What are the connections between population growth, sustainability, and environmental health? How can I find academic sources on this topic?
All three of these issues are intimately connected. One way to explore the relationships among them is through the concept of "carrying capacity." In biology, this refers to the population a region can support indefinitely without degradation of environmental quality. For species other than humans, populations usually attain a certain degree of equilibrium based on carrying capacity. For example, if more deer are born than can survive from the available grazing in an area, some of them will starve, returning the population back to sustainable numbers. Most natural environments have a sustainable balance of plant life, herbivores, and predators that is self-regulating and sustainable. One great worry among scientists is that humans' ability to alter our natural environment may enable us to exceed the carrying capacity of our world.
For example, population growth has been sustained by a "green revolution" in agriculture, which uses selectively-bred and now genetically-modified high-yield crops which require irrigation and fertilizer. While these temporarily sustain higher populations—leading to a cycle of even greater food production—there are worries that irrigation-intensive agriculture is depleting water tables and causing land degradation. Global climate change is another form of environmental degradation due in part to population growth.
Environmental health is affected by overpopulation in several ways. The most obvious is the spread of epidemic diseases in overcrowded environments with poor sanitation. Overpopulation and unsustainable agricultural and industrial policies cause degradation of water supplies, leading to various illnesses due to drinking contaminated water.
To find academic resources on these issues, you should search databases of academic articles. If you go to your university library's website, you can search the JStor database; a reference librarian or your library help desk can help you learn how to use advanced search tools. Another good place to look is on Google Scholar. Although it uses the same search interface as Google, it searches exclusively in scholarly rather than popular resources.