Abstract
Non-migratory resident species should be capable of modifying their foraging behavior to accommodate changes in prey abundance and availability associated with a changing environment. Populations that are better adapted to change will have higher foraging success and greater potential for survival in the face of climate change. We studied two species of resident central place foragers from temperate and equatorial regions with differing population trends and prey availability associated to season, the California sea lion (Zalophus californianus) (CSL) whose population is increasing and the endangered Galapagos sea lion (Zalophus wollebaeki) (GSL) whose population is declining. To determine their response to environmental change, we studied and compared their diving behavior using time-depth recorders and satellite location tags and their diet by measuring C and N isotope ratios during a warm and a cold season. Based on latitudinal differences in oceanographic productivity, we hypothesized that the seasonal variation in foraging behavior would differ for these two species. CSL exhibited greater seasonal variability in their foraging behavior as seen in changes to their diving behavior, foraging areas and diet between seasons. Conversely, GSL did not change their diving behavior between seasons, presenting three foraging strategies (shallow, deep and bottom divers) during both. GSL exhibited greater dive and foraging effort than CSL. We suggest that during the warm and less productive season a greater range of foraging behaviors in CSL was associated with greater competition for prey, which relaxed during the cold season when resource availability was greater. GSL foraging specialization suggests that resources are limited throughout the year due to lower primary production and lower seasonal variation in productivity compared to CSL. These latitudinal differences influence their foraging success, pup survival and population growth reflected in contrasting population trends in which CSL are more successful and potentially more resilient to climate change.
Highlights
As Darwin [1] stated: ‘‘It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change’’, species that are more capable of adapting to changing environmental conditions will have a greater capability of responding to long term changes in climate
The magnitude of seasonal change differs with latitude; it is more pronounced at higher latitudes than in equatorial regions, where there are typically only two seasons: a wet and a dry one
Colder marine systems are typically more productive and have a greater abundance of prey [9]. These latitudinal differences influence animal populations in many ways, e.g. foraging efficiency of Australian forest mammals is lower in tropical than in temperate ecosystems [10], tropical island terns reduce clutch investment at lower latitudes [11] and pinnipeds living in equatorial and temperate environments are more at risk of extinction than species living at higher latitudes [12]
Summary
As Darwin [1] stated: ‘‘It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change’’, species that are more capable of adapting to changing environmental conditions will have a greater capability of responding to long term changes in climate. Colder marine systems are typically more productive and have a greater abundance of prey [9] These latitudinal differences influence animal populations in many ways, e.g. foraging efficiency of Australian forest mammals is lower in tropical than in temperate ecosystems [10], tropical island terns reduce clutch investment at lower latitudes [11] and pinnipeds (seals, fur seals and sea lions) living in equatorial and temperate environments are more at risk of extinction than species living at higher latitudes [12]. These studies conclude that latitudinal differences in population size result from lower food availability and lower oceanic primary productivity, and prey depletion associated with resident-central place foraging behavior along with the reduced seasonality at lower latitudes
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