Grassland songbirds often nest in the same grassland-wetland complexes as waterfowl, particularly in a certain part of those complexes, namely, upland habitats surrounding wetlands. Although some wildlife management procedures directed at waterfowl, such as habitat enhancement or restoration, may also benefit songbirds , the impact of others, especially the control of waterfowl predators, remains difficult to predict. For example, most predators of waterfowl nests prey opportunistically on songbird nests, and removing these predators could directly increase songbird nesting success. Alternatively, small mammals such as mice and ground squirrels are important in the diet of many waterfowl-nest predators and can themselves be important predators of songbird nets. Thus. Removing waterfowl-nest predators could affect songbird nesting success through subsequent increases in small-mammal populations.
In 1995 and 1996, researchers trapped and removed certain waterfowlnest predators. primary raccoons and striped skunks, then observed subsequent survival rates for songbird nests. Surprisingly. They observed no significant effect on songbird nesting success. This may be due to several factors. Neither raccoons nor striped skunks consume ground squirrels, which are important predators of songbird nests. Thus, their removal may not have led to significant increases in populations of smaller predators. Additionally, both raccoons and striped skunks prefer wetlands and spend little time in upland habitats; removing these species may not have increased the nesting success of songbirds in the uplands enough to allow detection.
It can be inferred that the habitat preferences of raccoons and striped skunks affected the results of the experiment described in the passage for which of the following reasons?
Songbird nests in the wetlands are usually located in places that most waterfowl-nest predators cannot reach.
Raccoons and striped skunks are not usually found in areas where songbird nests tend to be located.
Mice and ground squirrels tend to avoid predation by raccoons and striped skunks by remaining exclusively in the uplands.
The populations of small mammals in the wetlands are usually controlled by larger waterfowl-nest predators such as raccoons and striped skunks.
The waterfowl on which raccoons and striped skunks prey in the wetlands compete with songbirds for food.