Corvids as ecosystem engineers

A new review of how jays and crows distribute plant seeds around the world has highlighted how essential they are for the survival and spread of forests.

Though it has been known for a long time how jays spread acorns and inadvertently create new oak forests, a new review paper has underlined how necessary many corvid species are for the maintenance of the world's depleted forests.

The review in The Condor: Ornithological Applications explores how oaks and pines depend on the crow family, including ravens, crows and jays, to reproduce and spread, and how birds may be the key to helping these valuable trees weather the challenges of habitat fragmentation and climate change.

Corvids store seeds in small caches spread across the landscape, a behaviour termed 'scatter-hoarding'. Birds cache more seeds than they eventually eat, so some seeds are able to sprout, and thus scatter-hoarding becomes seed dispersal, helping trees colonise new areas. Many oak and pine species have specific adaptations to encourage dispersal by birds, producing large, nutritious seeds with protective chemicals that prevent them from rotting, in turn encouraging scatter-hoarding behaviour by eliminating the need for animals to eat the seeds immediately.

In Europe, Eurasian Jays are proving to be a crucial ally for oaks as habitat fragmentation and climate change increasingly impact European hardwoods. In the western USA, researchers have shown that repeated long-distance dispersal events by Clark's Nutcracker are essential to establish and maintain Ponderosa Pine populations, and that Pinyon Jays help maintain the same tree's genetic diversity. In the eastern USA, Blue Jays speed forest fire recovery by increasing their caching effort after fires and selecting canopy gaps as cache sites.

In Europe, Eurasian Jays are proving to be a crucial ally for oaks as habitat fragmentation and climate change increasingly impact European hardwoods (Photo: Jamie MacArthur)

Harnessing this behaviour may aid habitat restoration. Europeans have been aware of the relationship between Jays and oaks for centuries, and managers in some areas of Western Europe are planting small stands of seed-source trees and relying on corvids to help disperse them across the landscape. In America, conservationists are exploring the possibility of reintroducing Island Scrub-Jays to islands where they were extirpated, to speed the recovery of oak and pine vegetation after livestock removal.

"In light of the globally changing climate and increasing habitat fragmentation, these winged dispersers that transport seeds over long distances are likely to become more important, as they enable plant populations to shift their range," said Mario Pesendorfer, lead author of the study. "Since oaks and pines are important keystone species that themselves provide habitat for hundreds of animal species, such dispersal can have ecosystem-wide benefits."