Abstract:
J. F. BRODIE AND H. K. GIBBS ( BUSHMEAT HUNTING AS CLIMATE THREAT, Letters, 16 October 2009, p. 364) argue that bushmeat extraction threatens the carbon stocks of tropical forests because (i) bushmeat hunting reduces abundances of large-bodied vertebrates; (ii) tree species with large seeds reproduce poorly without large-bodied vertebrates on which they depend for seed dispersal; (iii) large seed size is correlated with high wood density in tropical trees; and (iv) trees with high wood density contribute disproportionately to the carbon stock. Their first point is well-established, but evidence regarding the others is mixed. Killing animals reduces seed dispersal of vertebrate-dispersed trees (1 4) but does not necessarily reduce the reproduction of large-seeded trees (5), perhaps because large-bodied animals also function as seed predators and herbivores (2, 6). Likewise, the correlation between seed size and wood density in tropical trees is at best weak (7). Finally, plots with trees of higher wood density do not necessarily have higher total tree carbon stocks; depending on the site, carbon stocks may be positively related, negatively related, or unrelated to mean wood density, because of the usually countervailing effects of tree volume (8). Lianas (woody vines that climb into the tree canopy) provide an alternative possible link between bushmeat hunting and carbon storage. Hunting is a disadvantage for species with seeds dispersed by animals, and therefore gives a comparative advantage to species with seeds dispersed by wind (5, 9). This strategy is much more common among liana species than trees (60 versus 20%). Liana leaves displace an equal mass of tree leaves (10), and lianas store much less carbon per leaf area than trees (11). Thus, hunting may favor lianas, and an increase in lianas is likely to reduce carbon storage. Whatever its effect on forest carbon stores, the bushmeat crisis is unarguably a major threat to tropical biodiversity (2, 12, 13). This by itself is reason to fight it.
