Human activities linked to declines of big seeds

(Press-News.org) Seeds in Madagascar’s forests are getting smaller, and new research published in Ecology Letters suggests that human activities are playing a role in this shift. Researchers from the German Centre for Integrative Biodiversity Research (iDiv) and Leipzig University combined data from more than 2,800 plant species, 48 living and 15 extinct fruit-eating animals (frugivores) – including birds and lemurs. The results show that both past human-driven frugivore extinctions and current human pressures are shaping seed size across the island.

The researchers found that increased human footprint (a cumulative index of human pressure) is associated with smaller maximum seed sizes in plant communities, both directly – for example through selective logging of large-seeded trees – and indirectly, by depleting populations of large frugivores that would otherwise disperse them.

Frugivore downsizing leaves ecological ghosts behind

Because seed size and frugivore body size are linked – larger animals can swallow and disperse larger seeds – a loss of large animals can lead to the loss of large seeds, and hence seed downsizing. “Frugivore downsizing has been particularly severe in Madagascar, where all animals with a body mass over 10 kg (megafauna) rapidly went extinct around 1,000 years ago,” says Yuanshu Pu, doctoral researcher at iDiv and Leipzig University and first author of the study. “The loss of large frugivores can lead to secondary extinctions of species with large seeds. Alternatively, large-seeded species can evolve smaller seeds to adapt to the remaining frugivore community.”

However, the study also shows that several Madagascan plant species, such as Borassus madagascariensis and Tsebona macrantha, produce seeds too large for any living frugivore on the island to disperse. These “ecological ghosts” appear to reflect interactions with now-extinct giant lemurs and other megafrugivores. “It remains unclear how such plants have persisted, but they may have found alternative dispersers, such as introduced zebu cattle, bushpigs, humans, or water,” says Yuanshu Pu.

Seed size matters

Large seeds are often found in tree species that grow slowly and live long, sometimes playing important roles in carbon storage. But large trees are particularly vulnerable to human activities such as logging – and the slow growth and recruitment rates of large trees with large seeds further constrain their recovery.

Seed size is linked to many key plant traits, such as tree height, wood density, and lifespan. These characteristics shape forest structure and underpin important ecological functions such as potential carbon storage. Shifts toward smaller seeds could therefore further alter forest structure and ecosystem function.

“The futures of plants and the animals that disperse them are intertwined. Protecting Madagascar’s enigmatic lemurs – many of which eat the fruit and disperse the seeds of plants – is therefore vital not only for animal conservation, but also for maintaining a diversity of large-seeded food plants and the critical ecological functions they support,” says senior author Dr Renske Onstein, junior group leader at iDiv and head of the Biodiversity Hotspots research group at Naturalis Biodiversity Center.

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