Ants and birds help to spread out seeds by acting as gardeners

When you decide that you want to live in another place, you just pack and go. This may sound simple for animals, but it is not a feasible task for plants. For the large majority of plant species, individuals establish themselves early in lifetime (in the form of seeds) in definite sites. Their chance of moving out ends up. This means that before settling down, seeds have to count on environmental agents to give them a ride to places far away from where their mother plant produced them. But, from a seed’s point of view, what is the advantage of departing? Scientists have found that seeds removed from their birthplace have higher chances of escaping from being eaten by rodents. This is because the floor around an adult fruiting plant where many fruits and seeds fall resembles an enormous feast in which hungry rodents wish to take a seat.

Which are the agents that could act as seed movers in this process named seed dispersal? Birds and ants are included in the group of animals able to do that. In a recently published paper researchers described their observations and experiments involving birds and ants as seed dispersers of a plant species. Each animal group took part in separate occasions: birds in the branches of the trees and ants on the floor. The researchers recorded 12 bird species going to the trees and swallowing the whole fruit, which meant that the birds carried the seeds in their stomach. They would defecate the seeds thereafter because seeds consist of an inconvenient and non-nutritious load. However, it is impossible to follow the observed birds to know what will happen to the seeds they discharge. The researchers then attracted ants to the scene by simulating a bird dropping. They fed captive birds with fruits removed from the trees, collected the bird feces with seeds inside and placed the feces on the forest floor. To avoid having the seeds removed by animals other than ants, the researchers covered the feces with metal cages staked to ground. The cages were covered with a wire mesh that allowed nothing but small insects such as ants to go in and out. When checking the droppings, the researchers noted that 62% of the seeds were removed by five ant species. But, if the ants carried the seeds to their colonies, would these seeds become a plant? To answer this question, the researchers compared the number of young plants (seedlings) growing on the soil of 23 colonies with the number of seedlings growing on small areas without ant colonies. There were seedlings growing only on ant colonies. However, although living at their new home, these seedlings are vulnerable to death caused by low rainfall or grasshopper attack. It was then asked: how many seedlings would be still alive one year after the seeds had reached the spots? If an ant colony is a good place to live on, more seedlings are expected to be alive on it than elsewhere. The researchers searched for and located seedlings that were growing far from any ant colony to compare their survival rate with that of the colony’s residents. While 83% of the young plants growing on nests were alive, only 63% of those growing far away from colonies survived.

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This study showed that birds give the seeds a chance of escaping from mortality by carrying them to an apparently less dangerous neighborhood. The group of ant species that removed seeds from birds’ droppings is known to pose difficulties to insects trying to feed on seedlings growing around their colonies. Therefore, these ants may be greatly responsible for the survival of the young plants born of the ant-rescued seeds after birds have moved them away from the mother plant. Will at least some of the young plants be a reproductive adult one day? This is quite difficult to test. At least for while we know that birds and ants take turns as gardeners. By doing their job, they help to renew the plant population and ultimately contribute to keep the forest alive.

By Milene Martins.

Account based on the scientific article: Camargo P.H.S.A; Martins M.M.; Feitosa R.M.; Christianini A.V. 2016. Bird and ant synergy increases the seed dispersal effectiveness of an ornithochoric shrub. Oecologia 181(2): 507-518.

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