Fragmentation modifies seed trait effects on scatter-hoarders’ foraging decisions

Abstract

Scatter-hoarding animals are crucial in seed dispersal of nut-bearing plants. We used the holm oak Quercus ilex—wood mouse Apodemus sylvaticus mutualism as a model system to evaluate the relative importance of seed size and fat content on scatter-hoarders’ foraging decisions influencing oak dispersal and potential recruitment. We performed a field experiment in which we offered holm oak acorns with contrasting seed size (2 vs 5 g) and fat content (3 vs 11%). Moreover, to test if the strength of these seed trait effects was context-dependent, experimental acorns were placed in small fragments, where natural regeneration is scarce or absent, and forest habitats. In small fragments, rodents had to face increased intraspecific competition for acorns and reduced anti-predator cover during transportation. As a result, they became more selective to ensure rapid acquisition of most valuable food items but, in turn, transported seeds closer to avoid unaffordable predation risks. During harvesting and caching, larger acorns were prioritized and preferentially cached. Fat content only had a minor effect in harvesting preferences. In contrast, in forest sites, where rodent abundance was four times lower and understory cover was well-developed, rodents were not selective but provided enhanced dispersal services to oaks (caching rates were 75% higher). From the plants’ perspective, our results imply that the benefits of producing costly seeds are context-dependent. Seed traits modified harvesting and caching rates only when rodents were forced to forage more efficiently in response to increased intraspecific competition. However, when landscape traits limited cache protection strategies, a more selective foraging behavior by scatter-hoarders did not result in enhanced dispersal services. Overall, our result shows that successful dispersal of acorns depends on how specific traits modulate their value and how landscape properties affect rodents’ ability to safeguard them for later consumption.