New Emperor Penguin Numbers Are Double Previous Estimates
Fretwell, P.T., LaRue, M.A., Morin, P., Kooyman, G.L., Wienecke, B., Ratcliffe, N., Fox, A.J., Fleming, A.H., Porter, C. and Trathan, P.N. 2012. An Emperor Penguin Population Estimate: The First Global, Synoptic Survey of a Species from Space. PLoS One 7: e33751 doi: 10.1371/journal.pone.0033751.
In an effort to remedy this situation, Fretwell et al. (2012) surveyed the entire coastline of Antarctica by utilizing both medium resolution and Very High Resolution (VHR) satellite imagery to locate emperor penguin colonies. In places where colonies were identified, VHR imagery from the 2009 breeding season (September-December) was analyzed to estimate the number of birds present. Care had to be taken to distinguish penguins from snow, shadow and guano (feces). After the satellite image estimates were completed, eleven colonies were over-flown with aircraft for a 'ground-truth' count and these counts were used to convert the estimates from the satellite images into numbers of penguins for the entire Antarctic coast.
There were a number of sources of error (contributing to both over and under estimates) associated with this new method, including statistical errors (potentially contributing about 8.7% error in the estimate), inter-annual variation (10-30%), errors in interpreting the images (2.5-15%), errors in the 'ground-truth' counts (about 10%), and errors in assumptions about the ratio of breeding to non-breeding penguins in the colonies (error not estimated). In addition, it was determined that colonies of less than 200 birds could not be identified via satellite imagery, so small colonies might have been missed. What is more, the method could not account for failed colonies in the year of the count (local storms can cause all birds to lose their chicks and abandon the colony; usually, however, they return the next year to try again) and as a result, the authors were not able to estimate how many breeding penguins might have been missed in the count. However, while this method of estimating penguin numbers is associated with both known and unknown sources of error, it lacks the intrusiveness of other methods, such as flipper-band marking, which has been shown to have direct negative effects on breeding success in king penguins (Aptenodytes patagonicus) (Saraux et al., 2011). So what did the authors learn from their analysis, in spite of such potential errors?
Fretwell and colleagues found four new colonies that were overlooked during other surveys and confirmed the location of three additional, suspected penguin sites. In total, they recorded 46 breeding colonies. From their estimates of the breeding population present during 2009, they estimated a total population of approximately 238,000 breeding pairs, which is almost double the previous estimate of 135,000-175,000 pairs. Adding a percentage of this value to account for non-breeders (since only breeding birds were counted in their survey), they estimated a total population of approximately 595,000 ± 81,753 adult birds (however, note that the natural, inter-annual variation of 10-30% expected is not accounted for in the error value of this estimate).
Summing up their findings in the abstract of their paper, the authors conclude: "our work now provides a comprehensive estimate of the total breeding population [of emperor penguins] that can be used in future population models and will provide a baseline for long-term research." They also state, however, that "to disentangle global, regional, or colony population trajectories associated with climate change from other influences will require long term ecological research." In other words, having a baseline estimate of the total population of emperor penguins, even one with inherent errors, means these birds can continue to be monitored via satellite to assess future changes in numbers. However, determining what portion of this might be attributed to climate change will require at least some work on the ground.
Jenouvrier, S., Caswell, H., Barbraud, C., Holland, M., Stroeve, J. and Weimerskirch, H. 2009. Demographic models and IPCC climate projections predict the decline of an emperor penguin population. Proceedings of the National Academy of Science USA 106: 1844-1847.
Saraux, C., Le Bohec, C., Durant, J.M., Viblanc, V.A., Gauthier-Clerc, M., Beaune, D., Park, Y-H., Yoccoz, N.G., Stenseth, N.C., and Le Maho, Y. 2011. Reliability of flipper-banded penguins as indicators of climate change. Nature 469: 203-206.