Identify Assumption in Urban Wildlife Argument

A recent analysis of urban wildlife populations indicates that cities with larger green spaces tend to have higher diversity of bird species.

Environmentalists assert that increasing the size of urban parks will naturally enhance the diversity of bird life in those areas. However, local city planners argue that other factors, such as pollution levels and food availability, play a significant role in determining bird diversity.

Based on this information, which of the following is an assumption made by the environmentalists in their argument?