Untangling the opposing effects of ants on plants: lessons from the field

ants harvesting

Ants harvesting.   Photo: A. Fargi-Brener and M. Tadey

A blog post highlighing the Insectes Sociaux Best Paper Award Winner for 2017

Alejandro Farji-Brener and Mariana Tadey received the prize for their paper “Consequences of leaf-cutting ants on plant fitness: integrating negative effects of herbivory and positive effects from soil improvement”. This paper appeared in the February 2017 issue of the journal and can be found here.

Written by Alejandro Farji-Brener and Mariana Tadey

At the same time that I [Alejandro] was working on the ecology of the leaf-cutting ant, Acromyrmex lobicornis, in the north-western part of the Patagonian steppe (Argentina), Mariana was investigating the indirect effects of livestock on pollination in the Patagonian Monte Desert, 400km away from my study site. Mariana told me about the great abundance of leaf-cutting ants in her study area and soon we started our first ecological investigations there. It was interesting because in the Patagonian steppe, leaf-cutting ants were restricted to roadsides and in the Monte Desert, they were everywhere!

Looking at the ants, it was very impressive to see them harvesting a lot of plant species and dumping huge amounts of organic waste on the soil surface. We knew about the literature describing the potential negative effects of leaf-cutting ants as plant-damaging herbivores and the literature describing their positive effect on vegetation as soil improvers via dumping their nutrient-rich organic waste. Given these opposing effects of ants in an ecosystem, we wondered what their net effect was on the nearby plants.

Acromyrmex nest mound (above) and its waste piles (below)

Acromyrmex nest mound (above, pictured with measuring tape) and its waste piles (below). Photo: Farji-Brener and Tadey

Up to this point, several studies demonstrated the trade-off between herbivory and nutrient intake under greenhouse conditions. However, there were few studies investigating what happens in nature were plants are subjected to both situations, the loss of green tissue by ant defoliation and the enhanced plant growth caused by the ant-generated organic waste piles. Given that in this water poor habitat, both the loss of photosynthetic tissue and nutrient availability are key factors for the health and survival of plants, we thought that the net consequence on plant fitness should depend on the relative importance of these opposite ecological effects. Hence, we started our research about the fitness of native plants growing on, or near, ant organic waste piles (our nutrient hot spots) and on bare soil.

In our study site, plant species were differentially subject to defoliation by leafcutter ants, but there was no clear pattern of growth compensation for defoliation due to plant growth on organic waste. We proposed that this lack of compensation was caused by the water limitations imposed by the aridity of this environment, which restricted nutrient uptake for the plants. Overall, our work suggests that the interpretation of nice outcomes from controlled experiments may be unsupported by work under field conditions, highlighting the importance of the ecological context in scientific studies. Studies like ours allow us to include the particularities of ecosystems into theoretical frameworks so we can improve our understanding of how ecosystems work. Our study also shows how collaboration among scientists can lead to a big change our understanding of how organisms interact in nature.

ants carrying leaves

Ants carrying leaves. Photo: Farji-Brener and Tadey.

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