Trophic Cascades and Megafauna Loss: Ecological Consequences of Large Mammal Defaunation in Nigerian Forest Reserves

The loss of large mammal species from Nigerian forest reserves has proceeded at an alarming rate over the past half-century, yet the cascading trophic consequences of this defaunation on plant community composition, seed dispersal networks, and ecosystem function have not been empirically investigated at landscape scale in the Nigerian context. This study investigates trophic cascade effects and ecological consequences of large mammal defaunation across a gradient of defaunation intensity in eight forest reserves in Edo, Ondo, and Cross River States. Defaunation intensity was quantified using camera trap occupancy surveys for nine focal large mammal species and compared against historical records and hunter recall data. Seed dispersal network analysis compared mutualistic interaction diversity between high- and low-defaunation reserves using a combination of direct observation, seed trap data, and DNA barcoding of seeds in animal faecal samples. Vegetation structure and recruitment analyses examined size class distributions and regeneration adequacy for 45 large-seeded tree species known to be dependent on large mammal dispersal. Trophic cascade effects were evaluated by comparing understory herbivore densities and consequent impacts on seedling survival across the defaunation gradient. Results confirm significant trophic cascade propagation across all levels. Large-seeded tree species showed severely truncated size class distributions in high-defaunation reserves, consistent with dispersal failure. Seed dispersal network complexity declined by 61% between low- and high-defaunation sites, with disproportionate loss of functionally unique interactions. Understory herbivore densities increased significantly in the absence of apex predators, creating secondary trophic effects on seedling survival. The study proposes a defaunation impact index for Nigerian forest reserves and recommends fauna-sensitive management planning as a legal requirement. Keywords: trophic cascades, defaunation, seed dispersal networks, forest reserves, large mammals