The main factor that influences population growth in a given environment is the availability of resources, including food, water, and shelter. These resources determine the carrying capacity of an environment, which is the maximum population size that can be supported sustainably. Other factors such as predation, competition, and environmental conditions (like climate) also play significant roles. Understanding these influences is critical in fields like ecology and conservation biology, as they help manage ecosystems and protect endangered species by ensuring sufficient resources are available for populations to thrive.