Seed distribution by birds can be too much of a good thing

Birds can help plants spread their range by carrying seeds away, but how helpful are they? Majorie Sorensen and colleagues examined how birds interacted with Swiss Stone Pine (Pinus cembra). The seeds are carried away by the Spotted Nutcracker (Nucifraga caryocatactes) and cached. The team examined where these caches were to see how they could benefit the pines. However, they found the most distant caches outside known pine habitats. The discovery explains how pines can migrate to new locations.

The authors found that the birds often carried seeds over 5 miles (8 kilometres) away. This distance would be beneficial for colonising new sites, but there was another factor in the scatter-hoarding, said Sorensen and colleagues. “In our study, sites chosen for seed caching were located at low elevations outside the elevational range of the pine. In another scatter-hoarding system, European jays transport seeds from oak to pine stands which is considered beneficial for the viability of oak populations and the diversity of overall forest structure. We find a similar pattern, dispersal between areas dominated by different tree species, however, seed dispersal to spruce forests is likely not very beneficial for the pine, as pine is outcompeted by spruce at its lower elevational range edge.”

Scatter-hoarding birds are particularly helpful for studying seed dispersal, as they intentionally deposit seeds in stores that scientists can identify later. The ecologists used mist nets to catch Nutcrackers and then gave them GPS tags. The study used 5-gramme backpacks strapped to 20 nutcrackers, each weighing over 180 grammes, so the backpacks weren’t a huge encumbrance. These tags allowed the team to track the birds visiting pines around Davos, Switzerland. Examining the results revealed where the caches were. 

Following the birds, the ecologists found that the nutcrackers were selecting sites best for storage rather than germination. That means that the nutcrackers were not always helping disperse the pines, but Sorensen and colleagues say this might not always be the case.

“Although our results suggest that effective long-distance seed dispersal in the spotted nutcracker and Swiss stone pine system does not occur en masse in a typical year, masting years and rare seed dispersal events may result in sufficiently effective seed dispersal for maintaining population viability. Masting years, cyclical and spatially synchronized bumper crops, may cause consumer populations to be overwhelmed resulting in a larger number of unrecovered seeds that survive until reproduction.”

“Rare seed dispersal events may also make up for the unfavorable caching sites observed in this study. Rare events, occurring in less than 1% of seed dispersal events, may be sufficient to maintain genetic links between subpopulations and to allow for regeneration of pine populations. It has been shown previously that rare long-distance dispersal events may be disproportionately important for plant fitness and migration rates.”

READ THE ARTICLE

Sorensen, M.C., Mueller, T., Donoso, I., Graf, V., Merges, D., Vanoni, M., Fiedler, W. and Neuschulz, E.L. (2022) “Scatter-hoarding birds disperse seeds to sites unfavorable for plant regeneration,” Movement Ecology, 10(1), p. 38. https://doi.org/10.1186/s40462-022-00338-1