In disturbed forests, Atelidae primates, such as howler monkeys, are known to play a key role in the dynamics of secondary succession. However, little is known about their role in primary succession. Thus, the aim of the present study was to evaluate the role of the howler monkey Alouatta caraya in the primary succession dynamics of the Paraná flooded forest, in northeastern Argentina. This was achieved by studying the recruitment of saplings in the latrines of A. caraya groups inhabiting the forest in the first stages of succession and the distribution of plant species in relation to the age of the forest. The results showed that: (a) young forest areas were dominated by tree species dispersed by water, wind, birds, bats and fishes, whereas older forest areas were dominated by species dispersed by A. caraya, with individuals of some species present in young forests; (b) the floristic composition of saplings growing in A. caraya latrines of young forest areas was similar to that of old forest areas; and (c) saplings growing in young forest areas were heavily associated with A. caraya latrines and corresponded to species from older forest areas. The results allow the conclusion that A. caraya plays a role in the last phase of primary succession of the Paraná flooded forest, as well as in contributing to an increase in the structural complexity and species diversity in the young areas of the Paraná islands.

The species composition of the forest was evaluated in both young and old areas of the "Brasilera" island in Paraná River. Twenty-five plots of 20 x 20 m were located at random in the forest of the young areas of the island (characterized by presence of Tessaria integrifolia and Salix humboltiana, (Marchetti et al. 2013, Brumnich et al. 2019)), whereas the other 25 were located in the central and southern bars of the island to sample old forests. All trees with more than 10 cm of diameter at breast height were recorded. To find latrines of Alouatta caraya monkeys, three transects of 500 m long were used in the old forests and one in the young area. Eighteen latrines were found in the old forests and ten in the young area. All saplings, small individuals with less than a third diameter than adult trees, higher than 25 cm growing in them were recorded.