Rivers and freshwater lakes are the channels whereby macronutrients, including carbon and nitrogen, make their way from biomass on land to biomass in the sea. Each year, carbon and nitrogen usually enter freshwater systems after being leached out of the fallen leaves of forest trees. Once dissolved in lake water, both of these macronutrients influence ecosystem structure.
But now, a study led by scientists from the University of Cambridge highlights the disruptive influence of outbreaks of leaf-eating caterpillars.
Outbreaks 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 eat their way through so many leaves that the resulting decrease in leaf-fall changes the nutrient cycles between the land and the nearby lakes.
In addition, the caterpillars produce huge quantities of nitrogen-rich droppings (known as frass) that wash into the waterways and act as fertilizer; this changes the water chemistry.
Microbial growth is often limited by the availability of nitrogen, and so adding frass to rivers and lakes may also favor the growth of bacteria and algae in these systems. Increased microbial growth also leads to an increased production of carbon dioxide, as the microbes respire and undergo metabolic reactions.
“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 study senior author Professor Andrew Tanentzap in the University of Cambridge’s Department of Plant Sciences.
The study is believed to be the most extensive research ever conducted on how insect outbreaks impact freshwater carbon and nitrogen cycles. The researchers used 32 years of government data on insect outbreaks in 12 river catchments across Ontario, Canada.
The experts related this data to surveys of lake water chemistry and used satellite images of the forests to track changes in monthly leaf area cover. The results of the study show that, in years in which there are caterpillar outbreaks, the leaf area of forests was reduced by an average of 22 percent, due to the foraging activity of the caterpillars.
Correlated with these insect outbreaks, the nearby lakes contained 112 percent more dissolved nitrogen and 27 percent 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 are favored by the caterpillars over the leaves of coniferous trees like pines.
In years when there are no outbreaks of leaf-eating insects, carbon and nitrogen usually enter lakes from decaying leaf and needle litter, and this peaks in autumn. In fact, a previous 26-year study of 266 lakes across the northern hemisphere showed that carbon accumulates naturally 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.
However, when a caterpillar outbreak occurs, the leaf-fall is reduced to such an extent that carbon accumulation all but ceases for that year, which significantly improves the quality of the lake water.
“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 study first author Sam Woodman.
The researchers are concerned that as climate change favors the northwards range expansion of insect populations, the risk of outbreaks of leaf-eating caterpillars will increase. This may lead to greater production of CO2 from nitrogen-enriched lakes.
The caterpillar phenomenon is even more worrying since there is an abundance of freshwater lakes further north. Furthermore, climate change is also expected to favor broadleaved deciduous trees around the lakes, which will amplify the effects of the insects.
Outbreaks of defoliator insects are clearly a double-edged sword. In outbreak years the large quantities of frass will favor the growth of greenhouse gas-producing bacteria in lakes at the expense of algae that remove CO2 from the atmosphere. On the other hand, the reduction of dissolved carbon in the lakes during outbreak years when there is less leaf-fall has a positive effect on water quality.
“From a water quality perspective they (outbreaks) are a good thing, but from a climate perspective they’re pretty bad – yet they’ve been completely overlooked in climate models,” concluded Woodman.
The study is published in the journal Nature Communications.