Some ornithologists believe that many species of songbirds are threatened by deforestation. Yet they also claim that, despite recent reforestation, matters continue to worsen, since it is fragmentation of forest rather than reduction of forest size that endangers songbird species. The introduction of open spaces and corridors in forests reduces the distance of songbird nests from unforested areas and thus reduces the songbirds’ natural shield from predators.

The claim that there has recently been reforestation plays which one of the following roles in the ornithologists’ argument?