Density dependence and changes in the carrying capacity of Alaskan seabird populations

Abstract

Evidence for regulation of animal populations by negative density dependence is ubiquitous across the animal realm, and yet the dynamics of carrying capacity (K) are often overlooked. K acts as a threshold below which population size tends to increase and above which it tends to decrease. Documenting changes in this threshold is particularly important to population viability analysis (PVA). We reconstructed the population sizes of five longlived seabird species in Alaska, USA, and analyzed their population dynamics from the past four decades: Black-legged (Rissa tridactyla) and Red-legged Kittiwakes (R. brevirostris), Common (Uria aalge) and Thick-billed Murres (U. lomvia) and Tufted Puffins (Fratercula cirrhata). We evaluated a set of models that allowed for either density independence or density dependence, with or without a time trend in K. The best approximating models indicated that these seabird populations behaved in a negative density-dependent fashion. K increased significantly for murres, and remained relatively stable for Red-legged Kittiwakes. It decreased significantly (N 40%) for Black-legged Kittiwakes and Tufted Puffins, particularly in the Gulf of Alaska, following the 1989 Exxon Valdez oil spill. Although we have less confidence in the puffin data, our PVA suggests that, in the next 100 years, Tufted Puffins may become extirpated from the few colonies that are monitored in the Gulf of Alaska. Negative density dependence can help to prevent population crashes, but time lags and serial correlation in rates of change could suppress the recovery of contracted populations. Therefore, estimating the magnitude of population fluctuations around a changing carrying capacity is essential to managing and conserving declining populations.

Will the California Current lose its nesting Tufted Puffins?

“…The Tufted Puffin ( Fratercula cirrhata ) is an iconic species that is experiencing dramatic population declines across the southern portion of its geographic range ( Piatt & Kitaysky, 2002 ). While Tufted Puffin populations in the Alaska Current have remained relatively stable (but see Goyert et al, 2017 ), populations in the California Current large marine ecosystem (area of the eastern Pacific Ocean spanning nearly 3,000 km from southern British Columbia, Canada to Baja California, Mexico) have declined by approximately 90% relative to early 20th century estimates, and are currently declining 9% annually ( Hanson & Wiles, 2015 ). The number of occupied breeding-colony sites in Washington State has declined by 60% relative to the 1886–1977 average, and 45% relative to the 1978–1984 average ( Hanson & Wiles, 2015 ).…”

An assessment of climate change vulnerability for Important Bird Areas in the Bering Sea and Aleutian Arc

“…The climate-vulnerable IBAs and species identified here warrant further consideration, which may include assessment of other compounding stressors like intensified vessel traffic or other industrial activities, or complications related to ocean acidification. These areas could also benefit from additional climate modeling studies (especially those predictive of forage fish density), retrospective analyses relating population trends to forage availability (e.g., [91]), increased monitoring, and/or increased conservation measures.…”

Persistent annual migration patterns of a specialist seabird

“…If red-legged kittiwakes are targeting spawning myctophids or squids across their wintering range, this would provide further evidence of foraging specialization, that in most years is likely highly profitable. There is some evidence that through the 1990s, the abundances of myctophids in the subarctic North Pacific increased (Beamish et al 1999), potentially corresponding to upward population trends in red-legged kittiwakes and the stability of the ecosystem's apparent carrying capacity for this marine predator (Goyert et al 2017). …”

“…Red-legged kittiwakes are surface-foraging seabirds endemic to the Bering Sea, nesting at only a few colonies along the Aleutian Islands, in the Commander Islands, and in the Pribilof Islands, where > 70% of the entire population nests on the tall cliffs of St. George Island (~235 000 pairs; Byrd et al 2008a, Goyert et al 2017. Red-legged kittiwake populations at St. George Island experienced a significant decline in the 1980s leading to an IUCN 'Vulnerable' designation (BirdLife International 2017).…”