Ants nesting in dry fallen petioles of Cecropia obtusifolia Bertol. (Urticaceae): vertical stratification and nest site limitation

Cecropia (Urticaceae) trees continuously shed their old large leaves, which fall directly on the forest floor (ground stratum) or accumulate on the aboveground vegetation under Cecropia canopies (vegetation stratum). Since twig-nesting ants are expected to be more nest site limited in the higher forest strata than at ground level, we predict that richness, abundance, and colonization frequency of ant nests in Cecropia petioles are greater in the vegetation stratum when compared with ground stratum. During June 2019, fifteen trees of C. obtusifolia were sampled in a tropical rainforest, in Veracruz, Mexico. For each tree, we collected ten dry fallen petioles per stratum, for 300 petioles in total. We recorded 44 ant nests from 21 ant species. Forty-two nests were found in individual petioles, although in one case a single petiole housed two different ant species. Thirty-four (23%) petioles were colonized in the vegetation stratum and only nine (6%) in the ground stratum. At tree scale, ant nest richness and abundance were significantly greater in the vegetation stratum. Furthermore, frequency of petiole colonization per tree was greater in the vegetation stratum (14 trees; 93.3%) when compared with the ground stratum (5 trees; 33.3%). A stratified pattern of ant occupation in natural nesting sites was observed, supporting that twig-nesting ants are more nest site limited in the upper forest strata than at ground level. Because nesting sites are a limiting resource for arboreal twig-nesting ants, we suggest that Cecropia trees are key components of tropical forests that maintain arboreal ant diversity by promoting nesting sites through dry fallen petioles.