Nonadditive effects of two contrasting introduced herbivores on the reproduction of a pollination‐specialized palm

Abstract

Plant–animal interactions fall within a mutualism–antagonism continuum, exerting a wide range of effects on plant reproductive success. These effects become even more complex and diverse when several disparate animal species interact with the same plant species. Despite the increasing number of studies about the influence of herbivory on plant performance, the outcomes mediated by pollination and the combined impact of multiple herbivores on pollination‐specialized plants are underexplored. In this study, we chose the Mediterranean dwarf palm Chamaerops humilis (Arecaceae) to illustrate the isolated and joint effect of two contrasting introduced herbivores, the palm borer Paysandisia archon (Lepidoptera, Castniidae) and feral goats, on pollinator abundance and plant reproductive success. To this aim, we monitored moth herbivory and goat herbivory in four palm populations in Mallorca (Balearic Islands) during 2019 and 2020. The effect of herbivory varied widely depending on both the herbivore and the pollinator species. Moth herbivory had a positive effect on pollinator abundance and fruit initiation, whereas goat herbivory had a negative effect on inflorescence production, pollinator abundance and fruit initiation. In addition, both herbivores exerted unexpected nonadditive effects on palm reproduction. Palms attacked by both herbivore species produced many more inflorescences (up to 18‐fold) but had a lower fruit initiation success (close to zero) than unattacked palms or those attacked by a single herbivore species. Interestingly, only one of the two main pollinator species (the nitidulid beetle Meligethinus pallidulus) was impacted by herbivory. Our study highlights the need to investigate the possible nonadditive effects of all coexisting herbivores on plant performance, especially when establishing conservation plans and pest control strategies.

From seed dispersal service to reproductive collapse: density‐dependent outcome of a palm–mammal interaction

“…Nonetheless, an extreme herbivore pressure could completely suppress plant performance, as illustrated by our study. This impact, added to habitat fragmentation and the effect of invasive species like the palm borer Paysandisia archon (Muñoz‐Gallego et al 2022), could be leading to a slow regression of the insular C. humilis populations. Disentangling the net long‐term impact of introduced ungulates on plant communities, considering both positive and negative contributions (Sax et al 2022), is crucial for establishing proper management strategies and their ecologically sustainable densities.…”

“…In Mallorca (Balearic Islands, Spain), its populations are mostly restricted to the mountain areas Serra de Tramuntana and Serra de Llevant (Garcia and Morey 1989, CAIB 2023). For decades these island populations have been threatened by disturbances such as habitat loss and fragmentation, alien species, dense herbivore populations, fires and extreme climatic conditions (drought) (Ruiz et al 20120188, Muñoz‐Gallego et al 2022).…”

“…Although the introduction of domestic goats in Mallorca dates back thousands of years (Bover and Alcover 2008), rural abandonment in the 1960s led to the feralization and expansion of their populations (Vives and Baraza 2010, Roque 2017). Moreover, human pressure (urbanization, tourism), mainly through food supplementation (Côté et al 2004, Lecomte et al 2016), has fostered an overabundance of goats in specific locations on the island, causing severe damage to native vegetation (Cursach et al 2013, Capó et al 2021, Muñoz‐Gallego et al 2022). Goats can, however, exert both costs (e.g.…”

“…Goats can, however, exert both costs (e.g. florivory; Muñoz‐Gallego et al 2022) and benefits (seed dispersal; Muñoz‐Gallego et al 2019) on C. humilis performance, providing the opportunity to evaluate how the net outcome of the palm–goat interaction varies with the intensity of goat activity.…”

Seedling responses to moderate and severe herbivory: a field‐clipping experiment with a keystone Mediterranean palm

“…However, little is known about the responses of the dwarf palm to herbivory (Muñoz‐Gallego et al . 2022), especially at the seedling stage. Thus, we conducted a field‐clipping experiment, simulating different intensities of ungulate herbivory on naturally established dwarf palm seedlings, in two human‐disturbed sites in Mediterranean Spain.…”