A century of change in Glaucous-winged Gull (Larus glaucescens) populations in a dynamic coastal environment
Summary (3 min read)
INTRODUCTION
- Thus, it is plausible that the species responded favorably to human-induced environmental changes in the early 1900s, with these changes facilitating population growth above a baseline supported by the availability of natural foods (Vermeer 1992, Hayward et al. 2010).
- Under such a scenario, the recent decline would simply represent a reversal of these influences and a return to historical conditions.
- The authors examined potential causes of population change based on three hypotheses developed to account for temporal trends in gull populations generally (Grandgeorge et al.
Food Limitation
- Food availability may have driven both increases and decreases in Glaucous-winged Gull populations during the last century.
- Expanding human populations in the Georgia Basin may have increased the availability of food for gulls in the form of garbage, and by doing so may have facilitated gull population growth after 1920 (Vermeer 1992, Hayward et al. 2010), as has occurred in other systems (Spaans 1971, Pons and Migot 1995).
- Alternatively, changes in forage fish populations may have decreased food availability (Therriault et al. 2009, McKechnie et al. 2014), which may explain recent declines in gull populations in the Georgia Basin.
- Food shortages may also lead to increased egg cannibalism at some colonies (Hayward et al. 2014), which has the potential to contribute to clutch size declines.
Study Area and Colony Counts
- The Georgia Basin refers to the Canadian portion of the Salish Sea, an inland body of water encompassing the Strait of Georgia, BC; Puget Sound, Washington, USA; the eastern portion of the Juan de Fuca Strait; and the region’s islands and terrestrial watersheds .
- The area is influenced by several major urban centers and is heavily affected by human activity, being ranked as ‘very high impact’ in a global assessment of anthropogenic impacts of coastal ecosystems (Halpern et al. 2008).
- In a few cases the authors used median values as their colony counts, when historical estimates were provided as ranges (n¼ 15).
- As with previously published studies (Vermeer and Devito 1989, K. Vermeer personal communication), the authors counted only active nests (containing 1 egg, or evidence of depredation), because Glaucous-winged Gulls often build multiple nest cups prior to laying.
Temporal Trends
- Glaucous-winged Gull colony numbers in the Georgia Basin ranged widely from 1900 to 2010.
- The GAM indicated that a smoothed function with an estimated 5.5 degrees of freedom significantly reduced model deviance (F ¼ 34.14, P , 0.001).
- The smoothed function indicated that mean colony counts increased steadily from 1900 to the mid-1980s, reaching a maximum in 1986, and declining thereafter .
Demographic Models
- Population growth rates calculated from the time series of colony counts indicated positive growth rates (k . 1) that ranged from 1.03 to 1.05 until 1954.
- The Condor: Ornithological Applications 117:108–120, Q 2015 Cooper Ornithological Society Downloaded From: https://bioone.org/journals/The-Condor on 30 May 2022 Terms of Use: https://bioone.org/terms-of-use.
- This scenario did result in a reduction in population growth rates following the 1960s, as occurred in the trajectory of observed population growth rates, but overall appeared to overestimate the population growth rates derived from colony counts since the early 1990s.
Colony Counts
- The authors compiled 507 records of colony counts from 87 localities in the Georgia Basin over the study period, with 11 colony counts available prior to 1925.
- The Georgia Basin’s 2 largest (historically and at present) colonies were The Condor: Ornithological Applications 117:108–120, Q 2015 Cooper Ornithological Society Downloaded From: https://bioone.org/journals/The-Condor on 30 May 2022 Terms of Use: https://bioone.org/terms-of-use represented in these early counts, so that their early population size estimates are likely to be representative of regional totals for that period.
- Colonies for which early count data were available were also among the most frequently surveyed in the region over the entire study period, and a visual inspection of the long-term trends for these and other sites showed agreement with their estimated population trend .
Colony Counts and Temporal Trends
- Numbers of Glaucous-winged Gulls breeding in the Georgia Basin fluctuated widely during the 111 years from 1900 to 2010, increasing rapidly from 1900 to the mid-1980s, and then declining so that 2010 levels were ~50% of peak abundance.
- The authors study incorporated Glaucous-winged Gull colony count data from all available sources, and represents one of the more comprehensive sets of long-term census data available for a marine bird in Canada.
- Compilations of marine animal count data for time periods exceeding 100 years are generally rare, and thus are valuable in studies of animal population dynamics (Pauly 1995).
- No large colonies are currently known to exist in urban settings in their study area, and none are known from historical records (prior to the ~1950s, when urban nesting was first documented in the region; e.g., Bowles 1906, Eddy 1982, Vermeer et al.
Causes of Population Trends
- The authors used demographic models to construct population trajectories over time as a plausible, heuristic approach to conceptualizing drivers of long-term population change, and to test hypotheses about population trends , recognizing that more than one driver was likely to have influenced multidecadal trends.
- The Condor: Ornithological Applications 117:108–120, Q 2015 Cooper Ornithological Society Downloaded From: https://bioone.org/journals/The-Condor on 30 May 2022 Terms of Use: https://bioone.org/terms-of-use.
- In contrast, other studies have shown that gulls respond to dietary garbage by increasing productivity (Spaans 1971, Pons 1992, Weiser and Powell 2010).
Conclusions
- The authors results indicate that Glaucous-winged Gull popula- tions in the Georgia Basin have fluctuated considerably over the 111 years of their study, with strong increases from 1900 to the early 1980s, followed by population declines to 2010.
- These trends could be related to declines in clutch size and reproductive success that might have resulted from changes in food resources, particularly the availability of forage fish; to changes in predation rates from recovering Bald Eagle populations; and to the cessation of egg harvesting after the implementation of the 1916 Canada–U.S. Migratory Bird Convention.
- Local reports and anecdotal accounts from the early 1900s suggest that Glaucous-winged Gull numbers in the region were not ‘at baseline’ at that time, but instead were limited by human exploitation, so that twentieth century increases at least partially represented a population recovery.
- The authors results highlight the value of long-term, retrospective studies for providing unique perspectives on causes of population change, and indicate the need to exercise caution in assuming that historical data represent ‘pristine’ conditions by virtue of their age.
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References
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...The GAM approach is an extension of the General Linear Model, in which predictors are smoothed functions rather than linear relationships (Wood 2006)....
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...Year 1900 was set to a value of 0, and we used package ‘mgcv’ in R (Wood 2006) to fit a GAM that included a Poisson error distribution and a unique colony identifier as a random effect to account for the correlated non-normally distributed errors (all R code is available in the Supplemental…...
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...The GAM approach is an extension of the General Linear Model, in which predictors are smoothed functions rather than linear relationships (Wood 2006)....
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...Year 1900 was set to a value of 0, and we used package ‘mgcv’ in R (Wood 2006) to fit a GAM that included a Poisson error distribution and a unique colony identifier as a random effect to account for the correlated non-normally distributed errors (all R code is available in the Supplemental…...
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...…their potential demographic impacts on gulls by fitting simple matrix population models and comparing trajectories of hypothesized population growth rates to observed population growth rates following the general approach of Walters (1986), Hilborn and Mangel (1997), and Walters and Martell (2004)....
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