In a new article, researchers from Aarhus University
describe how the waste left by ants on plant leaves serves as a valuable
fertiliser for the plants -- handed on a silver platter.
You have often seen ants wandering about on leaves
-- even in tall trees. In fact, it is the plants themselves that attract them
by secreting sugar-containing nectar, which the ants eat with great pleasure.
And on their journey around trunks and leaves, the ants snap insects that could
otherwise damage the plants.
This has been known for many years and Danish
researchers now use this knowledge in the battle against harmful insects in
organic apple orchards. They simply move wood ants from the forest and create
new anthills in the orchards.
Now researchers have found yet another positive
effect of the ants' visit to the trees. Their urine or faeces, excreted
together, contain amino acids and urea -- substances that are commercially used
to spray on leaves to fertilise the plants.
A small coffee plantation in the laboratory
In tropical areas, there are many different species
of ants that live exclusively in the tree crowns. They do not come down to the
ground and therefore cannot get nutrition there. This applies, for example, to
weaver ants that live in the crowns of many different trees and bushes --
including coffee trees. Each tree can have up to 60,000 ants.
In the laboratory, the researchers built a
mini-coffee plantation with several individual coffee trees. The central coffee
tree held a colony of weaver ants. All the coffee trees were placed in water,
so the ants could not move from tree to tree unless there was a bridge to take
them across. Accordingly, the researchers built suspension bridges between
some, but not all, of the trees.
On the central tree, the ants were fed with an amino
acid -- glycine -- where the nitrogen atom consisted of the heavier nitrogen 15
(15N). The researchers were able to follow the labeled nitrogen in the
neighbouring trees to which the ants walked over via the suspension bridges.
And the results were quite amazing.
Intravenous nutritional supplement
First, the researchers observed that the visited
trees had a higher content of nitrogen than the trees to which the ants did not
have access. The trees visited by the ants also had larger crowns than the
trees without ants.
On the 'visit trees' some of the leaves were wrapped
so that the ants could not leave their waste here. But also in these leaves,
the researchers were able to trace the labeled nitrogen.
"For the first time, we have shown that
nutrients from ant waste are taken up by the leaves and transported to other
places in the tree," says senior scientist Joachim Offenberg, Department
of Bioscience, Aarhus University, who was in charge of the studies.
"This has great ecological importance. The
ants, which primarily feed on insects in the trees, digest the insects and hand
the nutrients on a silver platter to the plants. You can almost say that the
plants receive the nutrition intravenously exactly where they need it,"
explains Joachim Offenberg.
Great ecological importance
The ants appear frequently on new shoots and on
fruits -- both areas of the plant that can benefit from an additional nutrient
input.
The nutritional supplement to the leaves can be a
great advantage for many different plants, and the researchers will now
investigate how widespread the phenomenon is.
"We know that globally there are lots of plants
inhabited by ants. The nutritional supplement for their leaves can have a major
ecological significance and may also have been decisive for the evolution of
ant-plant interactions," says Joachim Offenberg.