Combined impact of multiple exotic herbivores on different life stages of an endangered plant endemism, Medicago citrina

Abstract

Summary Herbivores and granivores represent one of the most influential drivers of plant abundance and population dynamics. Their effect can be, in turn, modulated by biotic or abiotic factors such as community composition, habitat characteristics or space heterogeneity. Recent approaches to the study of herbivore and granivore impacts on plants have considered the combined action of multiple herbivore species, the effect of herbivores on several plant life stages or the effect of environmental gradients on these interactions. However, studies addressing the effect of multiple herbivore species on different plant life stages are still lacking. We estimated the combined effect of multiple exotic herbivores (European rabbits, Oryctolagus cuniculus; black rats, Rattus rattus; and house mouse, Mus musculus) on four different life stages of an endangered plant species (Medicago citrina, Fabaceae). Mortality for seed, seedling and sapling was estimated at three types of plots (open, rat exclusion and rat + rabbit exclusion) replicated at four sites (N = 3 per site and treatment) within Cabrera Island (Balearic Islands, western Mediterranean). Browsing of reproductive adults was simulated under common‐garden conditions (Sóller Botanic Garden, Mallorca Island) and its effect on reproductive effort and success measured. European rabbits and black rats had complementary impacts on the different life stages of M. citrina. These included independent effects on different life stages (seed predation by rats, seedling predation by rabbits), which resulted in multiplicative increases in plant mortality, and concurrent effects on the same life stage (sapling predation). In addition, the simulated‐herbivory experiment showed that a low rate of canopy removal (25% of initial biomass) already causes a strong decrease in fruit set (from 54% to 30%), but increasing rates of canopy removal do not increase this effect. Synthesis. Our results stress the importance of considering the combined effects of different herbivores on several life stages of the plant's life cycle and their consecutive effects on population dynamics. From an applied point of view, future reintroduction attempts of M. citrina in Cabrera Island should consider measures to either control the populations of both exotic herbivores or mitigate their impacts on the earlier recruitment stages of the plant (seeds, seedlings and saplings).