Investigating the efficacy of defence offered by obligate and cheat ant species.

Written by Lara Jackson.
Research conducted by Lara Jackson, Ruth Law, Natalie Porter-Bird and Luke Vassor.The Tropical Ecology Field Trip | University of Southampton

As a child, I was absolutely fascinated by stories of the rainforest. Of the sprawling trees that grew to heights beyond my imagination, of the colourful birds that flew above the canopy, of the elusive jungle cats that stalked their prey through the undergrowth, and of the people, whose lives were so different to my own. So, when the opportunity arose to conduct a small research project in the Belizean Rainforest, I jumped at the chance!

Tropical Rainforests cover less than 2% of the Earth’s surface, and yet they boast an astonishing 50% of all terrestrial life. These unprecedented levels of biodiversity and the ability of organisms to fill new and unique niches have resulted in a number of unusual interactions that occur almost exclusively in the tropics. One example is an obligate mutualistic relationship – a biological interaction unique to the tropics that describes the high dependency between two species that require each other for survival.

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Rainforest in Belize – By Heath Alseike, Flikr.

In Belize, a well-documented example of this relationship occurs between a group of trees called Acacias, and ants of the Pseudomyrmex genus. The Acacia tree is inhabited by a colony of aggressive ants that defend the plant against damage from other herbivores and encroaching vines. In return, the ants acquire a protected nesting space within the hollow spines of the Acacia tree and gain nutritional benefits from modified leaf tiplets called beltian bodies. These beltian bodies nourish the larvae by providing protein, carbohydrates and lipids.

This interaction may seem unusual, but over 1/3 of the world’s plants employ ants as a defence mechanism! Such interactions are abundant in tropical ecosystems and the degree of association can vary from a loose connection (the relationship is beneficial, but the species do not require each other for survival) to an obligate mutualism.

In the area surrounding our research station in Belize, Acacia gentlei was the dominant species. It has evolved an obligate symbiosis with Pseudomyrmex ferruginea, an aggressive species of ant that forms single-queen colonies reaching numbers of 15,000.

However, the relationship between ants and acacias is horizontally transmitted, meaning that a time lag occurs between the growth of the tree and the establishment of an ant colony within it. This leaves a window of opportunity where the mutualism is vulnerable to cheaters and may be invaded by an exploitative species that reap the rewards offered by the host, whilst offering little or no protection in return.

A. gentlei may be invaded by a ‘cheat species’ (Pseudomyrmex gracilis) that excludes the obligate mutualist (P. ferruginea) and destabilises the symbiotic relationship. Therefore, our study investigated the difference in ant-plant defence offered by the protective P. ferruginea and the exploitative P. gracilis, by measuring the frequency of ant attack to phytophagous herbivores.

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Pseudomyrmex ferruginea – By Ryan Somma, Wikimedia Commons.

We used proxy caterpillars made of a non-toxic modelling clay to assess the efficacy of defence. Whilst lacking the characteristics of a live caterpillar, previous studies have shown that models suffer similar levels of attack and therefore, offer a comparable measure of defence.

In total, 800 model caterpillars were placed on 80 trees. Forty A. gentlei trees were randomly selected and 10 model caterpillars were uniformly distributed on the petiole of the leaves. As a control, forty Acacia polyphylla trees (that lack a mutualism with ants but share morphological characteristics with A. gentlei) were selected and 10 caterpillars were uniformly distributed.

Trees standing more than 0.5m in height were used to control for ground-dwelling herbivores and the species of ant (P. ferruginea or P. gracilis) inhabiting A. gentlei was recorded. After 48 hours, the caterpillars were examined for evidence of bite marks left by the ants (figure 1 and 2) and a percentage frequency of defence by ants on the model caterpillars was calculated.

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The caterpillars placed on the control tree, A. polyphylla, experienced a ‘background’ defence frequency of 10.6%. In comparison, the defence frequency experienced by the model caterpillars on A. gentlei was recorded at 48.9%, corroborating the consensus that mutualistic relationships between ants and plants are strong and highly effective (figure 3).  

The two species of ants inhabited A. gentlei independently of each other. P. ferruignea (the obligate mutualist) exhibited a frequency of defence of 60.2% which was significantly higher than P. gracilis (the cheat species) at 30.1% (figure 4). Thus, confirming that P. gracilis forms a mutualism that is less effective to A. gentlei as they offer the host reduced protection.

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This study demonstrated that ant-acacia mutualisms are highly effective and that P. ferruginea successfully defends their host from the threats posed by other herbivores and encroaching vines. In fact, without occupants, Acacias are subject to severe defoliation from phytophagous herbivores and insects as they lack other forms of natural defence like alkaloids and toxins. Furthermore, without the ants to counteract the competitive nature of other vegetation and maintain a column of plant-free growing space, the acacia is used as a support by intruding vines and is often over-topped and killed.

The presence of P. gracilis can destabilise an effective symbiosis because they actively exclude the obligate species (P. ferruginea) from inhabiting A. gentlei. The vulnerability of ant-acacias to be exploited by cheat species means that to remain evolutionary stable, the obligate mutualists must develop a long-lasting relationship.

Rather than causing the extinction of the mutualists like A. gentlei and P. ferruginea, it’s thought that the presence of cheaters can increase the number of commodities offered by the mutualist species; this is because the exploiters provide a selective background which mutualists can utilise to demonstrate their superior quality. Therefore, the persistence of obligate mutualism may reflect a history of coevolution under the selective pressure of exploitation.

To our knowledge, this study presents the first test of strength protection provided by Pseudomyrmex ants on A. gentlei and publication is currently being worked towards!

To find out more about Lara and the projects she works on, follow her on Instagram and twitter @lara_wildlife or visit her website http://www.larawildlife.co.uk.

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