The way birds hoard seeds for winter helps forests regenerate, a new paper has argued. Crows and jays are ecosystem engineers, allowing trees to colonize territory made newly habitable by climate change or retake areas destroyed by fire or clearfelling.
Corvids, the bird group that includes crows, ravens and jays, are known to be exceptionally smart. Their most important contribution to the environment may not rely on their astonishing tool use, however, but on their memories, and sometimes lack thereof.
Wisely, corvids don't put all their winter nuts in one basket. Instead, they spread them in small caches, requiring an impressive ability to remember where the food has been stored. Sometimes, however, the birds forget and the seeds sprout in the spring. This behavior, known as scatter-hoarding, allows trees to spread their seed to places wind or rodents could not take them.
Birds make better seed dispersers than rodents because they skip barren areas, rather than being stopped by them. Pesendorfer et al.
Many trees produce particularly nutritious seeds to encourage corvids to disperse them in this way. PhD student Mario Pesendorfer of the University of Nebraska-Lincoln has examined the contribution this can make when forests have a chance for unusual expansion or recovery.
The corvids' role is comparable to the one played by large fruit-eaters in tropical forests. However, in temperate forests large trees such as oaks produce bird-sized seeds, making them reliant on species that are less vulnerable to hunting pressure by humans than the fruit-eaters of the tropics. Therefore, these temperate species are much more suited to survive the Anthropocene.
In the journal The Condor: Ornithological Applications, Pesendorfer provides a review of numerous studies of corvid contributions worldwide. He reports that some birds disperse 5,000 nuts a year over distances of many kilometers and others enhance the chances of acorns germinating. This allowed oak and beech forests to move rapidly north at the end of the last Ice Age.
The capacity to disperse seeds to new areas will "likely become even more important for conservation of oak and pine ecosystems as suitable climates shift rapidly in the decades ahead," the paper argues.
Corvids play a role in forest regrowth and expansion over a huge area of the planet. Pesendorfer et al.
"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," Pesendorfer said in a statement. "Since oaks and pines are important keystone species that themselves provide habitat for hundreds of animal species, such dispersal can have ecosystem-wide benefits."
Consideration is being given to reintroducing birds to areas where they are locally extinct so they can assist with re-vegetation, but the paper notes, "differences in habitat composition and usage patterns can lead to differential seed deposition," indicating that thought needs to be given to the species best suited to the role required.
"Seed selectivity, transportation distance, hoarding frequency, and cache placement affect seed dispersal effectiveness, a measure of quantity and quality of dispersal," the paper adds, making it important to choose the most appropriate corvid species to encourage.