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Plunging floater survival causes cryptic population decline in the Common Loon
Citation:
Piper, W., J. Grear, B. Hoover, E. Lomery, AND L. Grenzer. Plunging floater survival causes cryptic population decline in the Common Loon. Ornithological Applications. Oxford University Press, Cary, NC, 122(4):duaa044, (2020). https://doi.org/10.1093/condor/duaa044
Impact/Purpose:
The common loon was the the subject of an important case study for ORD's wildlife research strategy in the late 1990s and early 2000s. The strategy was designed to develop methods for incorporating population-level impacts on wildlife into EPA Office of Water's decisions about aquatic stressors. That work included development and publication of a population model for the common loon that allowed assessment of population impacts from potential changes in loon survival and reproduction that could be caused by exposure to mercury or other stressors. This manuscript provides an updated application of that model and reveals that the fitness of Wisconsin loon populations is declining. The ultimate causes of this decline remain unknown. However, a broader impact of this work is that significant changes in population fitness, including those potentally caused by aquatic stressors, can be difficult to detect with simple counts of population abudance. Detection of fitness change in this population was only made possible by long-term monitoring of popuation structure and indicates significant risk of local extinction in the coming decades.
Description:
Populations of many vertebrates are declining and geographic ranges contracting, largely as a consequence of anthropogenic threats. Many reports of such decline, however, lack the breadth and detail to narrow down its causes. Here we describe population decline in the Common Loon (Gavia immer), a charismatic aquatic bird, based on systematic resighting and measurement of a marked population. During our 27-year investigation, age-adjusted chick mass has fallen by 11%, mortality among young and old chicks has increased by 31% and 82%, respectively, and fledging success has declined by 26%. Meanwhile, the return rate of marked nonbreeders (“floaters”) has plunged by 53%, and the adult population overall has declined by 22%. Consistent with the thinning ranks of floaters, the rate of territory eviction has decreased by 52% during the study. Despite the decline in floaters, territory occupancy remains unchanged. However, a matrix model, updated with recent estimates for breeding success, juvenile survival, and senescence, yields a recalculated deterministic population growth rate (λ) of 0.94 for our study population, which suggests that declines in vital rates could lead to a loss of 52% of the current population and a decline of 37% in territory occupancy by 2031. Lack of data on floaters in other upper Midwest and New England loon populations leaves their status in doubt.
URLs/Downloads:
DOI: Plunging floater survival causes cryptic population decline in the Common Loon
https://academic.oup.com/condor/article/122/4/duaa044/5897435