Foraging in the Tropics.  Is it Possible?

(from Bailey, R. and T. Headland [1992]. “The tropical rain forest: is it a productive envrionment for human foragers?” Human Ecology 19(2): 261-285.  This special issue of Human Ecology has contributions from experts in reaction to this target article)
 
According to Headland and Bailey foraging in the tropics is impossible or difficult because: