Isotopic niche partitioning between two apex predators over time.
Stable isotope analyses have become an important tool in reconstructing diets, analysing resource use patterns, elucidating trophic relations among predators and understanding the structure of food webs. Here, we use stable carbon and nitrogen isotope ratios in bone collagen to reconstruct and compare the isotopic niches of adult South American fur seals (Arctocephalus australis; n = 86) and sea lions (Otaria flavescens; n = 49) - two otariid species with marked morphological differences - in the Río de la Plata estuary (Argentina - Uruguay) and the adjacent Atlantic Ocean during the second half of the 20th century and the beginning of the 21st century. Samples from the middle Holocene (n = 7 fur seals and n = 5 sea lions) are also included in order to provide a reference point for characterizing resource partitioning before major anthropogenic modifications of the environment. We found that the South American fur seals and South American sea lions had distinct isotopic niches during the middle Holocene. Isotopic niche segregation was similar at the beginning of the second half of the 20th century, but has diminished over time. The progressive convergence of the isotopic niches of these two otariids during the second half of the 20th century and the beginning of the 21st century is most likely due to the increased reliance of South American fur seals on demersal prey. This recent dietary change in South American fur seals can be explained by at least two non-mutually exclusive mechanisms: (i) the decrease in the abundance of sympatric South American sea lions as a consequence of small colony size and high pup mortality resulting from commercial sealing; and (ii) the decrease in the average size of demersal fishes due to intense fishing of the larger class sizes, which may have increased their accessibility to those eared seals with a smaller mouth gape, that is, South American fur seals of both sexes and female South American sea lions.
Departamento de Ecología & Evolución, Centro Universitario Regional Este (CURE), Universidad de la República, Tacuarembó s/n, 20000, Maldonado, Uruguay.
Massimiliano
0000-0003-2764-9849
M
Drago
IRBio y Departamento de Biología Evolutiva, Ecología y Ciencias Ambientales, Universidad de Barcelona, Av. Diagonal 643, 08028, Barcelona, España.
Luis
L
Cardona
Departamento de Ecología & Evolución, Facultad de Ciencias, Universidad de la República, Iguá 4225, 11400, Montevideo, Uruguay.
Valentina
V
Franco-Trecu
Laboratorio de Mamíferos Marinos, Centro para el Estudio de Sistemas Marinos (CESIMAR - CCT CONICET-CENPAT), Blvd. Brown 2915, U9120ACD, Puerto Madryn, Argentina.|Universidad Nacional de la Patagonia, Blvd. Brown 3600, U9120ACD, Puerto Madryn, Argentina.
Enrique A
EA
Crespo
Laboratorio de Mamíferos Marinos, Centro para el Estudio de Sistemas Marinos (CESIMAR - CCT CONICET-CENPAT), Blvd. Brown 2915, U9120ACD, Puerto Madryn, Argentina.
Damián G
DG
Vales
Investigaciones Arqueológicas y Paleontológicas del Cuaternario Pampeano (INCUAPA-CONICET), Universidad Nacional del Centro, Av. Del Valle 5737, B7400JWI, Olavarría, Argentina.
Florencia
F
Borella
IRBio y Departamento de Biología Evolutiva, Ecología y Ciencias Ambientales, Universidad de Barcelona, Av. Diagonal 643, 08028, Barcelona, España.
Lisette
L
Zenteno
Museo Nacional de Historia Natural, 25 de Mayo 582, 11000, Montevideo, Uruguay.
Enrique M
EM
Gonzáles
Departamento de Ecología & Evolución, Centro Universitario Regional Este (CURE), Universidad de la República, Tacuarembó s/n, 20000, Maldonado, Uruguay.
Pablo
P
Inchausti
D000818:Animals; D001118:Argentina; D001267:Atlantic Ocean; D001842:Bone and Bones / Q000737:chemistry; D002247:Carbon Isotopes / Q000493:pharmacokinetics*; D005260:Female; D046069:Fur Seals; D008297:Male; D009587:Nitrogen Isotopes / Q000493:pharmacokinetics*; D046070:Sea Lions
D002247:Carbon Isotopes; D009587:Nitrogen Isotopes
marine mammals; pinnipeds; stable isotopes; standard ellipse area; trophic ecology