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

Abstract

Interspecific ecological interactions are inherently context‐dependent. They may vary in both magnitude and sign depending on the biotic and abiotic conditions, depicting a mutualism–antagonism continuum. However, how population abundances and the activity of interacting species modulate these interactions remains underexplored. Here, we chose the interaction between the Mediterranean palm Chamaerops humilis and the feral goat Capra hircus in Mallorca (Balearic Islands, Spain). We selected three study plots with low, intermediate and high intensities of goat activity where we characterized palm distribution, seed rain, seed predation and early palm recruitment during two consecutive years. Since goats can cause both costs (e.g. florivory) and benefits (e.g. seed dispersal) to C. humilis performance, we investigated the following three questions: 1) does the spatial distribution of adult palms vary depending on the intensity of goat activity? 2) Does the intensity of goat activity influence seed rain and its potential spatial association with adult palms? 3) To what extent does the intensity of goat activity determine post‐dispersal events such as seed predation and seedling emergence? We found that adult palms showed a more clumped and complex distribution (double‐cluster process) in plots with low and intermediate goat activity compared to that with high goat activity (simple‐cluster process). In the low goat activity plot, dispersed seeds were spatially aggregated around adult palms, showing twice as much insect‐seed predation and nearly three times lower seed germination success than those in the intermediate goat activity plot. Palm seed dispersal and recruitment were almost nil in the high goat activity plot due to heavy consumption of palm inflorescences and developing fruits by goats. Our findings demonstrate how the net outcome of plant–animal interactions can change from mutualism to antagonism, from reproductive service to reproductive collapse, depending on the abundance and the activity of the interacting species.