Why do environmentalists such as Paul Ehrlich view unchecked population growth

Environmentalists recognize that much of Earth's natural capital cannot be effectively replaced. Land area is one of these. We cannot expand Earth, increasing its surface area to accommodate an unlimited population. Overpopulation also leads to degradation of natural systems and loss of biodiversity. We cannot always devise a prosthesis for lost ecosystem services and cannot replicate the exact function of missing species in ecosystems. Lost species may perform vital ecosystem services, such as pollination, and also have educational, aesthetic, or medicinal value that is irreplaceable.
 
While Ehrlich's predictions of the 1960s have not fully materialized in the 2000s the fact remains that many ecosystem services are in a state of decline, and despite the view of some Cornucopian economists that we can safely ignore his warnings, the pressure of a growing population on essential resources and biodiversity remains unmistakable.