One evening you go to dinner with several people, including a local wildlife manager. During dinner the manager tells you that she is setting up a new protected area where no wildlife will be killed. She explains that where people kill wild animals, their numbers and sizes have decreased. Moreover, where there has been heavy hunting and other types of human activities, wild places have fewer kinds of animals and plants. To help remedy these problems, the manager explains, her agency will fully protect wildlife in a few carefully chosen areas administered by the government.
Most people agree with that conclusion.
All of the scientists that I know who study protected areas have concluded the same thing, for both terrestrial ecosystems and marine ecosystems. Of course, the degree such management regimes are successful depends on a variety of factors regarding how they are designed and implemented, and how well external threats to the species are controlled. Nevertheless, the basic hypothesis that if you protect them they do better seems to be common sense and it is supported by scientific evidence. Of course it is more complicated than that, and more research really is needed to demonstrate how, where, and when this management tool should best be used. There is considerable experience in managing such areas for terrestrial ecosystems including as parks and wildlife refuges.