One of the first signs of overpopulation is unemployment i.e. too many people and not enough stuff for them to do.

Third cause; This correlation does not imply causation.

This is only a valid symptom within certain preassumed conditions, and in other contexts fails horribly as a metric.

For illustration, a certain fictional island could have rather poor and difficult source of food (farmland, fish populations, etc.) that simply cannot sustain a population past X, regardless of whether the remaining (N-X) humans have work to do (infrastructure, repair, killing the Giant Death Crabs that prey on the farmers, etc.). Similarly, its sister island could have a large population of wild fruit-bearing trees that can freely sustain any number of humans up to Y, whether or not some of them find themselves without "work" or any skills valuable to other inhabitants.

Both of the above parables are related to real, but complex, issues with natural resource distribution and acquisition in the real world.

Overpopulation, by contrast, is when the net resources of a timespace / group / system can no longer reliably support the population by whatever metrics are necessary or employed by the population -- food, clothing, shelter, kinship, entertainment, and yes, even work, employment and unemployment.

But all of these depend on the context. If you fix the problem of humans requiring food, then food no longer needs to be considered as a metric for overpopulation. If you fix the problem of humans requiring employment to gain access to the food (which retains its own separate overpopulation threshold, generally held to be higher than employment in some places), then employment no longer needs to be considered as a metric for overpopulation.