Ecology: Seed dispersal by migratory birds limits the response of plants to climate change

Migratory birds have limited potential to help European fleshy-fruited plant species to respond to global warming through the long-distance dispersal of their seeds towards cooler, northern latitudes, suggests a study published in Nature this week. Only 35% of the studied plant species are expected to benefit from such dispersal. Seed dispersal by migrating birds has the potential to influence plant adaptations to climate change.

To track the rapid pace of climate change, plants must disperse their seeds across long distances towards cooler latitudes, often tens of kilometres away. Migrations of fruit-eating birds could help to provide this all-important, long-distance seed dispersal service. The direction of dispersal is determined by whether the timing of fruit production coincides with southward or northward migrations, but this relationship has received limited investigation.

Juan Pedro González-Varo and colleagues studied 13 seed-dispersal networks comprising 949 interactions between 46 fruit-eating bird species and 81 fleshy-fruited plant species from European woodland communities. Data on fruiting periods and migratory patterns were recorded. Analysis revealed that for most plant species (86%), birds are dispersing their seeds towards drier and hotter regions as they migrate southwards, towards the equator, in the autumn. By contrast, spring migrations towards northern, cooler latitudes are responsible for the dispersal of seeds from just 35% of plants, such as species of juniper (Juniperus) and ivy (Hedera). The dispersal of this latter subset of plants — which are characterized by long or late fruiting periods that advantageously fall within the same time window as early spring — is closely related to northward bird migrations, which are mainly conducted by only a handful of common bird species that winter in central and southern Europe. This collection of birds notably includes the blackcap (Sylvia atricapilla) and the blackbird (Turdus merula).

The current study suggests that divergent seed dispersal by birds in Europe limits the potential of plants to establish new communities in climatic conditions that they can tolerate. Future research should explore the generalizability of these findings to other plant–bird systems — such as aquatic plant dispersal by waterbirds — and whether climate-driven changes in plant or bird behaviour could affect this situation.