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

Declines and recovery in endangered Galapagos pinnipeds during the El Niño event

Currently, the Galapagos sea lion (GSL, Zalophus wollebaeki) and Galapagos fur seal (GFS, Arctocephalus galapagoensis) are among the most important endemic species for conservation in the Galapagos Archipelago. Both are classified as “Endangered” since their populations have undergone drastic declines over the last several decades. In this study we estimated the abundance of both otariids, and their population trends based using counts conducted between 2014 and 2018 in all their rookeries, and we analyzed the influence of environmental variability on pup production. The GSL population size in 2018 in the archipelago was estimated to be between 17,000 to 24,000 individuals and has increased at an average annual rate of 1% over the last five years after applying correction factors. The highest number of GSL counted in the archipelago was in 2014 followed by a population decline of 23.8% in 2015 that was associated with the El Niño event that occurred during that year. Following this event, the population increased mainly in the northern, central and southeastern rookeries. The GSL pup abundance showed a decreasing trend with the increase in intensity of the El Niño. The GFS population in 2018 was counted in 3,093 individuals and has increased at an annual rate of 3% from 2014 to 2018. A high number of GFS counted in 2014 was followed by a population decrease of 38% in 2015, mainly in the western rookeries. There was interannual population fluctuations and different growth trends among regions of the archipelago. GSL and GFS pup abundance has a strong decreasing tendency with the increase in the subthermocline temperature (ST) and the El Niño 1 + 2 index. Our results provide evidence that both species are highly vulnerable to periodic oceanographic-atmospheric events in the Galapagos Archipelago which impact prey abundance and the flow of energy in the unique Galapagos ecosystem

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

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

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