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

Sara Labrousse, David Iles, Lise Viollat, Peter Fretwell, Philip N. Trathan, Daniel P. Zitterbart, Stephanie Jenouvrier, Michelle LaRue

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)151-165
Number of pages15
JournalRemote Sensing in Ecology and Conservation
Volume8
Issue number2
DOIs
StatePublished - Apr 2022

Bibliographical note

Publisher Copyright:
© 2021 The Authors. Remote Sensing in Ecology and Conservation published by John Wiley & Sons Ltd on behalf of Zoological Society of London.

Keywords

  • Emperor penguin
  • VHR imagery
  • intra-seasonal variability
  • population estimates
  • population trend
  • satellite imagery

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