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RESEARCH PRODUCT

Short seed-dispersal distances and low seedling recruitment in farmland populations of bird-dispersed cherry trees

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Summary In Central Europe, many plant populations are patchily distributed in human-modified landscapes and depend on animal vectors for seed dispersal. To predict seed-dispersal distances and locations of seeds of wild cherry trees (Prunus avium L.) in forest and farmland habitats in a human-modified landscape, we integrate movement data and seed regurgitation times of the Common Blackbird (Turdus merula L.) in a simulation model. We performed feeding trials with Common Blackbirds and wild cherries to determine the distribution of regurgitation times. We captured 32 male blackbirds and equipped them with radio tags to follow their movements in forest and farmland habitats. To simulate the movement of cherry seeds through the landscape, we combined the distribution of regurgitation times with bird movement data and modelled seed-dispersal distances and locations of seed deposition for forest and farmland birds. According to our simulations, more cherry seeds were deposited under foraging trees in farmland (20.8%) than in forest populations (9.5%). Median seed-dispersal distances (50.8 vs. 68.2 m) and the proportion of long-distance dispersal events (distances > 100 m) were predicted to be lower in farmland (14.9%) than in forest populations (28.2%). In the model, forest-dwelling blackbirds dispersed more cherry seeds into suitable habitat (98.7%) than farmland blackbirds (85.4%). In both habitats, seed deposition in suitable habitat was much higher than expected from the proportion of suitable habitat within blackbirds' home ranges, indicating directed dispersal. To test whether differences in seed-dispersal locations were related to recruitment success, we recorded seedling densities in farmland and forest populations of P. avium and determined survival probabilities of seedlings. Seedling densities and survival were much lower in farmland than in forest populations, even accounting for strong environmental effects on seedling recruitment. Synthesis: Our findings show that behavioural differences of animal seed dispersers between habitat types can result in substantial changes in seed-dispersal distances and locations in human-modified habitats. These changes in seed-dispersal services for bird-dispersed plant species may be related to reduced seedling recruitment in farmland populations making such populations prone to extinction in the long term.