The loss of large frugivores leads to seed dispersal loss and regeneration failure of numerous large-seeded trees near mother trees. Although Janzen–Connell effects are considered as the primary underlying cause, other factors remain understudied. Here, we used a field experiment to test the impact of flesh persistence on the recruitment of two large-seeded Sapotaceae species that lost their dispersers. In the rainforest of Mare Longue (Réunion), we sowed 3840 seeds in a four-factor design: seed treatment (seed cleaning; flesh persistence), canopy closure (understory; gap), year of sowing (01/2018; 11/2019) and species (Labourdonnaisia calophylloides, Mimusops balata). We also used camera traps to evaluate the impact of extant vertebrates. Seed treatment was by far the most influential factor: flesh persistence led to seedling recruitment divided by 3,2 on average, mainly due to failure of germination or seedling emergence. There were also significant variations in recruitment between species, years and canopy closure levels, notably due to the behaviour of the invasive fauna, especially giant snails that could unexpectedly restore recruitment by feeding on fruit flesh. Together, our results demonstrate strongly depleted recruitment due to flesh persistence and the importance of field experiments to understand the processes at work in complex ecosystems with novel plant–animal interactions.

A community of frugivorous weevils was studied by quantitative rearing of 57 weevil species represented by 10485 individuals from 326 woody plant species in lowland rain forest in Papua New Guinea. Only fruits from 35% of plant species were attacked by weevils. On average, weevils were reared from only 1 in 33 fruits and 1 kg of fruit was attacked by 2.51 individuals. Weevil host specificity was relatively high: 42% of weevil species fed on a single plant genus, 19% on a single plant family and only 16% were reared from more than one family. However, monophagous specialists represented only 23% of all reared individuals. The average 1 kg of fruits was infested by 1.84 individuals of generalist weevils (feeding on allogeneric or allofamilial host species), 0.52 individual of specialists (feeding on a single or several congeneric species), and 0.15 individual of unknown host specificity. Large-seeded fruits with thin mesocarp tended to host specialist species whereas those with thick, fleshy mesocarp were often infested with both specialists and generalists. Weevils tended to avoid small-seeded, fleshy fruits. The low incidence of seed damage (3% of seeds) suggests that weevils are unlikely to play a major role in regulating plant populations via density-dependent mortality processes outlined by the Janzen–Connell hypothesis.

The spatial patterns of canopy tree and seedling life stages in the African Tropical Tree Milicia excelsa were examined to test the Janzen–Connell hypothesis (escape hypothesis). The Janzen–Connell hypothesis states that the mortality of seeds and seedlings is highest near the adult tree and declines with distance away from the adult tree of the same species. This hypothesis argues that this effect is partially responsible for maintaining the high tree species diversity of tropical forests. Solar radiation and fruit dispersal were investigated as possible explanations of observed patterns in seedling distribution. Milicia excelsa trees ≥30 cm DBH were found to be randomly dispersed and occurred at a density of ≤1 ha. Milicia excelsa seedlings were located in small clumps between parent trees and up to 150 m from parents at a mean density of 65 seedlings ha−1. Fruits were in abundance (>9000 on the ground) beneath female trees. Light levels at locations of natural seedlings ranged from 2–29% of full irradiance. Planted seedlings did not suffer greater mortality closer to adult conspecifics, and survived at light levels between 2–37% of full irradiance. Phytolyma spp., gall-forming psyllids which cause high mortality in Milicia seedlings in large openings and plantations, were not observed on any natural or planted seedlings, although galls were present in the crowns of dominant trees.