Cybele

**Ant-Plant Interaction: from Mutualistic to Antagonist Interaction**

flat = **Introduction** =

Ants have been known for their social organizations and skills in the utilization of their environment. Their interaction with plants has not been studied extensively. However, in recent years, it has been shown that plants can have both beneficial and antagonistic interactions with plants.

An example of a beneficial interaction is the relationship between ants and the acacia trees in the African savanna and in South America. This interaction is mutualistic and ants provide the host plant with protection while the plant provides shelter and nutrients. A case of antagonistic interaction can be found in the Amazonian forest where a species of ant can inhibit the growth of plants. Both of these interactions are significant to the ecosystem as they dramatically change the habitat.

= **Organisms involved** =

We will look at two different interactions between trees and ants. For the first interaction, the plant involved is the acacia tree //Acacia drepanolobium//, commonly known as the whistling-thorn tree. It is named so due to the fact that the tree grows swollen and hollow thorns at its branch nodes. This species of acacia tree also can produce extra-floral nectar from structures known as nectaries. It grows in the savanna of North East Africa. Interacting with the whistling-thorn trees are four different species of ants: //Crematogaster mimosae//, //C.sjostedti//, //C.nigriceps// and //Tetraponera penzigi//. These four species of ants are typically found in Kenya, around the same areas as the whistling-thorn tree.   The second interaction occurs between the tree //Duroia hirsuta// and the specific ant species of //Myrmelachista schumanni//. //D.hirsuta// present swollen stems called domatia. The tree often grows within mono-species portions of land in the Peruvian rainforest. The areas where only //D.hirsuta// grow are called the ‘Devil’s Gardens’ by locals. Legend has it that these trees are cultivated by evil spirits, because they contrast so heavily with the biodiversity of the Amazonian forest. //M.schumanni// is commonly known as the lemon ant. Like many other ant species, they have formic acid inside their abdomen which can be used as a chemical weapon. This species of ant can be found all around the Amazonian forest and notably inside the Devil’s Gardens. 

= **Biological Interaction** =

In both ant-plant interactions, the plant provides rewards to the ants while the ants help protect the tree from predators (large herbivores). However when we look at them more closely, the interactions are far more different than what we would expect.

In our first interaction, between the acacia tree and the four different species of ants, the interaction is slightly different between the whistling thorn tree and each species of ants. This is a purely mutualistic relationship. //C.mimosae// use the swollen thorns of the tree as nurseries and they also consume the nectar produced by the tree as their main source of carbohydrates. //C. mimosae// is the species that interacts most closely with the acacia tree. In exchange for food and shelter, the ants fearlessly defend the tree against predation. When a branch is attacked by an herbivore, ants will gather and attack the predator. //C.sjostedti// also uses nectar as a source of carbohydrates but does not take advantage of the thorns. It defends the tree against herbivores but not as aggressively as //C mimosae//. //C.nigriceps// also enjoys the rewards from the tree and kills the apical meristem of //A.drepanolobium// which allows for larger lateral growth of the tree. Finally, //T.penzigi// uses the thorns of the tree but destroys all the nectaries that it does not use as a food source. This is done to prevent other ant species from colonizing the tree.

In our second interaction, the relationship between the ant and the plant starts when an //M.shumanni// ant queen decides to colonize a //D.hirsuta// tree. Similar to the previous interaction, the ant will use the swollen stem as shelter and also protect the tree against herbivory. In addition, the ants from the newly formed colony will use a specific mechanism to inhibit the growth of all other plant species around //D.hirsuta//. They do so by biting on the leaves of non- //D.hirsuta// plants, then injecting formic acid into the newly formed lesions. This action acts as an herbicide inducing rapid necrosis of the targeted plant. Even though formic acid is present in the abdomen of many ant species, this is the only instance in which the chemical is used as an herbicide by the ant. With time, only //D.hirsuta// trees will grow around the originally colonized tree and the colony of ants will expand. This particular ant species sets itself aside from other ants on a social organization perspective. Usually, there is only one queen for one colony. However for //M//.//shumanni//, there can be several queens in one colony and this is what contributes to the large size and longevity of the Devil’s Gardens. These interactions are extremely important for the success of //D.hirsuta// as it can become the dominant species in areas where it would be hard to expand otherwise.

= **Impact/Importance** =

These interactions between plants and ants directly impact the ecosystem. In our first interaction between ants and the acacia trees, it has been shown that there is an increase in tree growth and longevity, which of course has a direct impact on the species living around it, predators and humans included. Acacia has long been used by Africans for their ornamental value. Also, the sieve is sometimes used as gum by locals. Acacia trees produce tannins which have medicinal properties that can be used as a cure for rabies. For all the reasons above, acacia trees in general carry economic and cultural values. The importance of our second interaction resides more in an ecological perspective. In the Amazonian forest, the proximity of all species makes for a fragile balance; everything is interconnected. If a Devil’s Garden develops in an area, then the fauna, in addition to the flora in this area, will be greatly disturbed.

=Current Research=

Research has been done on both interactions in recent years. A study has been performed by Palmer et al. in 2008 on the importance of large herbivores in the acacia-ants interaction. The researchers excluded large herbivores from certain areas of the savanna. They then studied the impact of this exclusion on the ant-plant interaction. The results were surprising, showing that this removal had negative consequences for the trees. The explanation for this result lies in the fact that herbivore browsing in part drives the mutualistic relationship between the ant species and the whistling thorn tree. When an herbivore feeds on a tree, it stimulates the production of nectaries and swollen thorns by the tree. This production is essential to the dominance of //C.mimosae// on acacia trees as it is the species that will not only protect the tree the best but also promote its growth. If the herbivores are absent, there will be a shift in ant dominance and the tree will not grow as well.

The first hypothesis that scientists came up with to explain the Devil’s Gardens was allelopathy. Allelopathy is when a species of plant secretes chemicals into the environment to inhibit the growth of other plant species. However, this theory was refuted by Frederickson in 2005 when she proved that the gardens were due to the ants’ actions. The same group of researcher came back to Peru in 2007 to study the effect of Devil’s Gardens on herbivory. This research was done with the overall aim of explaining why the Devil’s gardens were not taking over the whole Amazonian forest. The results showed that herbivory increases proportionally to the number of trees in a Devil’s Garden. Therefore, the size of a Devil’s garden is limited by higher predation. Many questions have yet to be answered about the Devil’s Gardens. For example, how does the //M.shumanni// discriminates between //D.hirsuta// and other plant species? Or, why does herbivory increase in larger Devil’s Gardens?

=References=

· Todd M. Palmer, Maureen L. Stanton, Truman P. Young, Jacob R. Goheen, Robert M. Pringle, Richard Karban. 2008. Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna. //Science//. 319: 192-195.   · M.Heil, J.Rattke, W.Boland. 2005. Postsecretory Hydrolysis of Nectar Sucrose and Specialization in Ant/Plant Mutualism. Science. 308: 560-563.   · //Young//, T.P., //Stubblefield//, C.H., //Isbell//, L.A. //1997//. Ants on swollen-thorn acacias: species coexistence in a simple system. Oecologia 109: 98-107. · Megan E. Frederickson, Michael J. Greene, Deborah M. Gordon. 2005. //‘Devil’s Gardens’ bedevilled by ants//. Nature Brief communications. 437|22: 495-496.   · Megan E. Frederickson and Deborah M. Gordon. 2007. //The devil to pay: a cost of mutualism with// Myrmelachista schumanni //ants in ‘devil’s gardens’ is increased herbivory on// Duroia hirsuta //trees//. Nature. 274: 1117-1123.

Page authored by Cybele Arsan, student of Dr. Michelle Lum at Loyola Marymount University, Los Angeles.