Drivers of Spatial Variation in the Role of Ants as Secondary Seed Dispersers.

The spatial variation in the outcome of the interaction between secondary dispersers and seeds is superimposed upon the variation produced by primary dispersers. Investigating the factors that drive the outcome of the interactions with secondary seed dispersers thus represents an essential refinement to our understanding of the complete seed dispersal process. We studied the interactions between two ponerine ants (Pachycondyla striata Smith, 1858 and Odontomachus chelifer (Latreille, 1802)) with fruits experimentally set on the ground, and estimated the effects of ants on seedling establishment in three areas distributed along a 2-km stretch of a Brazilian Atlantic rainforest that differ in soil properties and vegetation physiognomies. We tested the hypothesis that interactions are more frequent, resulting in greater seedling establishment at the site with harsher abiotic and biotic conditions. Both ant species removed fruits frequently and have a positive effect on seedling establishment in all study areas, but fruit removal did not differ among areas, while seedling establishment was more pronounced at the site with stressful abiotic conditions. The two ant species differed in important aspects of their seed dispersal services, including the propensity to interact with seeds. As a result, both the species of ant and abiotic conditions interact at the scale of 2 km to determine the fate of seeds interacting with ants, thus creating a mosaic of outcomes with variable benefits to plants.

Palm harvesting affects seed predation of Euterpe edulis, a threatened palm of the Brazilian Atlantic Forest.

The palm tree Euterpe edulis is endemic to the Atlantic Forest, where it constitutes an economically important forest product. The often unplanned and illegal harvesting of palm hearts has led to drastic reductions in the populations of E. edulis in many areas where this palm used to be the dominant understorey tree species. We investigated the effects of harvesting on seed and seedling predation of E. edulis. We tested the predictions of the dominance-predation hypothesis according to which predator satiation leads to an inverse relationship between the amount of predation and the dominance of a tree species. During two consecutive years, seeds were set experimentally on an unharvested (> 250 adult palms/ha) and a neighboring harvested site (few, if any, adult palms) located in the Atlantic Forest of SE Brazil. Seedling mortality was studied at both sites for a six-month period in each of two consecutive years. Seed predation caused by rodents was higher at the harvested site, while insects caused more damage to seeds placed at the unharvested site. The proportion of seeds preyed upon by rodents varied annually, while insect predation did not. Seedling mortality did not differ between harvested and unharvested sites. The dominance-predation hypothesis was confirmed for generalist rodent seed predators, but not for specialist insect predators. This result shows that density-dependent mortality, not only at the individual level but also at the population-level scale, is a function of the class of predators and their types of foraging behavior.

Palm harvesting affects seed predation of Euterpe edulis, a threatened palm of the Brazilian Atlantic Forest

O palmito-jussara, Euterpe edulis, é uma árvore endêmica da Mata Atlântica e constitui-se em produto extrativista economicamente importante. A exploração ilegal e descuidada do palmito tem levado a drásticas reduções em suas populações ao longo de toda sua distribuição, que outrora fora abundante. Neste estudo investigamos o efeito da extração do palmito sobre a predação de suas sementes e plântulas por roedores e insetos. Especificamente, testamos a hipótese segundo a qual, em razão da saciação dos predadores, deve haver relação inversa entre a magnitude da predação sobre as sementes e a abundância da planta em determinada área. Durante dois anos consecutivos, sementes de E. edulis foram colocadas em uma área preservada (> 250 indivíduos adultos de E. edulis/ha) e em uma área vizinha explorada com baixíssima densidade de plantas adultas, ambas localizadas na Mata Atlântica do Estado de São Paulo. A mortalidade das plântulas foi estudada e comparada entre ambas as áreas por um período de seis meses. A predação das sementes por roedores foi maior na área explorada que na área preservada e variou entre os anos. A predação por insetos foi maior na área preservada e não variou entre os anos. A mortalidade das plântulas foi similar em ambas as áreas. A relação inversa entre a abundância da planta e a predação de suas sementes foi confirmada para predadores generalistas (roedores), mas não para predadores especializados (insetos). Os resultados mostram que a mortalidade de sementes dependente de densidade, não apenas em nível individual, como já demonstrado, mas também em escala populacional, depende do tipo de predador considerado.