Plunging floater survival causes cryptic population decline in the Common Loon

Area of Science:

• Ornithology
• Wildlife Ecology
• Conservation Biology

Background:

• Global vertebrate populations face declines and range contractions due to anthropogenic threats.
• Detailed studies are crucial for identifying specific causes of population decline.
• The Common Loon (Gavia immer) is an indicator species for aquatic ecosystem health.

Purpose of the Study:

• To investigate the causes and extent of population decline in a marked Common Loon population over 27 years.
• To assess the impact of changes in vital rates on population growth and future territory occupancy.
• To evaluate the role of nonbreeder ('floater') dynamics in population stability.

Main Methods:

• Systematic resighting and measurement of a marked Common Loon population over a 27-year period.
• Analysis of demographic parameters including chick mass, mortality rates, fledging success, and adult return rates.
• Development and application of a matrix population model incorporating vital rates and senescence.

Main Results:

• Significant declines observed: age-adjusted chick mass (-11%), chick mortality (+31% young, +82% old), fledging success (-26%).
• Marked nonbreeder ('floater') return rate decreased by 53%, and overall adult population declined by 22%.
• Matrix model predicts a population growth rate (λ) of 0.94, forecasting a 52% population loss and 37% decline in territory occupancy by 2031.

Conclusions:

• Declining vital rates, particularly reduced floater availability and lower breeding success, are driving the Common Loon population decline.
• The current trajectory suggests a substantial population and territory occupancy loss within the next decade.
• Urgent conservation efforts are needed, and further research on loon populations in other regions is warranted.