Overpopulation is a
troublesome measurement in a
human society. In an animal grouping, it's quite easy to define; it's the point at which the available
resources (including
space/privacy) are
overwhelmed by the demands of the group, and as a result, some members
die off until the group (population) is back below its supportable level.
In humans, however, the situation is never that clear-cut. For one thing, we've developed - ta-da! - transportation, which means that we can (theoretically) either a) leave the area or b) truck resources in. So who's to say when 'overpopulation' comes in? It's almost impossible to use this term without providing a context or a criteria with which to measure it. For example, you might say that New York City is overpopulated for the number of access points into Manhattan. Or, you might say that Mexico City is overpopulated because it cannot transport in enough resources to support its population (not true yet, as far as I know).
These days, the term is usually used to describe an entire ecosystem, and in our case, the only one we got - the Earth (mom Gaia) herself. Some environmentalists, human rights advocates, and other types claim that the Earth is overpopulated or is heading inexorably in that direction. However, there is evidence to the contrary; in some nations, notably European ones which enjoy relative internal security and calm polities, it has been observed that economic pressures have given rise to a 'flattening out' of the population growth curve. Germany, for example, at one point in early 1990s when her economy was flat, achieved ZPG or 'Zero Population Growth.' The hypothesis to explain this was that as people became unable to viably support more children, the average family size shrank to the point required to sustain the population level and no more.
So as humans, perhaps we can remain in harmony with our 'environment' - it's just that our environment is a bit more subjective than that of the beasts and birds.