Seed abundance affects seed removal of an alien and a native tree in the Brazilian savanna: Implications for biotic resistance

Invasive species are among the major threats to global biodiversity. Plant invasions are often driven by great propagule pressure that overcomes barriers to early colonisation and establishment, such as the biotic resistance from pathogens and seed predators. However, the way in which biotic resistance interacts with propagule pressure and affects seed survival remains poorly understood. Here, we experimentally assessed the effect of seed abundance on seed removal in the Brazilian Cerrado (savanna). We tested whether the invasiveLeucaena leucocephalaand the nativeAnadenanthera macrocarpa(both Fabaceae) differ in post-dispersal seed removal - an indicator of granivore pressure - by different guilds of seed predators, varying seed abundance (2, 5 or 10 seeds of each species per treatment) and distance from a disturbance (5, 20 and 50 m from a firebreak) in two fragments of Cerrado. Treatment encounter, that is the chance of at least one seed being removed, increased 72% from 2 to 10 seeds per treatment, but seed removal increased only by 32%, suggesting a threshold in granivore response to seed availability. Native seeds suffered higher removal than exotic seeds, but removal rates did not vary with distance to disturbance or interact with vertebrate and invertebrate exclusions. Our results suggest a patchy pattern of seed predation influenced by resource abundance and emphasise the efficiency of high propagule pressure in overcoming the density-dependent biotic resistance during early stages of exotic plant invasions in Cerrado. Seeds of fast-germinating species, such asL. leucocephala, that reach appropriate spots for regeneration may still overcome the biotic resistance due to the patchy nature of seed predation. in Portuguese is available with online material.