment-blind” stochastic population projections and calculate the elasticity of population growth to increased temporal variance in vital rates. Here, we show that the utility of this widely used demographic tool is greatly limited by shifts in vital rate correlations that occur as environmental drivers become increasingly variable. The direction and magnitude of these shifts are unpredictable without environmentally explicit models. Shifting vital rate correlations had the largest fitness effects on life histories with short to medium generation times, potentially hampering comparative analyses based on elasticities to vital rate variance for a wide range of species. Shifts in vital rate correlations are likely ubiquitous in increasingly variable environments, and further research should empirically evaluate the life histories for which detailed mechanistic relationships between vital rates and environmental drivers are required for making reliable predictions versus those for which summarized demographic data are sufficient.

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Ectoparasite burden influences the denning behavior of a small desert carnivore

2019-05-17 , Kluever, Bryan M. , Iles, David T. , Gese, Eric M.

Quantifying the impacts of parasitism on a host can be arduous and is generally understudied for ectoparasites, with known works being either laboratory‐focused, correlational‐based, or only focusing on a few species and spatial extents. Many mammalian species have evolved the modality of denning behavior, a lifestyle that can lead to higher ectoparasite burden, and it has been posited that animals may alter their denning behavior in an attempt to reduce exposure to ectoparasites. We conducted a test of the ectoparasite release hypothesis for kit foxes (Vulpes macrotis) and fleas in the Great Basin Desert of the western United States, a hypothesis that has remained relatively untested for over half a century. We experimentally administered a flea reduction treatment to a subset of kit foxes. We then measured and compared the number of unique den usages and residency time across treatment and control foxes (no flea reduction treatment) while accounting for other factors known to influence denning behavior. Foxes treated with the flea medication reduced the number of unique dens and increased their residency times at dens. All kit foxes continued to use multiple dens on the landscape, suggesting several factors in addition to flea burden influence denning behavior. Our results confirm the long‐dormant ectoparasite release hypothesis and suggest ectoparasites may shape the behavior of burrowing vertebrates to a greater extent than previously recognized.

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