Abstract
Understanding how sympatric species with similar requirements coexist is a major issue in ecology. A community with more resource sharing or greater niche overlap, will support more species than one with less. In this regard, the principle of competitive exclusion predicts that species will exhibit segregation mechanisms to coexist. In the Humboldt Current System (HCS) in Peru, South American sea lions (SASL, Otaria byronia) coexist with South American fur seals (SAFS, Arctocephalus australis). However, trophic, temporal and spatial partitioning while foraging remains unclear. This study takes place in Punta San Juan (PSJ) national reserve (15 ° 22'S, 75 ° 12'W) in Peru, there are important breeding colonies of the two species of otariids, making it an ideal setting to study coexistence mechanisms. The main goal of this thesis is to describe and compare the ecological niches of sympatric otariids in Peru, to determine intraspecific and interspecific niche differentiation/overlap. This will be addressed through three specific goals which include to describe and compare (i) the trophic ecology of sympatric pinnipeds; (ii) environmental variables that characterize ecological niches; and (iii) the foraging strategies (spatial and temporal dimensions) within the ecological niches of the sympatric pinnipeds, according to species and sex. In Chapter 1, analysis of population trajectories show how over a 20 year period both pinniped populations are now in a state of decline. A decrease in juvenile age-classes in the colony, suggests that resource limitation is a main contributing factor for current population decline. In Chapter 2, interspecific dietary segregation was detected, through the analysis of hard parts in scats. SAFS feed primarily on Peruvian anchovy (Engraulis ringens) and cephalopods while SASL consume mostly red squat lobsters (Pleuroncodes monodon) and Peruvian anchovy, in smaller proportions. In this study period, trophic overlap was detected during an Extraordinary El Niño event, suggesting that climatic changes can exacerbate competition. In Chapter 3, I analyzed the locations of 35 otariids (18 SASL, 17 SAFS) equipped with satellite tags to determine foraging strategies. Males segregate in space and time; whereas females exhibit spatial overlap but have temporal segregation mechanisms. Associated environmental variables suggest pelagic and coastal habitat features in female fur seals and sea lions, respectively. Whereas, males have higher interindividual variability that should be further explored. Regardless, results demonstrate how multiple foraging strategies at different scales aid resource partitioning. In Chapter 4, long-term niche differentiation was investigated by examining the isotopic signatures serially archived in whiskers SAFS and SASL. Otariids expanded their niche width (except SASL females) as a response to the increase in average sea level and warmer sea surface temperatures. The overall decline in the δ15N signals suggests an impoverished baseline of the food web, with bottom-up effects that has impacted tertiary consumers. Together findings explain the decline and help understand the response of top predators to changes in the marine environment.