Question

What is the most likely reason why the carrying capacity of the human population is difficult to estimate?

a. Humans are a K-selected species.
b. Human migration is higher than that in other species.
c. Human carrying capacity is influenced primarily by abiotic factors.
d. Humans consume more resources than other species.
e. Humans alter their own environment more than other species do.

Answer 1

I am thinking about d or e, which one do you think it is?

Answer 2

A case could be made for more than one.

It's definitely not K selection and the actual amount of resources that humans consume doesn't matter; the same limiting principle applies whether they (or we?) much or little. It's probably not migration, either, as humans a pretty much a globally distributed population with resources moved from one locality to another in vast scales (think of all the people in New York or Toronto trying to live on the natural resources available in the immediate vicinity).

I'd say at this point carrying capacity is limited by abiotic factors. I would argue that poverty and conflict are abiotic factors; they limit human population in many, many parts of the world. I also think a very good (and obvious) case could be made for E.

I realize that wasn't as helpful as perhaps you'd have liked. ;D