When two prey (or resource) species share a common predator (consumer), they can be in apparent competition. An increase in one prey species can increase the common predator density, which negatively influences the other prey species.
A textbook (Molles, Seventh edition, ISBN-10: 0077837282) gives an example of apparent competition that confuses me. It gives another definition "one species facilitating populations of a predator or herbivore of the second species." Thus, one of the species does not have to be consumed by the predator/herbivore.
The specific example is that there are two plant species. Species A becomes the habitat for the herbivore (but is not consumed by the herbivore). Species B is consumed by the herbivore. Therefore, an increase in species A will increase the herbivore density and has negative effects on species B. I thought that to form an apparent competition relationship, it has to be mutually negative relationship (-,-). In this example, there is no mutually negative relationship. Furthermore, the plants do not share the common herbivore.
This is simply a definition question. Different sources have variable definitions, but is the example really considered an apparent competition?