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| Stem cavity of the Ecuador laurel tree |
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| Azteca ant detects a caterpillar on a leaf of the Ecuador laurel tree. |
The Ecuador
laurel tree, Cordia alliodora,
developed an interesting and effective method to protect itself from herbivore
predators. During the dry season the tree has two main threats, lack of water
and herbivore consumers, so the tree devised a defense strategy to mitigate the
lost of leaves to predators. The Ecuador laurel tree created a mutualistic
relationship with Azteca pittieri ants that in exchange for living in the tree
offer protection against herbivores.
The ants nest in the stem cavities
of the tree and feed on the pink scale insects that also live within the tree.
The pink scale insects feed on the sap secreted by the tree and produce a
sugary substance called honeydew that serve as the ant’s food source. The ants are small in size compared to other
herbivores, but they work in groups to attack predators. For example, they will
surround a leaf-eating caterpillar and bite its underside until it falls off
the leaf.
This relationship guarantees the
tree protection, but it comes at an ecological cost. The pink scale insects
consume the tree’s sap, so the tree is constantly using its energy reserves to
produce more sap. It is arguable that the tree would seem better off to hoard
its sap during the dry season because if the sap supply is depleted, then the
tree will die. Furthermore, even if the tree did not perish, a low supply of
carbon would be detrimental for the rainy season. This relationship is unusual and requires a delicate balance for survival.
Researchers wanted to investigate
this relationship further, so they observed the tree and ant behavior in 26
sites, all with varying precipitation levels. They reported that in the drier
areas the tree and ant relationship was stronger than in the wetter regions. It
was also observed that in the drier sites there were more pink scale insects, thus supporting a larger ant population. The researchers observed that the ants in
the drier regions were much better defenders, with quicker reaction times and
defense movements, than in the wetter regions.
It can be gathered that the ants
serve as “tree insurance” because the cost of herbivory is high. The cost of
sustaining the ants is cheaper than replacing leaves that herbivores eat. The
tree would have to replace the leaves each time with energy from the sun. The
process of growing a new leaf is not practical, so the tree invests in defense.
Although this relationship is peculiar and defies common ecological trends, it
has led to the success of the Ecuador laurel tree.
Photo Credit to Science News
Zielinski, Sarah. In dry times, these trees invest in ants. Science News: Magazine of the Society for
Science and the Public. Science News, 7 Nov. 2013. Web. 9 Nov. 2013.
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