Comparing germination metrics of Opuntia ficus-indica and O. robusta between two sets of bird species (Pied Crows and two smaller species)

Abstract

The fleshy fruits of exotic Opuntia ficus-indica (L.) Mill and Opuntia robusta (Haw.) Haw (Cactaceae family) are consumed and dispersed by many vertebrates, which likely influences their invasion success. We tested whether seed ingestion by Pied Crows (Corvus albus) and other smaller birds (African Pied Starling Lamprotornis bicolor and the Red-eyed Bulbul Pycnonotus nigricans) improve the germination and speed in O. ficus-indica and O. robusta. Controlled germination trials for two Opuntia species were set up using the seeds extracted from faecal material, depulped seeds, and intact fruits. Overall, results show that seed germination for O. robusta was significantly higher than for O. ficus-indica. There were significant differences in seed germination between treatments of two Opuntia species, with the total mean germination of seeds defecated by the Pied Crows being equivalent to that of seeds defecated by the other smaller birds but significantly greater than the other treatments (i.e., depulped seeds and intact fruit). We noted that removal of the fruit pulp from seeds significantly improved germination in both Opuntia species compared to intact fruits. The O. robusta seeds defecated by the smaller birds had significantly greatest and most accelerated germination of all other treatments followed by the seeds defecated by the Pied Crows. Seeds of O. ficus-indica defecated by the Pied Crows had significantly greater germination than the seeds ingested by smaller birds. We conclude that seed germination success in fleshy fruits including those of alien species, varies with seed treatment by different vertebrate dispersal agents, and their net effect on seeds, may have consequences on species invasion success