Sterols and stanols as novel tracers of waterbird population dynamics in freshwater ponds

Abstract

With the expansion of urban centres in the mid-twentieth century and the post-1970 decrease in pesticides, populations of double-crested cormorants () and ring-billed gulls () around Lake Ontario (Canada and USA) have rapidly rebounded, possibly to unprecedented numbers. Along with the use of traditional palaeolimnological methods (e.g. stable isotopes, biological proxies), we now have the capacity to develop specific markers for directly tracking the presence of waterbirds on nesting islands. Here, we apply the use of lipophilic sterols and stanols from both plant and animal-faecal origins as a reliable technique, independent of traditional isotopic methods, for pinpointing waterbird arrival and population growth over decadal timescales. Sterol and stanol concentrations measured in the guano samples of waterbird species were highly variable within a species and between the three species of waterbirds examined. However, cholesterol was the dominant sterol in guano, and phytosterols were also high in ring-billed gull guano. This variability highlights a specialist piscivorous diet for cormorants compared to a generalist, omnivorous diet for gulls, which may now often include grain and invertebrates from agricultural fields. A ratio that includes cholesterol and sitosterol plus their aerobically reduced products (cholestanol, stigmastanol) best explained the present range of bird abundance across the islands and was significantly correlated to sedimentary δN. Overall, we demonstrate the use of sterols and stanols as a direct means for tracking the spatial and temporal presence of waterbirds on islands across Lake Ontario, and probably elsewhere.

Multicentury perspective assessing the sustainability of the historical harvest of seaducks

“…9). Thus, we conclude that the amount of cholesterol entering the ponds relative to the plant sterol, sitosterol, is a good proxy for tracking eider presence and abundance over time, as shown for other bird colonies (1, 9). These low-lying offshore Arctic islands are free of cliff-nesting seabirds and large animals (e.g., Arctic fox, caribou) that would deposit cholesterol directly into the pond or its catchment, although polar bears ( Ursus maritimus ) occasionally visit the islands to consume eggs (29).…”

“…This “seabird sterol index” provides a direct measure of seabird presence and, when measured in sediment cores, can be used to infer population trends through time. Furthermore, this proxy can be corroborated with stable nitrogen isotopes, an already established tool in assessing seabird-derived nitrogen release to coastal environments, both spatially and temporally (1, 4).…”

“…The factors that shape population dynamics of wildlife often act over time periods well beyond the scope of retrospective analyses of available field information such as population surveys (1). This is particularly true in remote locations like the Canadian Arctic and West Greenland where standardized bird surveys only began in the 1950s and even then occurred rarely with incomplete spatial coverage (2).…”

“…This is because nutrient flow from bird guano and related deposition is dynamic and reflects the number of birds present or absent as nesting populations change over time (4, 6). Sterols and stanols, found in varying concentrations and combinations in the feces of animals (7), are emerging as important proxies to directly track animal inputs to lake sediments over decadal-scale and century-scale time periods (1, 8). For example, the ratio of cholesterol (cholest-5-en-3β-ol) to cholesterol and sitosterol (β-sitosterol), a common phytosterol, demonstrated significant enrichment of cholesterol in pond sediments closest to a nesting colony of northern fulmars ( Fulmarus glacialis ) in the High Arctic and was significantly correlated with ornithogenic δ 15 N enrichment of sediment (9).…”

“…Once female eiders reach these breeding grounds (displaying high philopatry) and commence incubation, they remain on the nest 99.8% of the time (21), losing ∼45% of their body mass over ∼24 d (22). Waste products released from the nesting colonies become archived in pond sediments, which serve as historical records of seaduck densities on the island (1, 23).…”