Prey Switching - Definition

Definition

The term switching was first coined by the ecologist Murdoch in 1969 to describe the situation where a predator eats disproportionately more of the most common type of prey. Eight years earlier in 1962 the geneticst B. C. Clarke described a similar phenomenon and called it "apostatic selection". Since then the term prey switching has mainly been used by ecologists, while apostatic selection has been used by geneticists, and because of this they have been used to describe different aspects of frequency dependent selection.

One of the ways prey switching has been identified and defined is when a predator's preference for a particular type of prey increases as the prey increase in abundance. The result is a strong preference for prey which are common in the environment and a weak preference for prey which are rare. The definition of preference will therefore impact on understanding switching. The most common definition of preference is the relationship between the ratio of prey in the environment and the ratio of prey in a predator's diet. It has been independently proposed a number of times and is described by the equation:

P1/P2 = c (N1/N2); alternatively, c = (P1/P2)/(N1/N2)

where N1 and N2 are the abundance of prey types 1 and 2 in the environment and P1 and P2 are the abundances of the same prey types in the predator's diet. c is the preference for prey type 1. If the value of c increases over time with N1/N2, prey switching is presumed to occur. The opposite of prey switching is when a predator eats disproportionately more of the most rare prey than would be expected by chance. From the equation above this would occur when c (preference) decreases over time as N1/N2 (amount in the environment) increases. This opposite phenomenon has been called negative prey switching, or anti-apostatic selection when it refers to the choice between different morphs.

Prey switching has been in the scientific literature for around 40 years, but since his initial work Hassell has suggested that interest in prey switching has fallen since it is hard to demonstrate whether it has or is occurring.