Do penguins care about their neighborhood? Population implications of bioerosion in Magellanic penguin, Spheniscus magellanicus, at Martillo Island, Beagle Channel, Argentina

Abstract

Intrinsic and extrinsic factors, such as bioerosion at nesting sites, regulate population dynamics and are relevant for the long-term conservation of penguins. Colony trends (between 2004-2022) were studied in a Magellanic penguin colony on Martillo Island, Beagle Channel, Argentina and compared between zones with contrasting degrees of erosion (high, medium, low). Individuals from each zone were characterized for foraging ecology, stress, and reproductive performance during the 2017-2018 breeding season to better understand the colony dynamics. Changes in nest abundance varied in magnitude between nesting zones with different characteristics of occupation time, density and erosion. Declines in nest abundance in the densest, most eroded and longest occupied zone suggests that environmental degradation may be limiting the colony's carrying capacity. A higher percentage of late breeders (probably younger breeders) occupied the less eroded and more recently occupied zone. Foraging, breeding and stress barely differed between zones. New individuals recruiting into the breeding colony select less-eroded zones, either to reduce competition for nests or to avoid other effects of erosion and high-density areas. If this is the mechanism behind the shift in numbers throughout the island, we expect the island to be progressively occupied to the west. If competition or other density dependent factors are at play, a time will come when the vacant east side will begin to be recolonized by younger individuals. However, if erosion or other long-term effects spread throughout the island, recolonization may not occur and the colony may ultimately be abandoned as individuals search for new breeding grounds. Erosion at the breeding site may be a key factor in regional population trends of this burrow nesting species, by following an extinction / colonization of new sites process.

Fig 1. Bio-erosion categories at Martillo island, Beagle Channel, caused by Magellanic penguin (Spheniscus magellanicus).

Zones with low (0–30%), medium (30–50%) and high (> 50%) erosion percentages of the island during 2016 year (modified of [42]). Dots: point of the permanent grid. Location of Martillo Island (red star in upper maps) in the Beagle Channel, Tierra del Fuego, Argentina (grey).

Fig 2. Nest density (Dn: nests/ha) of the Magellanic penguin colony in Martillo Island each year, from 2004 to 2022.

Fig 3. Colony size of the Magellanic penguin in Martillo Island.

Total population (estimated number of breeding pairs for Distance Software) per year. Errors bars indicate ± SE. Arrows indicate the selected change points for model 2.

Fig 4. Growth trends of the Magellanic penguin colony on Martillo Island in each erosion zone and overall.

Counted nest number per year and erosion zone: High, Medium and Low degrees of erosion. Mean and ± SE (errors bars) estimated on Model 2 using package rTRIM.

Fig 5. Percentage of inactive nests of Magellanic penguins per year in zones with high (black), medium (grey) and low (white) degrees of erosion and total (diagonal stripes).

Mean ± SD (standard deviation = errors bars) are shown.

Fig 6. Stable isotope signatures of Magellanic penguins in 2017 at Martillo Island.

Carbon and nitrogen stable isotope biplot and standard ellipse areas corrected for small sample size (SEA_C) of blood adult n = 22 from zones with different erosion degrees: high (red), medium (green) and low (yellow) and fledging chicks n = 14 from high (blue) and medium (purple) erosion zone.

Fig 7. Percentage of early, medial or late breeders (nests) for each zone with high, medium or low erosion degrees.