Monitoring a marine ecosystem using responses of upper trophic level predators

Abstract

Summary1. This study examined the changing status of the marine ecosystem at the island of South Georgia (Southern Ocean) using up to 27 variables measured over 22 years from three upper trophic level predators that specialize in foraging upon krill ( Eupuasia superba Dana). These variables included population size, breeding performance, offspring growth rate, foraging behaviour and diet. A method was developed for reducing these multivariate time-series to a single vector, called a combined standardized index (CSI). 2. Sensitivity analyses showed that missing values had a large effect upon the accuracy of the CSI but this effect was reduced if the individual variables were highly correlated. The level of correlation and proportion of missing values within the empirical data set were within the acceptable range. Individual variables had widely varying influence upon the CSI but, in general, those with longer time-series had the greatest influence. 3. Principal components analysis showed that variables representing offspring growth tended to explain the greatest proportion of the variability in the CSI and this was followed by variables representing diet. 4. There were 3 years in which the CSI showed extreme and significantly low values. There was a significant non-linear functional response (similar to the Holling Type II functional response) between the overall CSI and krill biomass and a similar relationship existed when the CSI was calculated for each species individually. 5. Separate analysis of variables that were likely to be representative of changing population size showed the presence of a significant decline between 1977 and 1998. There was no trend in the CSI from variables representative of foraging conditions during the summer breeding season. The study has shown that the marine ecosystem at South Georgia shows acute but transient variability that is amplified in the response of upper trophic-level predators. There is less certainty that trends in populations are a consequence of shifts in the degree to which the ecosystem can support krill-feeding seals and penguins.

Segregation of foraging between two sympatric penguin species: does rate maximisation make the difference?

“…In addition, inter-annual variation in estimates of patch quality based on the diving behaviour of Antarctic fur seals at Bird Island was positively correlated with that of krill density measured in ship-based hydroacoustic surveys around Bird Island (Mori & Boyd 2004). Also, the effect of krill density on reproductive success was similar between Antarctic fur seals and the 2 penguin species at Bird Island (Boyd & Murray 2001). This indicates that the krill distribution experienced by fur seals could be the same as that experienced by the macaroni and gentoo penguins.…”

Foraging under contrasting oceanographic conditions: the gentoo penguin at Kerguelen Archipelago

“…Yet, such studies might provide original information on the local availability and characteristics of marine prey populations (Ridoux 1988) and help to monitor coastal marine ecosystems (Boyd & Murray 2001).…”

Diet variability and reproductive performance of macaroni penguins Eudyptes chrysolophus at Bird Island, South Georgia

“…Diets might be expected to reflect the relative abundance or proportion of prey items in the ecosystem, so it is possible to consider predators as integrated indicators of ecosystem structure. Diet data, along with measures of reproductive output can be used to understand predator responses to prey availability (Boyd & Murray 2001, Reid & Croxall 2001, Reid et al 2005a, and are essential for the development of plausible foodweb models (e.g. Murphy et al 2007a, Hill et al 2012.…”