Those that do not have any organism in a similar relationship to themselves are apex ones.  
This type of organism has a namesake pit that can occur when limited states exist. The functional responses of these organisms are divided into three types, with "type two" including saturation and extinction. The canonical differential equation for them includes a conversion efficiency factor. Changes in their population size lead to trophic cascades. Their interaction is described by the Lotka-Volterra equation. Mobbing is a response to them, as is aposematism and mimicry. Those that do not have any organism in a similar relationship to themselves are apex ones.  For 10 points, name this type of organism that kills prey for sustenance.
This type of organism has a namesake pit that can occur when limited states exist.
predator