1 + 1 = 1

Nutrient levels in soil don’t add up when food chains combine.

By | July 1, 2015

NOT ADDING UP: Three experimental ecosystems demonstrate the effects of herbivore (left), detritivore (middle), and combination (right) food chains on soil nitrogen levels (N). In the herbivore food chain, grasshoppers’ feces elevate nitrogen levels over those in a plant-only control ecosystem (not shown). In the detritivore and combination food chains, nitrogen levels are the same as the control, suggesting the food chains interact to dampen the nitrogen-elevating effects of the herbivores.PHEBE LI FOR THE SCIENTIST

EDITOR'S CHOICE IN ECOLOGY

The paper
R.W. Buchkowski, O.J. Schmitz, “Detritivores ameliorate the enhancing effect of plant-based trophic cascades on N cycling in an old-field system,” Biology Letters, doi:10.1098/rsbl.2014.1048, 2015.

Life on Earth may be carbon-based, but it wouldn’t exist without nitrogen. Soil microbes transform nitrogen from the air and from decaying organic matter into forms of the element available to plants and, in turn, the animals that eat them.

Within their respective food chains, detritivores—dirt-dwelling invertebrates that feed on decaying matter—and herbivores have been shown to raise soil nitrogen levels. And although it stands to reason that interactions between these food webs might act synergistically on nitrogen levels, it was unknown what their combined impact might be.

To examine this question, graduate student Robert Buchkowski and his advisor Oswald Schmitz at Yale University set up 45 mesocosms: cage-covered pots one foot wide by three and a half feet tall that hold self-contained ecosystems. Each mesocosm contained soil and one of nine different permutations of herbivore and detritivore food chains: a plant-only control; two herbivore-based food chains including plants, plant-eating grasshoppers, and grasshopper-eating spiders; two detritus-based food chains of plants, detritus-eating woodlice, and woodlice-eating spiders; and four combinations of herbivore and detritivore food chains. The researchers left their simplified ecosystems untouched for 46 summer days, then measured the soil’s nitrogen content.

At the end of the experiment, the herbivore food chain of spider, grasshopper, and plants increased the nitrogen content of the soil. The detritus-based food chains alone, on the other hand, did not appear to impact nitrogen levels (although the researchers suspect this could have been due to the experimental conditions). But what was striking was that the combinations of detritus and plant food chains also yielded no increases in soil nitrogen. The researchers concluded that the presence of the detritus-based food chain must have erased the nitrogen-adding impact of the plant-based food chain.

Buchkowski speculates that the ground-dwelling woodlice might be aggressively competing with the grasshoppers for space, forcing the grasshoppers to spend more time above the ground level and are thus more vulnerable to being eaten by spiders. The increased predation would therefore cut into the grasshoppers’ impact on soil nitrogen through their herbivory. In the future, Buchkowski hopes to test whether this is the case. “We need to consider the interaction between species that are in these two different food chains,” he says.

The real questions, says Richard Bardgett of the University of Manchester in the U.K., are whether these effects on nutrient cycling happen in different and more-complicated ecosystems in the wild and how the interactions among food chains alter nitrogen content on the ecosystem level.

“People very rarely look at these two food webs in unison,” says Bardgett, who studies interactions between aboveground and belowground organisms. “Nutrient turnover in ecosystems is a very complex issue that’s driven by not just decomposer organisms but also their interactions with the food webs aboveground.”

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