It's easy to think of population limiting factors only in terms of animals and plants, but these factors apply to humans as well. Some of these factors, such as earthquakes, floods and natural disasters, affect populations regardless of their density and are known as density-independent. Density-dependent factors, however, are those that have great impact only once populations reach a certain level.
The demand for energy sources is a factor that affects populations in a way that is proportional to their density. For example, if only one locust were to inhabit an area, chances are that food demand wouldn't be such a pressing issue. However, locusts live in swarms, and they will deplete an area of the food before migrating to a new area. Likewise, if the jackrabbits in one part of Death Valley National Park run short of food, they will begin to die off and have to migrate to another place where either food is plentiful or there aren't as many jackrabbits.
An imbalance in the predator-prey relationship can be a limiting factor as well. For example, if the Death Valley jackrabbits migrate, they will cause an imbalance in their relationship with the foxes, who will be left with less food than they need. As a result, the foxes will be unable to maintain their population density without making an adjustment, such as migrating. A surplus of predators can also affect this balance. For instance, if a wolf population in an area grows, or if a pack immigrates to the area, the additional hungry wolves can place a density limit on the local moose population.
Competition for food is a factor when at least one of two populations reaches a density where the two populations combined overwhelm the food supply. For example, when rainbow smelt were introduced into Lake Winnipeg, they put a strain on the flourishing population of emerald shiners because both species eat the same food. As a result, the population of emerald shiners is believed to have decreased because of the competition. Also, competition isn't limited to animals. Eurasian water milfoil is a freshwater aquatic plant that grows and spreads rapidly in ponds and lakes. It can use up much of the dissolved oxygen that other plants and fish need to survive.
Disease is density-dependent because organisms have to live close enough to one another for the disease to spread. In the context of humanity, it is easier to see how disease can spread in a city such as New York or Hong Kong as opposed to the rural setting of Wyoming. Furthermore, research conducted at The Ohio State University shows a link between population density and higher percentages of water-borne illnesses. This shouldn't be a surprise, as many high-population areas utilize integrated city water systems while many rural areas still use individual wells. The denser population breeds the need for a community water supply, which then serves as a transport for pathogens.