PREDATION BY VERTEBRATES AND INVERTEBRATES ON THE SEEDS OF 5 CANOPY TREE SPECIES OF AN AMAZONIAN FOREST

We studied the pre-germination loss of seeds to invertebrate and verte brate seed predators of 5 species of Amazonian trees (Astrocaryum macr ocalyx - Palmae; Bertholletia excelsa - Lecithydaceae; Calatola venezu elana - Icacinaceae; Dipteryx micrantha - Leguminosae (Papilionoidae); Hymenaea courbaril -Leguminosae (Caesalpinoidae)). These five species were selected from a large tree flora on several criteria. All posses s large (3-10 cm) well-protected seeds that might plausibly be attract ive to mammalian seed predators. The reproductive biology of three of the species, or close congeners, had been studied elsewhere in the Neo tropics (Astrocaryum, Dipteryx, Hymenaea); one is important to the eco nomy of southeastern Peru (Bertholletia); and one, despite large and a pparently edible seeds, appeared to suffer no pre-germination loss to predators (Calatola). We conducted the research in mature forests in t he Manu National Park of southeastern Peru where mammal densities are unperturbed by human activities. Densities of adult trees of the five species in our area range from very high (> 30 per ha: Astrocaryum) to very low (much less than 1 per ha: Hymenaea). Loss of seeds to all ca uses, and to mammalian seed predators in particular, was determined fo r seeds placed in 2-square meter mammal exclosures and in open control s located at 10 m (near) and 50 m (far) from a large mature individual of the target species (with minor variations in the design for Astroc aryum and Calatola). The exclosures were of two types: impermeable - d esigned to exclude all mammals, but not invertebrate seed predators, a nd semipermeable - designed to admit small (< 500 g), but not large ma mmals. Experimental and control plots were stocked with apparently via ble seeds during the dry-wet transition period (October-November) and scored one year later. A significant distance effect (higher predation near vs far from a large conspecific adult) was found in only one of the species (Astrocaryum), the only one to be attacked with high frequ ency by invertebrate seed predators. The absence of any detectable dis tance effect attributable to mammals suggests that mammals, over the c ourse of a year, thoroughly search the forest floor for seeds. Inverte brates may thus be responsible for most pre-germination distance (dens ity) effects. With respect to the treatments, we found three qualitati vely distinct results: seeds of three species (Astrocaryum, Berthollet ia, Dipteryx) were significantly protected by the impermeable, but not semipermeable exclosures, implicating small mammals in seed loss; the seeds of one species (Hymenaea) were significantly protected by exclo sures of both types, implicating large mammals; and the seeds of one s pecies (Calatola) exhibited 100% survival, whether or not protected by exclosures. The importance of large mammals as seed predators is gene rally underestimated in these experiments because semipermeable exclos ures may serve as foraging reserves for small mammals. Finally, we not ed no relationship between the intensity of mammalian seed predation ( as suggested by the survival of unprotected seeds) and the abundance o f adults of the five species in the environment. The diversity of resu lts obtained for the five species reveals that large-seeded tropical t rees may display a wide range of demographic patterns, and points to t he likely importance of post-germination bottlenecks in the population biology of many species, even those that may experience severe pre-ge rmination seed loss.