The role of ants in seed dispersal and regeneration in a degraded West African montane forest-grassland landscape, Ngel Nyaki Forest Reserve, Nigeria

Beneficial interactions between ants and plant diaspores, such as the dispersal and cleaning of diaspores have been widely reported from the neotropical, subtropical and temperate regions of the world. Remarkably, there are no reports of such interactions from the Afrotropics. Hence, it is not known whether the nature and outcomes of such interactions in this region mirror those from other regions. Therefore, this study was undertaken to document the species, and functional trait diversity of ground-dwelling ants, and determine the contribution of interactions between ants, and diaspores, to regeneration in a West African montane forest-grassland landscape in Nigeria, which is ecologically diverse but has been under considerable anthropogenic pressure. Ants were sampled from August 2015 to July 2016 using pitfall traps set along transects passing through forest, edge and grassland, to determine ant species richness and composition across the three habitats. Timed focal surveys at diaspore depots were conducted to record ant-diaspore interactions and an enclosure experiment in which vertebrates were excluded or allowed access to diaspore depots (with or without cages), was used to examine the effect of small, vertebrate competitors on the removal rate of diaspores of Trema orientalis by the two most abundant ant species. The effect of diaspore availability, size, texture, colour shade and chemical composition on ant-diaspore interactions were also investigated. Finally, the effects of manually cleaning seeds to mimic ant-cleaning, and sowing in soil taken from ants’ nests on seed germination success, were investigated in nursery germination experiments. Overall, ant species richness was low, with 17 species/morphospecies recorded across the three habitats. Species richness was higher in grassland and edge habitats, and species composition differed significantly between grassland and forest and between edge and forest, but not between grassland and edge habitats. Principal Component Analysis of functional trait values indicated trait diversity and adaptation of ant species to different levels of habitat complexity and resource exploitation. An unidentified Pheidole species dominated the forest samples while the grassland and edge habitats were dominated by Myrmicaria opaciventris and the Pheidole species from the forest samples. Only these two small, but abundant myrmicine ant species, interacted with diaspores of 10 plant species (moving six, and cleaning four); all non-myrmecochorous trees. Diaspores were cleaned on the spot or removed to ant nests/litter. Dispersal distances were low; ranging from 0.02 to 1.24 m with a mean of 0.29 ± 0.01 m. Diaspore size and texture significantly affected diaspore removal; ants only moved diaspores that weighed ≤ 0.3 g and preferred soft seeds to hard ones. Larger diaspores were cleaned on the spot and hand-cleaned diaspores germinated significantly quicker than uncleaned ones. Soils from ant nests were significantly more porous and had higher moisture content than non-nest soils, but ants did not significantly alter soil pH, nitrogen, phosphorus and potassium content. Cowpea seeds sown in soil from ant nests grew faster compared to those in non-nest soil, suggesting that ant nests may provide better microsites for germination. This first contribution on ant-diaspore interactions in an Afrotropical montane ecosystem, demonstrates that like in other regions, ants interact opportunistically, with non-myrmecochorous diaspores in potentially beneficial ways that could enhance seedling recruitment and regeneration in degraded habitats. But in contrast to those regions, the contribution of ants, to dispersal here, are minimal, reinforcing the reality that unless large vertebrate frugivores, which are the main long-distance dispersers in tropical forests are conserved, natural regeneration will be severely limite