Final answer:
When ants were removed, the number of rodents increased, and when rodents were removed, the number of ant colonies also increased, showing a competitive relationship. Genetic variation in voles could either increase if more food is available or predation decreases, but less selection pressure might also lead to less genetic variation over time.
Step-by-step explanation:
In Brown and Davidson's study, which focused on the relationship between seed-eating rodents and ants in Arizona, the dynamic between these two groups plays a crucial role in understanding their ecosystem. When ants were removed, the number of rodents increased, likely due to decreased competition for seeds which both groups feed on. Conversely, when rodents were removed, the number of ant colonies increased. This illustrates a competitive relationship, whereby the presence of one group limits the population growth of the other.
Turning to genetic variation, if there is more food available or the degree of predation decreases, we might initially expect the future generations of this group of voles to evidence greater genetic variation because a more stable and less predatory environment could support a wider range of traits within the population. However, without the selective pressure from predation or food scarcity, traits that would normally be selected against could persist, potentially leading to less genetic variation over time as these traits do not confer a significant survival advantage.