What does it mean when one of the least researched and understood marine birds in the Arctic turns up in Duluth, Minnesota 1,500 miles outside of its range? Locally, it ensures a birding rush of in-state and out-of-stater birders eager to see the rare bird, but what does it say about the global status of this unique bird? How can we use its presence to educate ourselves of human impact on the high Arctic? Is the Ivory Gull (Pagophila eburnea) an indicator species of a greater issue in the Arctic? The suspicion that their unprecedented, 80% population decline over the last 20 years may be linked to mercury suggests they are.
Population Free-fall of the Ivory Gull
Ivory Gulls are colonial birds, meaning that large numbers gather into groups to breed. By monitoring the nesting colonies of colonial birds, population trends may be established by researchers. However, surveys for Ivory Gulls were only conducted in 1985 (Thomas and MacDonald, 1987) making it impossible to understand population trends. Compounding the lack of population data, Ivory Gulls are considered to be one one of the least understood marine birds. This is partly due to wintering along the ice pack between Greenland and Labrador ensuring they are not a bird which is in-sight of many people. However, indigenous knowledge has suggested declining populations since the 1980s (Mallory et al. 2003). In light of this, researchers flew surveys of known nesting islands as well as newly found Islands in 2002 and 2003 and found something shocking. The number of nesting Ivory Gulls had declined by 80% since the 1980s (Gilcrest et al. 2005).
Gilcrest et al. (2005) started to hypothesize at alternative reasons for the lack of gulls. They explored the possibility that the Ivory Gulls had simply shifted their nesting locations. However, a significant move is not inline with the known biology of the bird which generally move less than 1-2 kilometers. Food sources of fish and carcasses have remained relatively stable in their study area giving them little reason to move. They noted that Ivory Gulls were not seen flying along the survey paths. It seems that the Ivory Gull was truly dying off.
The Driver of Change
Since the startling revelation of population decline, researchers have been trying to understand why Ivory Gulls are disappearing. It is probable that ice-pack changes and altered forage have contributed to the population decline (Gilchrest et al. 2005), but researchers think a stronger factor is in play . In his interview with the BBC World Service (full interview below) Dr. Alex Bond hypothesizes that mercury is a leading stressor on Ivory Gulls based on findings that levels of mercury have risen 45 -50 times the levels found 130 years ago. There is strong evidence showing mercury levels in the eggs of Ivory Gulls is significantly higher than any other known marine bird. Braun et al. 2006 found that mercury in the eggs of Ivory Gulls were 2.5 times greater than even the next highest species, and were almost 3 times greater the amount which impairs reproductive success. Where is that much mercury coming from? And how exactly might it effect Ivory Gulls?
To understand where the mercury is coming from, its important to know the basics of the mercury cycle. Mercury falls into the oceans from atmosphere pollution originating from coal-fired power plants, or is directly input from Alkali metal processing . There are also natural sources of mercury like volcanic eruptions and “volitilization of the ocean” (USGS 2000). Once deposited in a waterbody, mercury becomes available to marine animals when it is transformed to methylmercury. Once in the that state, it moves up through the food chain into plankton, and then to fish, and finally to top level predators like birds and marine mammals. Levels of mercury grows in organisms through bioaccumulation and biomagnifcation. To clarify that jargon, bioaccumulation means that the older you are, the more mercury you have since it is difficult to get it out your system once ingested. Biomagnification means that if you feed higher on the food chain you gain mercury more quickly. Marine mammals like seals have very, very high levels of mercury due to the effect of both bioaccumulation and biomagnifacation. With that information in mind it is easier to understand why Ivory Gulls accumulate mercury; they scavenge on carcasses of marine mammals and feed on fish which have high levels of mercury. They also have a high metabolic rate and consume more fish (Braun et al. 2006).
To date, the effect of mercury on Ivory Gulls has not been studied, but we can gather clues from looking at other species. Common Loons (Gavia immer) also accumulate high levels of mercury due to eating fish (biomagnification) and having long lives (bioaccumulation). Evers et al. 2008 found a 41% decrease in fledged loon young in parents with >3 micrograms of mercury per gram of tissue compared to those with <1 microgram. They predict total reproductive failure of Common Loons if levels exceed 16.5 micrograms. Based on hundreds of hours of observation, they report that loons with elevated levels of mercury are lethargic and spend significantly less time foraging for food and less time taking care of their young. Each lead to fewer chicks growing to adulthood. It is important to note in their study that mercury levels of a species change throughout their range due to climate, forage, and many other factors. Transferring the lessons of Common loons to Ivory Gulls, variation in mercury levels changes are observed in Canada as well; in general levels of mercury increase from east to west in Canada. Although the effect of mercury on Ivory Gulls has not been directly studied and may effect gulls differently than loons, a good hypothesis for their decline is poor parenting and lethargy due to extraordinarily high levels of mercury. Only future research will help tease out the true effect of mercury on their decline.
When an Ivory Gull shows up in Duluth, Minnesota it is a chance to reflect. Reflect on the beauty of an animal. Reflect on the joy of seeing such a rarity. However, do not miss the opportunity to acknowledge that its prescense is out of the norm of the species and that an unseen driver which we do not fully understand is at play. Reflect on the fact that the impact of humans in a nearly un-inhabited region is undeniable. Human consumption of fossil fuels is depositing mercury into the Arctic at rates which may be directly effecting a species. The Ivory Gull is a red flag, an indicator that things are not right in the Arctic and that we should pay heed to what else may be going wrong that we just have not taken the time to study yet.
Braune, B. M., Mallory, M. L., & Gilchrist, H. G. (2006). Elevated mercury levels in a declining population of ivory gulls in the Canadian Arctic. Marine Pollution Bulletin, 52(8), 978-982.
Evers, D. C., Savoy, L. J., DeSorbo, C. R., Yates, D. E., Hanson, W., Taylor, K. M., … & Munney, K. (2008). Adverse effects from environmental mercury loads on breeding common loons. Ecotoxicology, 17(2), 69-81.
Gilchrist, H. G., & Mallory, M. L. (2005). Declines in abundance and distribution of the ivory gull (Pagophila eburnea) in Arctic Canada. Biological Conservation, 121(2), 303-309.
Mallory, M. L., Gilchrist, H. G., Fontaine, A. J., & Akearok, J. A. (2003). Local ecological knowledge of ivory gull declines in Arctic Canada. Arctic, 293-298.
Thomas, V.G., MacDonald, S.D., 1987. The breeding distribution and
current population status of the ivory gull in Canada. Arctic 40,
USGS. 2000. http://www.usgs.gov/themes/factsheet/146-00/