A researcher has documented a decline in the population of wood frogs in an area in which bullfrogs have been introduced. According to the researcher's data, the wood frog population began to decline soon after the introduction of the bullfrogs. The decline has accelerated since the bullfrog population began growing more rapidly. The researcher notes that bullfrogs are much larger and more aggressive than wood frogs and concludes that the wood frogs are being driven from their breeding areas. This conclusion is questionable because the researcher