Abstract
Considerable uncertainties often surround the causes of long-term changes in population abundance. One striking example is the precipitous decline of southern sea lions (SSL; Otariaflavescens) at the Falkland Islands, from 80 555 pups in the mid 1930s to just 5506 pups in 1965. Despite an increase in SSL abundance over the past two decades, the population has not recovered, with the number of pups born in 2014 (minimum 4443 pups) less than 6% of the 1930s estimate. The order-of-magnitude decline is primarily attributed to commercial sealing in Argentina. Here, we test this established paradigm and alternative hypotheses by assessing (1) commercial sealing at the Falkland Islands, (2) winter migration of SSL from the Falkland Islands to Argentina, (3) whether the number of SSL in Argentina could have sustained the reported level of exploitation, and (4) environmental change. The most parsimonious hypothesis explaining the SSL population decline was environmental change. Specifically, analysis of 160 years of winter sea surface temperatures revealed marked changes, including a period of warming between 1930 and 1950 that was consistent with the period of SSL decline. Sea surface temperature changes likely influenced the distribution or availability of SSL prey and impacted its population dynamics. We suggest that historical harvesting may not always be the "smoking gun" as is often purported. Rather, our conclusions support the growing evidence for bottom-up forcing on the abundance of species at lower trophic levels (e.g., plankton and fish) and resulting impacts on higher trophic levels across a broad range of ecosystems.
Highlights
Diagnosing the cause of long-term changes in the abundance of plants and animals is a basic tenet of ecology and is at the heart of many conservation efforts (Caughley and Gunn 1996)
Decline was the result of commercial sealing at the Falkland Islands.—To determine (1) sealing locations, (2) numbers killed annually, and (3) sex, we examined all available documents relating to sealing between the early 1900s and the end of sealing in 1966
For the remaining southern sea lions (SSL) tracked, the mean maximum foraging trip distance was 84 6 47 km and mean duration was 4 6 1 d (Table 2). These results clearly show that, while extended movements were observed in some subadults, there was no evidence of a migration to Argentina or the South American mainland (Fig. 4)
Summary
Diagnosing the cause of long-term changes in the abundance of plants and animals is a basic tenet of ecology and is at the heart of many conservation efforts (Caughley and Gunn 1996). For large vertebrates, harvesting and habitat loss have driven global declines in a broad range of marine and terrestrial species (Myers and Worm 2003, Lotze and Worm 2009), leading to the current view that a major anthropogenically induced global extinction event is underway (Barnosky et al 2011). In addition to harvesting and habitat loss, other processes are responsible for long-term changes in abundance, including shifts in predator–prey interac-. Manuscript received 15 October 2014; revised 19 February 2015; accepted 24 February 2015; final version received 19 March 2015.
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