Trophic and non-trophic pathways mediate apparent competition through post-dispersal seed predation in a Patagonian mixed forest.

CACCIA, F.; CHANETON, E.J.; KITZBERGER, T.

OIKOS

Blackwell Publishing

Lugar: Oxford; Año: 2006 vol. 113 p. 469 - 480

0030-1299

Post-dispersal predation can be a major source of seed loss in temperate forests. Little is known, however, about how predator-mediated indirect interactions such as apparent competition alter survival patterns of canopy tree seeds. Understorey plants may enhance tree seed predation by providing sheltered habitat to granivores (non-trophic pathway). In addition, occurrence of different tree seeds in mixed patches may lead to short-term apparent competition between seed types, because of the granivores’ foraging response to changes in food patch quality (trophic pathway). We hypothesised that understorey bamboo cover and mixing of seed species in food patches would both increase tree seed predation in a Nothofagus dombeyi-Austrocedrus chilensis forest in northern Patagonia, Argentina. Seed removal experiments were conducted for three consecutive years (2000–2002) differing in overall granivory rates. Seed patch encounter  and seed removal rates were consistently higher for the larger and more nutritious Austrocedrus seeds than for the smaller Nothofagus seeds. Seed removal was greater beneath bamboo than in open areas. This apparent competition pathway was stronger in a low-predation year (2000) than in high-predation years (2001-2002), suggesting a shift in microhabitat use by rodents. Patch composition had a significant, though weaker, impact on seed survival across study years, whereas seed density per patch enhanced encounter rates but did not influence seed removal.  Removal of the less-preferred Nothofagus seeds increased in the presence of Austrocedrus seeds, but the reciprocal indirect effect was not observed. However, this non-reciprocal apparent competition between seed species was only significant in the high-predation years. Our study shows that granivore-mediated indirect effects can arise through different interaction pathways, affecting seed survival patterns according to the predator’s preference for alternative seed types. Moreover, results indicate that the occurrence and relative strength of trophic vs non-trophic pathways of apparent competition may change under contrasting predation scenarios.