Final answer:
The most likely primary result of a decreased rabbit population on Macquarie Island due to a virus carried by fleas would be that feral cats would shift their diet to native birds. This can cause a significant impact on the native bird populations and alter the ecological balance. However, this could potentially benefit native plant populations as well.
Step-by-step explanation:
When the flea carrying a virus decreased the rabbit population on Macquarie Island, the most likely primary result was that the feral cats switched from eating rabbits to eating native birds. This is supported by ecological principles and is an example of an indirect consequence of biological control. The removal of a primary food source, such as the rabbits, can cause a predatory species to shift to a different prey, thus putting native bird populations at higher risk. Examples from other islands, such as the introduction of other species that had unforeseen effects on the local fauna and flora, demonstrate this kind of ecological chain reaction.
With rabbits no longer available as a food source, the feral cats may impact the native bird populations more significantly. This kind of ecological impact has been seen in other locations where a change in one part of the ecosystem can have cascading effects, such as an increase in one species leading to the decline of another.
In contrast, if rabbits were to decrease significantly, it could potentially lead to an increase in native plant populations on the island, as rabbits would no longer be consuming the vegetation. However, without specific studies or data, it's difficult to guarantee that plant populations would thrive immediately after the decline of an invasive herbivore.