Across parts of tropical and subtropical shrub and grass dominated savanna ecosystems, a peculiar mutualism between specific ants and Acacia trees is thriving. For millions of years, specific ant species have teamed up with these trees, providing nutrients and protection against herbivores. In some ecosystems where fire disturbances are quite regular, ants have evolved the behavior to clear flammable debris from the base of the tree, thereby reducing the chances their plant will catch fire. In return, the trees offer housing, and in some cases a sugar reward for the ant’s services. This symbiosis is well studied, but still, not much is known on how the neighboring plant community influences these two partners.
The aggressive and voracious Crematogaster mimosae.
Not all partners involved in this mutualism are created equal, with some ant species having a larger cost than others. For example, the ant species Crematogaster mimosae is one of the most vigorous protectors of the nutritious Acacia leaves. They can deter the largest herbivores with their aggressive nature and sheer number, but their services are costly. C. mimosae have an exceptionally high metabolism easily matched by their aggression. These voracious eaters scour their host plant for extrafloral nectaries, (nectar producing parts outside of flowers) feeding non-stop. So, to maintain this symbiosis, these plants must allocate a significant portion of their photosynthate to the ants.
Oppositely, the ant Tetraponera penzigi too lives in the specialized hollow thorns the plant produces called domatia. Only T. penzigi doesn’t consume Acacia sugars and receives all nutrients from external sources. The tradeoff here is that these ants are far less aggressive when a hungry herbivore waltzes in for a mid-morning snack.
An ant housing, thorn domatium produced by many Acacia species.
A recent paper (Todd Palmer et al. 2017) used these two aforementioned ant species to study how neighboring plants influenced these two separate mutualisms. Their first prediction was that grasses growing nearby would successfully compete for soil nutrients and reduce the amount of sugars the Acacia could produce. With this in mind, they further hypothesized that the mutualism between Acacia trees and the voracious C. mimosae would be disrupted, with ants relocating to another tree that can satisfy their sugar demand. Their final hypothesis was that the less aggressive T. penzigi would remain unaffected, since they do not depend on the extrafloral nectaries.
This team of scientists set off to a savannah ecosystem at the Mpala Research Centre in central Kenya to study these interactions. Over a 40-week period, a manipulative study was carried out. Instead of studying mature Acacia individuals, saplings were tested (15-70 cm) because these younger plants should have a more enhanced competitive interaction with the surrounding grass. Soil moisture content, ant colony survivorship and the proportion of active extrafloral nectaries were quantified, but all in all, this study was straight forward with just one treatment. The influence of grass removal was tested against the control, in which the competitive grass remained untouched.
Ant survivorship. C. mimosae survived better with grass intact. T. penzigi was unaffected by presence of grass. Palmer et al. 2017.
After analyzing their data, they realized their results actually went against their initial prediction. Amazingly, the grasses growing nearby had a facultative influence on this mutualism. Instead of competing for soil nutrients, these grasses shaded the soil, and brought up water from the soils depth, ultimately enhancing soil water content. The young Acacia trees fitness greatly improved with the presence of these grasses and were able to produce more sugar nectar for their ant symbiote- C. mimosae. C. mimosae had a significantly greater survivorship when occupying an Acacia associated with nearby grass. Like they predicted, the survivorship of T. penzigi remained unaffected, since they only use their host plant for housing.
The precence of grass enhanced soil moisture. With more available water, more sugar can be produced. Palmer et al. 2017.
The growth of seedlings occupied by both ant species was heightened by the presence of grass, but these results were not statistically significant. Again, because T. penzigi doesn’t feed on extrafloral nectaries, the plants it inhabits allocated more resources to growth, and grew taller than those occupied by mimosae.
Seedling growth of Acacia trees inhabited by both species of ant. In the presence of grass, Acacia seedlings grew more, but these results were not statistically significant. Palmer et al. 2017.
These counterintuitive effects show us yet again how complex the forest floor, or savannah floor really is. Only young Acacia trees were used in this study, so it is very likely these interactions change as the plant becomes older and more robust. These nearby grasses increase water availability, thereby enhancing the fitness of both the Acacia tree, and the voracious ants that utilize extrafloral nectaries. Before this paper, nearby grasses were thought to have a negative influence on this plant-ant mutualism. With a straight forward, one treatment manipulative study, we now know that grasses facilitate these species. This publication reveals that the power of science is immeasurable, and the importance of maintaining biodiversity is imperative in this modern human era, since we still don’t know how most neighboring species interact with each other.