Floristic Composition and Mutualistic Interactions in Ant Gardens of a Tropical Rocky Outcrop Forest in Peru

Tropical rocky outcrops are considered biological islands with high endemism and challenging environmental conditions. Ant gardens (AGs) represent a sophisticated ant–plant mutualism that may facilitate plant establishment in these restrictive habitats. This study conducted a comprehensive floristic inventory of Morro de Calzada (MdC), a tropical rocky outcrop in the Peruvian Andean–Amazonian piedmont, and evaluated the relationship between plant composition and the presence of AGs across six sampling units (912–1413 m a.s.l.). A total of 310 vascular plant species (77 families, 180 genera) were recorded. Melastomataceae (37 spp.), Moraceae (18 spp.), and Rubiaceae (18 spp.) were the most diverse families. Species richness decreased with elevation, with sampling unit 2 (951–1050 m a.s.l.) showing the highest richness (170 spp.) and sampling unit 6 the lowest (52 spp.). AGs were concentrated in the sampling unit SU4, which hosted the highest phorophyte richness (12 spp.) and number of gardens (22); this diversity was possibly driven by canopy and host-tree structure. The ant Azteca trailii tococae was the primary builder, showing strong associations with Meriania aff. microflora and Ficus citrifolia. Epiphyte composition was dominated by Anthurium gracile, with Epiphyllum phyllanthus as a notable xerophytic element. AG dimensions correlated positively with host-tree DBH and height. High floristic turnover among sampling units (Jaccard dissimilarity 0.78–0.94) contrasted with structural homogeneity of gardens, indicating ecological plasticity of Azteca trailii. The study highlights the role of AGs as ecosystem engineers in rocky outcrops and emphasizes the conservation value of MdC, which harbors endemic and threatened species. Sampling unit SU4 emerges as a functional hotspot for ant–plant mutualisms.