In recent decades, it has become a popular view that the loss of large apex consumers may exert pervasive influence on nature. The idea that an ecosystem may be shaped by top-down effects from the apex consumers dated back to the 19th century is now widely accepted and termed “trophic cascades”.

In 1970’s, it was found that the presence or absence of sea otters dramatically influenced marine communities in the Aleutian islands. Communities lacking sea otters were characterized by high density of sea urchins and a distinct lack of macrophytic vegetation. In contrast, communities with sea otters had relatively high kelp densities and low sea urchin biomass. However, in 1990’s, sea otter populations declined precipitously and unexpectedly over large areas of western Alaska and increased killer whale predation was supposed to be the likely cause of these declines.

In this talk, we propose simple models of communities with or without sea otters and finally with sea otters and killer whales. We will show that the existence of unpalatable plants which are competitive against macrophytes strongly affects community dynamics and that the trophic cascades can not necessarily have static but also dynamic influence on food chains depending on the number of trophic levels.