Comparison of southern elephant seal populations, and observations of a population on a demographic knife-edge

Abstract

The dynamics of animal populations are determined by several key demographic parameters, which vary over time with resultant changes in the status of the population. When managing declining populations, the identification of the parameters that drive such change are a high priority, but are rarely achieved for large and long-lived species. Southern elephant seal populations in the South Indian and South Pacific oceans have decreased by as much as 50% during the past 50 yr. The reasons for these decreases remained unknown. This study used a projected stochastic Leslie-matrix model based on long-term demographic data to examine the potential role of several life-history parameters in contributing to the declines. The models simulated the observed population trends that were independently derived from annual abundance surveys. Small changes in survival and fecundity had dramatic effects on population growth rates. At Macquarie Island for example, a small change (ca. 5%) in survival and fecundity rates resulted in the population reverting from a decreasing one to a population that increased. The vital rates that had the greatest impact on fitness were, in order of importance: (1) juvenile survival, (2) adult survival, (3) adult fecundity and (4) juvenile fecundity. Population viability analysis (PVA) for each of the 2 decreasing populations revealed that there was a high probability of the Marion Island population becoming extinct within the next 150 yr, while the probability of extinction at Macquarie Island was low. The estimated mean times to extinction for each population was 134 yr (95% confidence intervals: 105 to 332 yr) at Marion Island and 564 yr at Macquarie Island (the earliest time to extinction was 307 yr).

Cohort and tag-site-specific tag-loss rates in mark-recapture studies: A southern elephant seal cautionary case

“…Generally, small shifts in demographic rates of large‐mammal populations, especially adult female survival, are able to produce a change in population growth (Eberhardt and Siniff 1977, Pistorius et al 1999, McMahon et al 2005). This illustrates the importance of accurate estimation of survival rates in mark–recapture studies, as biased estimates of demographic rates may result in erroneous conclusions and implementation of inappropriate management strategies leading to failure in management objectives (Brook et al 1997, McMahon and White 2009).…”

“…The mark–recapture program forms the foundation of investigations into life‐history, demography, dispersal, and philopatry of southern elephant seals at Marion Island ( e.g. , Bester 1989; Pistorius et al 1999, 2004; Kirkman et al 2003, 2004; McMahon et al 2005). Two previous studies have estimated tag‐loss for this population, to incorporate tag‐loss adjustments into demographic data.…”

The effects of body size and climate on post‐weaning survival of elephant seals at Heard Island

“…Regardless of the lack of variation in sub‐adult survival, our estimates of sub‐adult survival ( ϕ s ) were twice as high as the yearling survival estimates. The lower and more variable yearling survival is a probable driver of the observed decline (McMahon et al ., 2005 b ). Lending further support to our conclusion that yearling survival is the principal driver of the observed decreases in seal numbers is that yearling survival rates in the 1940s–1960s at Heard Island were lower than those we observed for stable or increasing populations (0.51–0.73; Pistorius & Bester, ; McMahon et al ., 2003, 2005 b ; Condit et al ., ) and similar to those observed during the 1950s–1960s for the decreasing population at Macquarie Island [mean ± SD 0.46 ± 0.09 and 0.435 ± 0.06 for females and males, respectively (Hindell, )].…”

“…The lower and more variable yearling survival is a probable driver of the observed decline (McMahon et al ., 2005 b ). Lending further support to our conclusion that yearling survival is the principal driver of the observed decreases in seal numbers is that yearling survival rates in the 1940s–1960s at Heard Island were lower than those we observed for stable or increasing populations (0.51–0.73; Pistorius & Bester, ; McMahon et al ., 2003, 2005 b ; Condit et al ., ) and similar to those observed during the 1950s–1960s for the decreasing population at Macquarie Island [mean ± SD 0.46 ± 0.09 and 0.435 ± 0.06 for females and males, respectively (Hindell, )]. Yearling survival influence on population dynamics likely occurs via recruitment into the adult breeding population, combined with the differences in the reproductive performance of early and later breeding females (Desprez et al ., ; van den Hoff et al ., ).…”

Inferring population trends for the world's largest fish from mark–recapture estimates of survival

“…2005; Brook & Bradshaw 2006) and extinction risk faced by populations under various environmental scenarios (Fagan & Holmes 2006). However, demographic data alone cannot always divulge the mechanisms responsible for population trajectories, which is especially inconvenient when management actions are required to mitigate decline (McMahon et al . 2005).…”