The (soil) cycle of life

Decomposition is a great housekeeper: it takes care of dead plants and animals by breaking them down into simple molecules.  A variety of fungi, insects, worms, and bacteria are involved in decomposition.  The end result is that nutrients are returned to the soil for use by the next generation of plants.

Decomposition is important for maintaining the ecosystem, because there is a finite amount of matter in the world.  Think of it like a bunch of Legos: you can build a whole town, but then all your bricks are used up.  To build something new, you have to take apart the cars and buildings that you’re done playing with.  Then you can build something else with the freed-up pieces.

If you live in a town or city, you might not think about decomposition very often.  But out in the woods, you can’t miss it.  You’re often tromping on a thin layer of dead leaves that are square in the middle of being decomposed.  It’s long been thought that the main factor affecting the return of nutrients to the soil was simply how many plants were around to be decomposed.

Turns out it might be more complex than we thought.  In a study published in Science on June 15th 2012, Dror Hawlena, a post-doc in the Schmitz lab at the Yale School of Forestry, and his colleagues show that the rate of plant decomposition is affected by the predators in an ecosystem.

And it’s not because predators are eating the plants.

Hawlena and his colleagues stressed grasshoppers by raising them with a natural predator: a spider, but one whose mouth was glued shut.  The grasshoppers were under the impression that they could be dead at any minute.  Grasshoppers lived their lives, with or without the spiders present, died, and decomposed by the hand of microbes in the soil.

When prey animals, like grasshoppers, feel their lives are in danger, they want a constant supply of ready energy in case they have to make a break for it.  So, they eat more carbohydrates and break proteins in their bodies down into sugar.  While these behavioral and metabolic changes give the grasshoppers energy, they also alter the chemical composition of their bodies.  Carbohydrates have lots of carbon and not a lot of nitrogen, and the proteins that they dismantled were loaded with nitrogen. The combined effect of these changes is that stressed grasshoppers have a higher carbon-to-nitrogen ratio than unstressed grasshoppers.  Dead grasshoppers become part of the microbial diet.  True to the old adage, “you are what you eat,” that carbon/nitrogen imbalance is passed on to the microbes.

When the researchers piled dead grass on top of the soil where the grasshoppers had been, they found a surprise: microbes that broke down stressed grasshoppers were 200% less efficient at breaking down the grass.  The reason this is so surprising is that there was only a 4% difference in the carbon-to-nitrogen ratio between stressed and unstressed grasshoppers.  That small difference in body chemistry exploded into a huge effect on the rate of plant decomposition.

So, microbes eating stressed grasshoppers get less nitrogen.  Nitrogen is what microbes use to grow and to make the enzymes that break down dead plants.  Less nitrogen in the microbial diet probably means fewer microbes and fewer digestive enzymes in the soil, hence slower decomposition.

This doesn’t mean that we should mount a campaign against grasshopper stress.  Nature has a way of finding the right balance when left to its own devices.  But humans have a way of mucking with nature, so we need to keep in mind that the endangerment or extinction of high-level predators could have a ripple effect on the entire ecosystem; one that reaches deeper than we originally thought.

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More reading:

Fear of predation slows plant-litter decomposition. Science. 2012 Jun 15;336(6087):1434-8.