In Mauritius, the endemic blue-tailed day-gecko Phelsuma cepediana is currently the sole pollinator and seed disperser of the critically endangered endemic plant Roussea simplex (Rousseaceae). The flowers and fruits are often visited by the invasive ant Technomyrmex albipes, which forages on the nectar and fruit pulp, and tends honeydew-producing mealybugs on the fruits. Here, we experimentally explore how the presence of this alien ant species influences geckos foraging at flowers and fruits of R. simplex by removing and excluding ants from flowers and fruits. Gecko visitation rates to ant-free control flowers and fruits, and flowers and fruits where ants had been removed and excluded, were higher than those to ant-infested flowers and fruits. The resulting seed set of ant-infested flowers was greatly reduced, compared to ant-free control flowers. Similarly, for fruits with ants, very few seeds were likely to be ingested and dispersed by the geckos. Thus, T. albipes monopolizes flowers and fruits of R. simplex, and prevents access of pollinating and seed-dispersing P. cepediana geckos by aggressive interference competition. For a critically endangered plant like R. simplex, this double-disruption of two vital mutualistic interactions is of urgent conservation concern.