Empirical evaluation of directed dispersal and density-dependent effects across successive recruitment phases

The directed-dispersal (DrD) hypothesis constitutes one of the main explanations for the adaptive value of seed dispersal in spatially heterogeneous environments. Traditionally, the DrD hypothesis asserts non-random arrival to specific sites where establishment conditions are independently favourable. Yet, enhanced arrival might negatively affect the establishment in otherwise favourable sites through enhanced density-dependent mortality (DDM). Since both density effects and habitat suitability might differ among establishment phases, assessment of habitat suitability should encompass the entire establishment process. Here, we test the DrD hypothesis against a null model of random dispersal (RD) in an arid ecosystem where fleshy-fruited perennial Ochradenus baccatus shrubs are dispersed by two resident passerines. Acacia trees were predicted to present the favourable habitat in our system through the nurse-plant effect facilitating O. baccatus early establishment. We determined expected seed arrival by monitoring the dispersers' time allocation among the different habitats and assessed habitat quality for different establishment phases: seed-phase survival (post-dispersal survival and germination) was determined by line transects, while early seedling survival was estimated by monitoring seedlings throughout the first dry season. Additionally, subsequent recruitment success was assessed by comparing the distribution of adult shrubs to that expected from the observed germination patterns. When considering the expected seed arrival and early establishment (seed-phase and early seedling survival), the DrD hypothesis is supported: birds preferentially deposited seeds in habitats where conditions are favourable (e.g. under Acacia trees). Yet, considering recruitment success during later phases, the dispersal service provided by the birds was less beneficial than RD. Effective density was indexed using the zone-of-influence approach in which densities are corrected by phase-specific sizes. We suggest that intense seed deposition into favourable habitats has led to strong DDM (presumably through competition) in those habitats during late establishment phases and was responsible, at least partially, for changes in habitat suitability across the recruitment phases. Synthesis. Our results show that assessment of the DrD hypothesis depends on the context of the study (in terms of recruitment phases) since sites of enhanced seed arrival and high seed-to-seedling survival can ultimately prove unfavourable to the plant if density-dependent or other effects might increase mortality in these sites during later recruitment phases.