One plus one does not always make two, when plants add up costs and benefits

Mutualism is a relatively simple concept in plant science. Plants will reward other organisms that perform services for them, so both the plant and partner are better off then they would be alone. But what happens when you add more than one species to an ecosystem? Non-additive benefit or cost? Disentangling the indirect effects that occur when plants bearing extrafloral nectaries and honeydew-producing insects share exotic ant mutualists by Amy Savage and Jennifer Rudgers shows it’s not always as simple as adding up costs and benefits to find out the net effect species interaction has.

Being simple myself, the phrase ‘non-additive’ puzzled me, but there’s an excellent graph that explains what Savage and Rudgers mean. They think of a plant hosting two species. One is an ant. The ant tends to protect the plant from herbivores – except one. The second species is a sap-sucking insect that produces honeydew, a sugary substance that’s a bit like an insect’s version of nectar. What happens if you put the two species on a plant?

The Additive model simply adds up costs and benefits. So if ant protection boosts growth by two units, but the sap-suckers deplete it by one, then the net benefit is one unit to the positive. That’s the first graph below. But it might not be that simple.

Ants like honeydew, because of the complex sugars. But suppose ants REALLY like honeydew. So much that hosting the insects pulls a massive number of ants up to protect the leaf. Here is what is called a Non-additive benefit model. The next effect is far more positive than simply adding up the interactions of the two species with the plant. But this could go the other way too. Maybe the protection given by the ants really allows the sap-suckers the safety to plunder the plant. Then you have a Non-additive cost model – the third graph.

It turns out there hasn’t been much research on the combined effects of ants and honeydew producing insects (HPI) on plants. This is an issue because of another complex factor – what happens when one ant invades the territory of another?

Savage and Rudgers looked at ants on Savai’i, Samoa, in particular Anoplolepis gracilipes. A. gracilipes correlates with extra-floral nectaries on plants, and is more aggressive to other insects on its patch than other ants. The mutualism between A. gracilipes and plants could be aiding A. gracilipes kill off other insects in the area.

The study was on the lava flows of Savai’i, colonised by Morinda citrifolia a plant with floral, extrafloral and post-floral nectaries. It produces nectar all year round. They picked a variety of sites, some with A. gracilipes and some with less invasive ants present.

To test the effects of the ecological interactions they could remove ants from plants, by circling the base of a plant with a pest barrier. They could reduce nectar production by bagging the nectary bodies. They could also reduce the presence of HPI by brushing them off the plants. They could then manipulate some of these factors together.

For A. gracilipes, they found the key was nectar. Reduce nectar and you reduce the number of ants on the plant. Reducing HPI only had a small effect, which suggests A. gracilipes aren’t so bothered about honeydew.

For the plants, the presence of the ants makes a big difference. They hardly grew without any A. gracilipes around, but grew 3000% with the ants around. HPI had little effect without A. gracilipes, but also promoted growth then there were ants there too. This supports a non-additive benefit model. For the less invasive ants the effect the effects were less than additive.

One of the interesting findings was that the A. gracilipes had an effect on HPI, turning them from a negative to a positive for plant growth. It’s something that you could not predict simply from observing just A. gracilipes or HPI. The paper certainly provides plenty to think about on the complexity of cost and benefit analysis. You can now pick it up for free from Annals of Botany.