Top‐Down Control in Tropical Forests 1

IN HIS SEMINAL PAPER, TERBORCH (1992) BROUGHT TO THE ATTENTION of tropical biologists the possible importance of top-down control in Neotropical forests. Top-down structuring of biotic communities has been known for a long time for some aquatic systems (Hrbacek 1958, Paine 1966) and more recently, for simple desert ecosystems (Brown et al. 1986). Terborgh hypothesized that the loss of large predators such as jaguars and pumas due to hunting and/or forest fragmentation may result in higher abundances of large and medium-sized mammalian seed predators (>1 kg), which in turn may reduce populations of large-seeded tree species. He furthermore, albeit reluctantly, suggested that “Perhaps by hunting vertebrate seed predators, humans can attempt to substitute for the missing carnivores . ” Terborgh‘s ideas have attracted serious attention (Bierregaard et al. 1992, Molofsky & Fisher 1993, Meave & Kellman 1994, Peres 1994), but relevant empirical studies have produced equivocal results (Leigh et al. 1993, Terborgh & Wright 1994, Wright et al. 1994). Because Terborgh‘s ideas are of theoretical importance, and at the same time have important consequences for conservation and management, we offer comments on three points: (1) the correspondence between the sizes of terrestrial mammalian herbivores and the sizes of seeds and fruits the herbivores consume; (2) the relative importance of seed predation vs. dispersal by mammals for tree regeneration; and (3) recent trends in hunting pressure on large cats and mediumhge-sized mammalian herbivores. (1) Terborgh‘s arguments depend on the assumption that a correlation exists between seed size and mammalian herbivore body size. So far, significant positive correlations have been found between fruit size and gape width or body mass of fruit-eating tropical birds (Wheelwright 1985, Jordano 1992). Not surprisingly, however, large birds consume a wider range of fruit sizes than do smaller birds (Wheelwright 1985). A positive correlation between fruit mass and body mass has also been documented for frugivorous bats (Fleming 1988). Similar inferences could also be made for seed sizes and body masdgape widths of bats and birds, given the strong correlation between fruit size and seed size (Fleming 1988, IbarraManriquez & Oyama 1992). As yet, however, a correlation between the body mass of terrestrial mammals and size of seeds and fruits eaten has been assumed (Terborgh 1992, Fleming et al. 1993) but has never been rigorously documented. We analyzed qualitative records presented in 43 publications for 24 mammal species (20 terrestrial, 4 scansorial) and 112 tree species with known seed and/or fruit sizes from the Neotropics for documented cases of frugivory and granivory (seed predation). The minimum dimension-assumed to be the most restrictive one in terms of handling-was used in the analysis. The same analysis using maximum dimension yielded very similar results (not given). For information on seed and fruit dimensions, monographs by Fouque (1 976) and van Roosmalen (1985) were particularly helpful. All data sets can be obtained by request from the authors. We found no correlation between mammal body size and seed size (rr = 0.151, N = 61 mammal species-tree species combinations, P = 0.24) or between mammal body size and fruit size (rr = 0.144, N = 85, P = 0.19). A Mann-Whitney U test showed no significant difference in the sizes of seeds ( U = 395, P = 0.38) consumed by large (>1 kg, N = 35) and small (<1 kg, N = 26) mammals. We obtained similar results for fruits (U = 738.5, P = 0.18; N = 37 for small and 48 for large mammals). Furthermore, we did not find any significant difference between large and small mammals for maximum ( U = 36.5, P = 0.47; 7 small and 13 large mammal species) or minimum ( U = 28, P = 0.16) seed size consumed. Nor did we find any significant difference between large and small mammals for maximum ( U = 43.5, P = 0.68; 7 small and 14 large mammal species) or minimum (U = 39.5, P = 0.48) fruit size consumed. It is still possible that a significant trend will be found using data on more species and a d ~ ~ O U I I D of seeds and fruits of different tree species consumed by terrestrial mammals in Neotropical forests. Data of this kind are clearly needed. Lack of a correlation between body sizes of mammals and the sizes of