The fluctuations in the population abundance of Antarctic krill – small, crustaceous animals – are the result of internal self-regulation (i.e. competition for food) and not because of external climatological events, as has previously been believed. This is the conclusion of a new paper authored by an international research team which includes André de Roos, professor of Theoretical Ecology at the University of Amsterdam (UvA). The team’s results were published on 5 June in Nature Ecology & Evolution.
Antarctic krill (Euphausia superba) is one of the most abundant animal species on Earth and forms an essential food source for penguins, wales, seals and other fish species in Antarctic waters. Krill populations exhibit a population cycle of 5 to 6 years during which the population undergoes gigantic changes, with oscillations in biomass exceeding one order of magnitude. To date, the origin of these changes in krill densities every few years has been poorly understood. Previous studies have argued that the krill cycle is induced by periodic climatological factors.
As part of their study, the team, comprising researchers from the University of Oldenburg, the UvA’s Institute for Biodiversity and Ecosystem Dynamics (IBED) and the Alfred Wegner Institute in Bremerhaven (Germany), combined a mathematical model with field data in order to explain the origin of the cycle. This bioenergetics model predicts the characteristics of the fluctuations in the krill population cycle in remarkably precise agreement with the field data and presents an alternative explanation: that the krill cycle is caused by intraspecific competition between individual krill members in different life stages.
The new bioenergetics model describes the ontogenetic development of krill in the polar seasonal environment. ‘In this way, we could identify autumn as the main bottleneck of krill development’, says De Roos. ‘During summer and autumn, larvae and adult krill have extremely high food requirements because they need to accumulate lipids for overwintering. At the same time, phytoplankton concentrations (the main food source of krill) decline. This rate of decline is increased further by the grazing of an abundant krill population. As a result, a large krill stock ultimately leads to a longer starvation period and becomes an obstacle rather than an advantage for successful overwintering and the recruitment of a new generation. All of this shows that this intraspecific competition of larvae and adult krill is the main driving force of the krill population cycle.’
The research results run counter to the common view that the Antarctic winter season (defined by the timing of ice-edge advance and annual ice season duration) is the most critical phase for the survival of larval krill. Krill larvae develop during summer, autumn and winter. The winter season, when most of the Southern Ocean is covered by sea ice and the food in the water is extreme low, was so far seen as a main bottleneck in krill recruitment.
The results also have a far-reaching implication for the whole food web in the Southern Ocean. De Roos: ‘According to the model, a decline of apex predators (such as seals, penguins, or albatrosses) is likely to amplify the oscillations, potentially destabilising the marine food web and leading to a further decline of top-predators. In contrast, an increased predation pressure, caused for instance by recovering whale populations, paradoxically might stabilise krill populations.’ The model results show that the stability of the krill cycle depends crucially on the net-difference between krill reproduction and consumption. Any factor that changes this delicate balance might have drastic consequences for the entire Antarctic ecosystem.
A.B. Ryabov, A.M. de Roos, B. Meyer, S. Kawaguchi, B. Blasius: ‘Competition-induced starvation drives large-scale population cycles in Antarctic krill’ (5 June 2017 in Nature Ecology Evolution. Doi: http://dx.doi.org/10.1038/s41559-017-0177.