The dual role of red deer in yellow rockrose seed dispersal and predation in Mediterranean Spain

Endozoochory, seed dispersal after animal ingestion, is a mutualistic plant-animal interaction that often involves substantial costs (e.g. ingested seed destruction). However, few studies have simultaneously comprehensively assessed the costs and benefits. We investigated the interaction between overabundant ungulates and the Mediterranean shrub Cistus halimifolius in Doñana National Park. We evaluated frequency of ungulate visits to fruiting C. halimifolius individuals using camera traps. We then assessed seed occurrence and germination success via faecal analysis and germination experiments. Camera traps revealed that the red deer, Cervus elaphus, were the principal consumer of C. halimifolius fruits (85.7% of 333 ungulate visits). Although red deer frequently visited C. halimifolius, only 4.5% of faecal samples (n = 246) contained seeds, likely because red deer consumed immature fruits whose seeds were then fully digested and thus undetectable. Indeed, all seeds recovered within deer faeces were fully developed, mature seeds. 17% of the mature deer-ingested seeds (19 out of 114) germinated, a lower proportion than in control mature seeds (40%, n = 168). Mature seed passage through the deer digestive tract reduced Cistus seed germinability. Despite low germination rates, the highly mobile deer are potential long-distance seed dispersers of C. halimifolius, promoting population gene flow and the (re)colonization of vacant habitats. The interaction between C. halimifolius and red deer involves substantial costs, which likely changes spatio-temporally and also depend on densities of the interacting species. Future studies should quantify such changes and identify whether and how such pervasive interaction in Mediterranean ecosystems shifts along a mutualism-antagonism continuum.