Saturday, May 14, 2011

Airlifting Ants

Competition. A contest. An opposition. A rivalry for supremacy.

It may occur directly or indirectly between members of the same species or different species. Commonly you will see it over such resources as food, space, or mates. Usually something that is limited. Let’s focus on indirect competition. Here species clash over access but in such a way that one interferes with the other’s ability to utilize the resource. These interference interactions are common in social insects, and that is the topic of today’s post.

A recent article in Biology Letters reports such interference behavior in ants and wasps. Now, you might think indirect competition would be a common avenue of study as the evolutionary and ecological importance of exploitative competition has been well documented. Not so. It was only recently that behaviors such as ant avoidance when wasps catch prey, wasps robbing food from ants, and wasps guarding mutualists against ants has been published. Why even care? Don’t think of this topic as wasp vs. ant (although it is fun to envision such a showdown) but rather from the viewpoint of an invasive species biologist or manager. Understanding interactions between invasive wasps and native ants can inform us as to why a species (invasive or native) thrives or declines after an introduction.

This study takes a look at the social wasp Vespula vulgaris. This wasp is native to temperate regions of the Northern Hemisphere and is a major invasive species in New Zealand, particularly in beech (Nothofagus spp.) forests. Enter the limiting resource: honeydew. Mmmm. These beech forests have healthy populations of scale insects and scale insects produce large amounts of honeydew, which is rich in carbohydrates. The invading V. vulgaris consumes this honeydew as do a variety of native animals including ants (particularly Prolasius advenus). But honeydew isn’t the only resource of contention. Wasps and ants have also been observed scavenging on the same prey.

What peaked the scientists attention was the observation that wasps would pick up worker ants in their mandibles and then drop them some distance away from the food. The researchers hypothesized that this was an example of interference behavior and that by removing the ants the wasps were effectively decreasing the competition and freeing up more of the food for themselves. Sorta like shoving the weaker kid to the back of the cafeteria line. To test this they set up 48 bait stations containing canned tuna fish randomly among the leaf litter and filmed them. They counted and time-averaged the numbers of ants and wasps that visited the traps as well as the number of aggressive wasp-wasp interactions. Then they looked at each interaction between wasp and ants by looking at the film frame by frame, scoring them as one out of 12 behavioral categories. One of these categories included “ant-dropping” where a wasp would pick up an ant in its mandibles, fly backward, and drop it away from the resource.

They found that when few ants were present the wasps had more conflict among themselves, but if many ants were present it was more likely interference behaviors would predominate. They concluded that the ant-dropping behavior was not predatory as the wasps were never seen leaving the bait station with an ant and not returning, and the removed ants were not injured. They also ruled out a defensive response as most of the ants were not behaving aggressively towards the wasps. The results showed that even though a wasp weighs 212 times more than an ant, that the wasp more often hung back or moved away in the presence of a high abundance of ants. I feel it is important to mention that P. advenus belongs to the Formicidae, so named because of their ability to produce and spray formic acid along with their bite. You can see why the wasps limited their contact with the ants. They typically picked up an ant to remove it from the bait when it was feeding or walking around the food. In fact, ants were successfully moved away from the food 83.9% of the time, 47.3% of which those dropped ants did not return to the food. And if the ant did return the wasp preceded it in 75% of the cases.

As a competitive strategy, this behavior is pretty efficient, at least on an individual level. V. vulgaris lack nest-based food recruitment mechanisms and so individual workers must be independent and opportunistic. In strategies such as this you often find that individuals adopt behaviors that allow them to exploit a resource early and quickly. The results of this study support this. Ant-dropping is a short term advantage to individual wasps that allows them to garner additional food.

Here’s the paper:
Grangier, Julian and Philip J. Lester (2011) A novel interference behaviour: invasive wasps remove ants from resources and drop them from a height Biology Letters: published online 30 March. (DOI: 10.1098/rsbl.2011.0165)

and the link for the above video:

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