The Hawaiian crow, one of only two fruit-eating bird species left on the ‘Big Island’ of Hawaii, may play a vital role in the restoration of native plant species by helping to disperse their seeds more efficiently.
One of the major pains about being a plant is that you are fixed to one spot for your entire life. To overcome this problem, many plants rely on birds and other animals to carry out essential tasks for them, such as pollination and, most importantly, seed dispersal. But what happens to these plants when many of the bird species that carry out these crucial roles become extinct? This is the problem facing forest communities in Hawaii, where most of the native bird species have now vanished from the wild.
Oceanic islands such as Hawaii usually don’t have any native mammal species, as it is difficult for flightless animals to reach and colonise isolated areas on their own. But birds can, and they have diversified in places such as New Zealand and the Galápagos to fill the roles normally occupied by mammals on the continents. Because of this, birds play an extremely important role in maintaining island ecosystems by carrying out important functions such as seed dispersal for plants.
Since the arrival of humans to Hawaii, however, around half of the bird species native to the islands have disappeared completely. This is largely due to the introduction of numerous alien plant and animal species, as well as the large-scale changes in the landscape that have occurred over the years. Similarly, many Hawaiian plants are currently on the IUCN Red List for endangered species and only survive due to conservation efforts.
The Hawaiian Crow (Corvus hawaiiensis), or ‘Alalā, is one of only two fruit-eating birds that still survives on the Big Island of Hawaii. The last sighting of a wild Hawaiian Crow was in 2002. Today, the remaining population amounts to less than 100 individuals and exists only in captivity. Undoubtedly, these birds were once an important seed disperser for many native plants in Hawaii, but most information on their behaviour comes from short notes made by naturalists in the early 1900s. Now, however, research published in Ecological Applications has shown that the crow may play a significant role in facilitating the restoration of several rare forest plants on Hawaii.
Researchers collected fruits from 11 native Hawaiian plant species and offered them to a study group of 57 Hawaiian Crows. They then monitored the eating behaviour of the crows and recovered seeds from the bird droppings and pellets whenever possible. These seeds were then planted to see how well they would germinate in comparison with seeds that had not been eaten. Seeds often benefit from passing through the digestive system of an animal, as this helps to break the tough seed coat and improves its ability to sprout. They found that the birds ate all of the various fruits that were offered to them, including the fruit of two endangered species, loulu (Pritchardia schattaueri) which is a species of palm tree, and halapepe (Pleomele hawaiiensis) which is a tree-like plant of the agave family. Neither of these species have a known seed disperser, but it seems likely that they relied on the Hawaiian Crown in the past to disperse their seeds.
Two other plant species also benefited greatly from their seeds being ingested by the crows. Far more seeds of both ‘oha kēpau (Clermontia hawaiiensis) and hō‘awa (Pittosporum hosmeri) sprouted after being consumed in comparison with those that remained uneaten. Passage through the crows’ digestive system appears to have made it easier for the seeds to subsequently germinate. Both these plant species have large fruits, but no known native seed dispersers. Hō‘awa may be particularly reliant on the Hawaiian Crow. The fruit of this plant consists of a tough woody capsule which needs to be removed before the seeds inside can germinate, and the crow is particularly adept at opening these capsules. Currently, there are very few hō‘awa seedlings or saplings in the wild; the study suggests that this most likely due to the loss of the Hawaiian Crow, their primary seed disperser.
It is crucial for many plants to disperse their seeds as widely as possible. This helps to reduce competition with adult plants for fundamental resources such as light, nutrients and water. It also enables the plants to potentially colonise new areas that are more favourable for growth and development. In order to reach these distant locations, plants rely on animals to transport their seeds for them, particularly if the seeds are too heavy to be carried by the wind. This ensures that the next generation of plants have a greater chance of survival than they otherwise would.
The persistence of small numbers of these endangered Hawaiian plants in the wild despite the loss of important seed dispersers is a curiosity. However, it is known that native Hawaiian people have historically used material from these plants for a variety of purposes, including for medicine and to make parts for canoes, houses and weapons. So even though the bird population has declined, the seeds of the plants may still be dispersed, albeit indirectly, through human activities. Regardless, it now seems unlikely that these plant species will be able to survive for much longer in the wild without human intervention. The authors hope that the reintroduction of the Hawaiian Crow to Hawaiian forests will help to contribute to the restoration of these endangered plant species. However, such a project will be extremely challenging to say the least and will require concerted efforts from all involved.
Reference: Culliney S, Pejchar L, Switzer R, & Ruiz-Gutierrez V (2012). Seed dispersal by a captive corvid: the role of the ‘Alalā (Corvus hawaiiensis) in shaping Hawai’i’s plant communities. Ecological Applications, 22 (6), 1718-1732 PMID: 23092010