Whales maintained a high abundance of krill; both are ecosystem engineers in the Southern Ocean

Krill abundance was predicted to rise after the end of commercial whaling in the Southern Ocean due to the release of predatory pressure from 2 million whales that were killed between 1915 and 1970, but contrary to expectations, there has been a substantial decline in abundance of krill since the end of whaling. I presented a model 7 yr ago which explained how krill behaviour, in response to the threat of predation by whales, may provide an answer to this paradox. The original model contained a speculative link: a mechanism by which krill could detect the presence of whales over a wide area, and therefore could behave in response to a credible threat. Recently, iron has been implicated in a positive feedback cycle between whales, krill and primary production. The cycle depends on the buoyant faeces of whales fertilising surface layers. This is both a plausible way for krill to detect whales over a wide area and an explanation for enhanced feeding at the surface, but this was not incorporated in the original model. Thus, nutrient retention and behavioural control are probably an example of niche construction and ecosystem engineering by both krill and whales. In this paper I revisit and update the simple model of krill mentioned above. The model is calibrated against known system states and is used to imply the ecosystem level changes caused by commercial whaling. This improved model may explain the reduction in krill abundance after the end of commercial whaling. Untested hypotheses which can be falsified in designed experiments are listed.