Fear and the Ecosystem

Written by Anna Cates on Friday, May 01, 2015

Our feelings affect our physical bodies. So do grasshoppers’, it turns out. I think ecologically-minded entomologists have perhaps known for a long time that we should be paying more attention to the feelings of the small critters they study. But if, like me, you are not an entomologist, perhaps you have been in the dark about the fantastic impact of insect fear on the ecosystem. 

A group of scientists led by Dror Hawlena at Yale University removed spiders from a grassland, and measured grasshopper bodies in the spiders’ absence. Spiders eat grasshoppers, so it makes sense that there would be more grasshoppers, and bigger grasshoppers, when spiders are eliminated. But these grasshoppers’ were also less stressed out, and their bodies were qualitatively different. They had more nitrogen (N), which is used to build proteins and complex biological molecules. When organisms are stressed, they use energy to defend themselves from stress and simply maintain, rather than producing proteins. Stressed grasshoppers had a higher C:N ratio than non-stressed grasshoppers.

Insects’ physical response to stress is interesting in and of itself, but Hawlena’s group asked further, how does this physical change in grasshopper bodies affect their surroundings? Grasshopper bodies, once they've died, are prime microbe food. As microbes process grasshopper bodies they release CO2, so grasshopper decomposition is part of the global carbon cycle. Although CO2 released from grasshopper decomposition is a pretty minuscule contribution to greenhouse gases compared to industrial processes, cars, etc, there are a lot of grasshoppers out there, so it’s important to know how they’re going to add up. 

There was no difference in microbe decomposition of stressed and non-stressed grasshoppers. But decomposing grass in the soil after the non-stressed grasshoppers’ bodies released more CO2 than grass after the stressed grasshoppers. Grass is less ideal microbe food, so the researchers suggest that the earlier addition of extra N in non-stressed grasshoppers had helped the microbes work through the grass litter. 

To recap: grasshoppers with no fear of predation tend to have bodies richer in protein (N). When they die, that N stimulates soil microbes to decompose grass more quickly. We see higher CO2 flux from soils where non-stressed, high-N, grasshoppers have died and stimulated microbes. Over time, this could add up to a significantly greater contribution to global warming.

Keep in mind that in these experiments, stress shifted grasshopper bodies’ N very slightly (C:N ratio was 4% higher), and researchers added 140 times more grass litter than grasshopper bodies to the soil. It didn't take very much N from grasshopper bodies to radically increase microbial decomposition of grass. Imagine the effects of stress on larger bodies, and the quantity of N which could stimulate soil microbes when a squirrel, a bird, a fish or a deer dies. Whether or not these bodies are stressed could have big impacts on CO2 emissions from soil microbes.

The effects of a predator on a prey population are an elegant illustration of nature at work: a predator population rises, the prey population falls. This is a classic piece of grade school ecology. Due to less grazing by the prey population, plant communities may be affected, and you have a classic trophic cascade. But this study shows that different species are connected by more subtle links, to each other and to global processes. Even grasshoppers and spiders. Makes you want to become an entomologist, doesn't it?

Anna Cates, a 2014 graduate of the Agroecology Program, still thinks of herself as an agroecologist as she delves into the ecology of decomposition and soil organic matter in corn fields as a PhD student at UW-Madison with Professor Randy Jackson. Other interests include dry bean varieties, home butchering, and ribbons

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