However, the low number of species makes the Baltic Sea especially vulnerable, as the loss of one species may have a large effect on other parts of the ecosystem, as there may not be species to replace the niche of the lost species. For most areas it is considered in a "non-acceptable" state, Figure 27 (HELCOM, 2010). When looking at biodiversity indices only, some areas in the northern parts of the Baltic Sea reach acceptable status (HELCOM, 2010). This suggests that the Baltic Sea is one of the most threatened marine ecosystems worldwide (HELCOM 2007, 20136, SEPA 2009). It is thought that the relatively simple food webs and low biodiversity renders the Baltic vulnerable, since key functions may be supported by single species.