Galapagos Pinnipeds, Challenges to Their Survival

Pinnipeds endemic to the Galapagos archipelago are in endangered conservation status. The Galapagos sea lion, Zalophus wollebaeki, and Galapagos fur seal, Arctocephalus galapagoensis, have adapted to an ecosystem with high environmental variability and unpredictable marine productivity to survive. In addition to the environmental factors that pressure their populations, these species are exposed to anthropogenic influence, mainly in rookeries on islands with human settlements. It has been determined that the populations of Galapagos pinnipeds have different growth trends between regions of the archipelago, islands of the same region, and between rookeries of the same island. 58% of the Galapagos sea lion population is in the southeastern, with the largest rookery in direct contact with the inhabitants. Various strategies have been proposed to reduce the negative impacts of human–animal interaction, ensure the population’s viability over time, and reduce the species’ extinction risk

Feeding Habits of Introduced Black Rats, Rattus rattus, in Nesting Colonies of Galapagos Petrel on San Cristóbal Island, Galapagos.

Introduced rodents are responsible for ecosystem changes in islands around the world. In the Galapagos archipelago, their effects on the native flora and fauna are adverse, including the extinction of endemic rodents in some islands and the reduction in the reproductive success of the Galapagos petrel (Pterodroma phaeopygia) in its nesting zones. Understanding the feeding behavior of introduced rodents and their trophic interactions with native and non-native species on islands, can assist in the design of management strategies and conservation plans of invasive and endemic species respectively. Four petrel nesting colonies were monitored during June 2013 on San Cristóbal Island (El Plátano, El Junco, San Joaquín, and La Comuna). The feeding habits of black rats were evaluated by analyzing stomach contents and stable isotopes in hair. Three species of introduced rodents were captured. R. rattus was the most abundant at all sites (n=43, capture success (CS) = 55.8%), followed by the house mouse, Mus musculus (n = 17, CS = 37.8%), and the Norwegian rat, R. norvegicus (n = 4, CS = 4.5%), captured only at La Comuna. The omnivorous black rat ate mostly plants (98%) and arthropods (2%). Intact seeds of Miconia robinsoniana were the main food at all sites (relative abundance=72.1%, present in 95% of the analyzed stomachs), showing the black rats' possible role in the archipelago as endemic seed dispersers. There was no evidence of petrel's intake; however, its possible consumption is not discarded at all. The δ15N and δ13C analysis corroborated the primarily herbivorous diet of black rats. The isotopic signatures of the three rodent species reflect the inter- and intra-specific differential use of food resources. Black rat showed a wider diet in La Comuna, which was related to a lower availability of its primary prey and its ability to adapt to the available resources in its habitat

Means (± SD in ‰) δ¹³C and δ¹⁵N values in fur of black rats and their prey identified by the stomach contents analysis.

<p>The samples were collected in El Junco during June 2013. The sample size for each species is shown in parentheses.</p

Means (± SD in ‰) δ13C and δ15N values in fur of M. musculus (gray simbols), R. rattus (White symbols) and R. norvegicus (black symbol) collected during June 2013 in three breeding colonies of Galapagos petrel on San Cristóbal Island: El Junco (J) (circles), San Joaquín (SJ) (squares) and La Comuna (C) (triangles).

<p>The sample size for each species per collection site is shown in the parentheses besides the code of the species and the initial of the study site. Mm = <i>M</i>. <i>musculus</i>; Rr = <i>R</i>. <i>rattus</i>; Rn = <i>R</i>. <i>norvegicus</i>.</p

Percentage of frequency of occurrence (FO) and relative abundance (RA) of prey species and other food items found in the stomachs of <i>R</i>. <i>rattus</i> and <i>R</i>. <i>norvegicus</i> (last column) collected in nesting colonies of Galapagos petrel on San Cristóbal Island during June 2013.

<p>For <i>R</i>. <i>rattus</i>, the percentage values per site and total are shown. The sample size in each case is shown in parentheses. <i>R</i>. <i>norvegicus</i> was only captured in La Comuna.</p><p>*Endemic species in the Galapagos Islands.</p><p>◊ Native species in the Galapagos Islands.</p><p>† Introduced species in the Galapagos Islands.</p><p>Percentage of frequency of occurrence (FO) and relative abundance (RA) of prey species and other food items found in the stomachs of <i>R</i>. <i>rattus</i> and <i>R</i>. <i>norvegicus</i> (last column) collected in nesting colonies of Galapagos petrel on San Cristóbal Island during June 2013.</p

Relative abundance (%) of prey species and other food items found in stomachs of black rats collected during June 2013 in four nesting colonies of Galapagos petrel on San Cristóbal Island: El Plátano (white bars), El Junco (black bars), San Joaquín (gray bars), La Comuna (plaid bars).

<p>It is also shown the relative abundance of preys found in stomachs of Norwegian rats (striped bars) collected in La Comuna.</p

Map of San Cristóbal Island, Galapagos, showing the study sites: San Joaquín (0°53′90″ S, 89°30′93″ W), El Plátano (0°53′88″ S, 89°29′21″ W), El Junco (0°53′80″ S, 89°28′51″ W) and La Comuna (0°52′97″ S, 89°28′00″ W).

<p>Map of San Cristóbal Island, Galapagos, showing the study sites: San Joaquín (0°53′90″ S, 89°30′93″ W), El Plátano (0°53′88″ S, 89°29′21″ W), El Junco (0°53′80″ S, 89°28′51″ W) and La Comuna (0°52′97″ S, 89°28′00″ W).</p

Population Abundance of the Endangered Galapagos Sea Lion <i>Zalophus wollebaeki</i> in the Southeastern Galapagos Archipelago

<div><p>There is great concern regarding the population status of the endangered Galapagos sea lion (GSL) because it has drastically decreased over the last 30 years. We determined the population size and growth trend of the GSL in the Galapagos southeastern region (SER) at three population levels based on the available census data: 1) SER (2011–2015), including 13 rookeries on the four islands San Cristóbal (SC), Española, Floreana, and Santa Fe, comprising 58% of the archipelago’s population; 2) SC (2011–2015), including five rookeries, comprising 52% of the SER population; and 3) El Malecón (2005–2015), the largest rookery on SC and in the SER (43% of the population on SC and 22% in the SER). We also analyzed the influence of environmental variability on pup abundance in these rookeries. The current GSL population size in the SER, after applying correction factors to the counts, is estimated at approximately 2300–4100 individuals and has declined at an average annual rate (ʎ) of 8.7% over the last five years. A similar trend was determined for SC but at ʎ = 1.4% during the same period. For El Malecón, a count-based population viability analysis using a diffusion approximation approach showed that the population increased from 2005 to 2015 at ʎ = 2%. The interannual variability in pup abundance was associated with anomalies in sea surface temperature linked to oceanographic-atmospheric events, which impact the abundance and availability of prey, and ultimately may determine the population’s reproductive success. Since rookeries in the SER had different population trends, management actions should be implemented based on their specific conditions, giving priority to rookeries such as El Malecón, which, despite showing a slightly increasing population trend, still faces a high risk of extinction due to anthropogenic disturbances and environmental variability that may affect its growth and survival.</p></div