Hungry caterpillars an underappreciated driver of carbon emissions


Date:
November 3, 2021
Source:
University of Cambridge
Summary:
A study has found that periodic mass outbreaks of leaf-munching
caterpillars can improve the water quality of nearby lakes -
but may also increase the lakes' carbon dioxide emissions.



FULL STORY ==========================================================================
A study led by the University of Cambridge has found that periodic mass outbreaks of leaf-munching caterpillars can improve the water quality of
nearby lakes -- but may also increase the lakes' carbon dioxide emissions.


========================================================================== Outbreaks of caterpillars of invasive gypsy moths, Lymantria dispar
dispar, and forest tent caterpillar moths, Malacasoma disstria occur at
least every five years in temperate forests. The insects munch through
so many leaves that the resulting decrease in leaf-fall and increase
in insect excrement has been found to alter the cycling of nutrients, particularly carbon and nitrogen, between land and nearby lakes on a
huge scale.

Nitrogen-rich insect excrement, called frass, can wash into lake water
and act as fertiliser for microbes, which then release carbon dioxide
into the atmosphere as they metabolise. The researchers suggest that
in outbreak years the large quantities of frass will favour the growth
of greenhouse gas- producing bacteria in lakes at the expense of algae
that remove CO2 from the atmosphere.

"These insects are basically little machines that convert carbon-rich
leaves into nitrogen-rich poo. The poo drops into lakes instead of the
leaves, and this significantly changes the water chemistry -- we think
it will increase the extent to which lakes are sources of greenhouse
gases," said Professor Andrew Tanentzap in the University of Cambridge's Department of Plant Sciences, senior author of the paper.

Northwards range expansion and increased insect population growth
is anticipated as the climate changes. This puts northern forests at
increased risk of defoliator outbreaks in the future, potentially causing greater quantities of CO2 to be released from nearby lakes.

This northwards shift is also concerning because there are more freshwater lakes further north. And climate change is also expected to favour
broadleaved deciduous trees around the lakes, which will amplify the
effect of the insects.



==========================================================================
The study found that in years with insect outbreaks, the leaf area of
forests was reduced by an average of 22%. At the same time, nearby lakes contained 112% more dissolved nitrogen and 27% less dissolved carbon
compared to non-outbreak years. The effects were greatest when lake
catchments contained higher proportions of deciduous broadleaved trees,
such as oaks and maples, which the caterpillars favour over coniferous
trees like pines.

To get their results, researchers combined 32 years of government
data from insect outbreak surveys and lake water chemistry in 12 lake catchments across Ontario, Canada, and satellite remote sensing data
on forest type and monthly leaf area cover. The results are published
today in the journal Nature Communications.

This is believed to be the most extensive study ever undertaken into how
insect outbreaks impact freshwater carbon and nitrogen dynamics. Previous studies have been so small that it has been difficult to extract wider generalities.

A previous 26-year study of 266 lakes across the northern hemisphere
has shown that carbon is naturally accumulating in these lake waters,
in a process called browning. The trend is attributed to a variety of
factors including climate change, and recovery from historical acid rain
and logging activities.

Comparing the new results to this data showed that an outbreak of
leaf-munching caterpillars can effectively offset an entire year's worth
of carbon accumulation in nearby lakes -- significantly improving water quality.

In years without outbreaks of leaf-eating insects, carbon and nitrogen
entering lakes usually comes from decaying leaf and needle litter, and
peaks in quantity in autumn. In outbreak years, the study found that
nearby freshwater lakes contained an average of 27% less dissolved carbon.

"Outbreaks of leaf-eating insects can reduce the carbon dissolved
in lake water by almost a third when the trees around the lake are
mainly deciduous. It's just amazing that these insects can have such a pronounced effect on water quality," said Sam Woodman, a researcher in
the University of Cambridge's Department of Plant Sciences and first
author of the report.

He added: "From a water quality perspective they're a good
thing, but from a climate perspective they're pretty bad
-- yet they've been completely overlooked in climate models." ========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
text of this story is licensed under a Creative_Commons_License. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Woodman, S.G., Khoury, S., Fournier, R.E. et al. Forest defoliator
outbreaks alter nutrient cycling in northern waters. Nat Commun,
2021 DOI: 10.1038/s41467-021-26666-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211103082615.htm

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