The correct answer to complete the statement could be "Furthermore," as it connects the ideas about the environmental impact of outdoor cats and the dangers they face outside. The revised text would read:
"People who keep cats as pets should keep them indoors. Domestic cats pose a threat to the environment; furthermore, one study found that areas with a large population of outdoor cats have smaller populations of numerous small animals than areas with small populations of outdoor cats. Additionally, life outdoors is dangerous for the cats. They can be harmed by larger predators and are at risk of injury from cars."