A keystone species is one whose presence has a disproportionally large influence on its environment relative to its abundance. The effects can be both direct and indirect (Paine, 1995).
Antarctic krill (Euphausia superba) play a massive part in both the marine and terrestrial Antarctic ecosystems (Figure 5). These effects can be seen in trophic levels both above and below them. The overall biomass of E. superba is seen to overshadow that of many organisms and is the most conspicuous of the macroplankton species (Ward et al., 2012). As a consequence of this the food chain is seen to be krill dominated (Hempel, 1985).
Variability in abundance and distribution of a keystone species due to a variety of factors (both environmental and anthropogenic) can have strong bearings on marine ecosystems, with the decrease in abundance of species from lower trophic levels causing a cascade, reaching the apex predators (Forcada et al., 2005; Hill et al., 2006).
The changes observed in krill biomass are perceived to have a significant influence on the breeding efficiency and foraging success of krill dependent predators within the Southern Ocean. This includes species such as: albatrosses, penguin species and Antarctic fur seals (Croxall et al., 1999). The Scotia Sea and Southern Drake Passage are areas of extremely high E. superba abundance. Additionally, these regions are also the main places in which air breathing vertebrates assemble in order to breed and forage. The Antarctic fur seal (Arctocephalus gazella), Grey-headed Albatross (Thalassarche chrysostoma) and the Wandering Albatross (Diomedea exulans) are examples of these species (Hill et al., 2013). Other marine mammals such as cetacean species, in particular Baleen whales, are also affected by fluctuations in the levels of their primary prey species, E. superba (Figure 6) (Nicol et al., 2008).
Avian predators are also affected, for example; Adélie (Pygoscelis adeliae), and Gentoo (Pygoscelis papua) penguins, with the Chinstrap species (Pygoscelis antarctica) (Figure 7) showing the strongest responses to these instabilities (Lima & Estay, 2013). In addition, over the past 20 years, the Antarctic Shag (Phalacrocorax bransfieldensis) are beginning to turn to other prey whilst foraging, including both pelagic and demersal fish species (Casaux & Barrera-Oro, 2006).
Below E. superba within the marine ecosystem are the zooplankton, phytoplankton and algae, with phytoplankton (particularly diatoms) being the krill’s main food source. The grazing pressures exerted by the krill on the marine algae assist in controlling and regulating the levels of chlorophyll a and ammonium (Whitehouse et al., 2011) and additionally play a key part in reprocessing bioactive elements for phytoplankton (Trovar-Sanchaz et al., 2007). Furthermore, E. superba is a prominent source of dissolved organic matter production within the Southern Ocean, an amount comparable to that of phytoplankton (Ruiz-Halpern et al., 2011).