State-space modelling reveals proximate causes of harbour seal population declines

Abstract

Declines in large vertebrate populations are widespread but difficult to detect from monitoring data and hard to understand due to a multiplicity of plausible biological explanations. In parts of Scotland, harbour seals (Phoca vitulina) have been in decline for 10 years. To evaluate the contributions of different proximate causes (survival, fecundity, observation artefacts) to this decline, we collated behavioural, demographic and population data from one intensively studied population in part of the Moray Firth (north-east Scotland). To these, we fit a state-space model comprising age-structured dynamics and a detailed account of observation errors. After accounting for culling (estimated by our model as 14% of total mortality), the main driver of the historical population decline was a decreasing trend in survival of young individuals combined with (previously unrecognised) low levels of pupping success. In more recent years, the model provides evidence for considerable increases in breeding success and consistently high levels of adult survival. However, breeding success remains the most volatile demographic component of the population. Forecasts from the model indicate a slow population recovery, providing cautious support for recent management measures. Such investigations of the proximate causes of population change (survival, fecundity and observation errors) provide valuable short-term support for the management of population declines, helping to focus future data collection on those ultimate causal mechanisms that are not excluded by the demographic evidence. The contribution of specific ultimate drivers (e.g. shooting mortality or competitors) can also be quantified by including them as covariates to survival or fecundity.

Establishing the link between habitat selection and animal population dynamics

“…While many ecological questions on species distribution are motivated by population dynamics (e.g., viability of fragmented populations, spatial management of pests, species range shifts), an explicit connection between observed distributions and dynamics is rarely pursued (Railsback et al 2003, Guisan and Thuiller 2005, Zurell et al 2009, Gaillard et al 2010, McLoughlin et al 2010, Mieszkowska et al 2013. This, and other broadly recognized issues with SDMs have thus far been investigated with literature reviews, or comparative studies between existing frameworks that, in their majority, make the assumption that populations are at a state of equilibrium (Guisan and Zimmermann 2000, Arau´jo and Guisan 2006, Randin et al 2006, Elith and Graham 2009, Elith and Leathwick 2009, Zurell et al 2009, Hoffman et al 2010, Matthiopoulos and Aarts 2010. Such comparative approaches have their utility, but as ecological practitioners we should perhaps be less concerned about which heuristic SDM performs better in particular case studies.…”

Unravelling the relative roles of top‐down and bottom‐up forces driving population change in an oceanic predator

“…By considering specific species of seals and seabirds as occupants of intermediate trophic levels, we improve our ability to evaluate the drivers of their population dynamics. This is best assessed using integrated population models (Francis and Sagar 2011, Matthiopoulos et al 2013, Maunder and Punt 2013, Thomson et al. 2015, Tuck et al.…”

Harbour seal Phoca vitulina movement patterns in the high-Arctic archipelago of Svalbard, Norway

“…This species was listed as 'Least Concern' by the IUCN Red List in 2008; at that time the global population was estimated to be 350 000 to 500 000 individuals. However, there have been major declines documented for some harbour seal populations in the last 20 yr (Thompson & Här -könen 2008), with currently ongoing declines in some regions (Womble et al 2010, Hanson et al 2013, Matthiopoulos et al 2014. Southern harbour seal populations are generally well-studied, protected from exploitation for the most part, and are stable or increasing (e.g.…”

“…Chavez-Rosales & Gardner 1999, Jeffries et al 2003, Hassani et al 2010, Olsen et al 2010, Reijnders et al 2010. However, the situation is quite different for northern populations, where up-to-date regional harbour seal abundance information is often lacking (despite local harvesting activities), and some populations have shown marked declines in both the North Atlantic (Lucas & Stobo 2000, Hanson et al 2013, Matthiopoulos et al 2014 and the North Pacific (e.g. Frost et al 1999, Small et al 2008, Womble et al 2010.…”

“…Frost et al 1999, Small et al 2008, Womble et al 2010. The reasons for the observed declines are not clear in all cases, but several hypotheses have been proposed that suggest climate change-related linkages such as shifts in prey distribution, increased predation pressure, vulnerability to pathogens or competitive stress (Trites et al 2007, Matthiopoulos et al 2014. The declines in harbour seal numbers in the Aleutian Islands and the Gulf of Alaska over the last 30 yr have been linked to a large-scale oceanographic regime shift that has likely been induced by climate change (Small et al 2008, Womble et al 2010.…”