Aquatic insects in restored streams need more rocks to lay their eggs

Likening it to providing more runways at busy airports, researchers at North Carolina State University found in a new study that adding protruding rocks to restored streams can help attract female aquatic insects that lay their eggs on the rock bottoms or sides. 

More eggs that hatch into larval insects is great news for stream restoration because the re-establishment of organisms, such as insects, is often slower than expected in restored streams, says Brad Taylor, associate professor of applied ecology at NC State and corresponding author of a paper describing the research. A thriving population of stream insects generally portends good water quality, overall stream health, and provides food for fish, amphibians, reptiles, and even birds, he adds.

Most stream insects use rocks protruding above the water as runways to land on, then crawl underwater and attach their eggs to the underside of the rocks. Because restored streams sometimes fail to regain their abundance of aquatic insects even decades following restoration, researchers were interested in testing whether increasing egg-laying habitat   the rock landing areas   would increase the abundance and diversity of insect eggs and larvae. 

Taylor and NC State graduate student Samantha Dilworth selected 10 restored streams in northwestern North Carolina and added protruding rocks   gathered near the streams   to five of them; the other five restored streams did not receive additional rocks.

The results showed that the streams with added protruding rocks had almost twice as many egg masses compared to the untreated streams. After adding rocks, egg numbers in treated streams were similar to those in undisturbed streams downstream of state parks and national forests.

"This study was a successful proof of concept: adding rocks to restored streams enhances the abundance and diversity of stream insect eggs," Taylor said. 

The researchers also discovered that some rocks were more attractive to females and received most of the eggs; many rocks receive few or no eggs.

"The insects are really putting all their eggs in one basket, so to speak, or onto a few rocks," Taylor says. "A future goal is to figure out how and why females select specific rocks, so that any rocks added to streams get used, because moving large rocks around a stream is hard work."  

The study also showed that the response of the larval stages of streams insects was weaker than expected based on the increase in eggs in restored streams.

"We can restore aquatic insect eggs by adding rocks, but we didn t see a consistent increase of the larval insect stages," Taylor said. "The second year of the study was one of the wettest in recent years, so a lot of the rocks rolled. Insects in restored streams with no added rocks also declined, while those with added rocks did not change or increased, but not as much as expected.

"We observed that many rocks rolled, which could have caused eggs to be crushed or exposed to air where they would dry out and die. These sources of egg mortality may explain the weaker increase of larval insects, as some eggs may have never hatched into larvae. Also, higher larval mortality in restored streams could have been caused by factors unrelated to disturbance of the rock egg-laying runways. For example, priority or primacy effects by established insects or other organisms could be excluding newly arriving insects, food resources could be limiting larval insects, or other disturbances of larval habitats could be occurring."

Taylor plans to follow up with research on how best to stabilize protruding rocks using hydrologic modeling to determine the best locations. To extend the runway metaphor, what will make rocks more like O'Hare International Airport runways than a local grass or gravel airstrip?

"If we can get this information to restoration practitioners, they can add rocks that are both more attractive to female insects and stable," Taylor said. "This will make restoration efforts more cost efficient and effective. There are miles of restored streams, so there are lots of rocks needed. We want to make sure every rock gets eggs." 

The study appears in Ecological Applications. The study was funded by the North Carolina Dept. of Environmental Quality.

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Note to editors: The paper abstract follows.

"Facilitating the recovery of insect communities in restored streams by increasing oviposition habitat"

Authors: Samantha Dilworth and Brad W. Taylor, NC State University

Published: Dec. 10, 2023 in Ecological Applications

DOI: 10.1002/eap.2939

Abstract: Recruitment limitation is known to influence species abundances and distributions. Recognition of how and why it occurs both in natural and in designed environments could improve restoration. Aquatic insects, for instance, rarely re-establish in restored streams to levels comparable to reference streams even years post restoration. We experimentally increased oviposition habitat in five out of ten restored streams in western North Carolina to test whether insect egg laying habitat was limiting insect populations in restored streams. A main goal was to test whether adding oviposition habitat in the form of rocks that partially protrude above the water surface could be used to increase the abundance and richness of stream insect eggs and larval insects in restored streams. Adding egg-laying habitat enhanced several response variables (e.g., protruding rocks, number of eggs, egg masses, egg morphotype richness, and oviposition habitat stability) to levels similar to those found in reference streams. Following the addition of protruding rocks, egg mass abundance increased by 186 % and richness increased by 77 % respectively in restored-treated streams. Densities of larval insects that attached their eggs to protruding rocks showed an overall pattern consistent with treatment effects due to the combination of non-significant and significant increases of several taxa and not just one taxon. Our results indicate that these stream insect populations are limited by oviposition habitat and that adding egg-laying habitat alleviated this component of recruitment limitation. However, the weaker larval response indicates that additional post-recruitment factors, such as egg or larval mortality, may still be limiting a full recovery of larval insect abundances in these restored streams. This study shows the importance of integrating information from animal life histories, ecology, and geomorphology into restoration practices to improve the recovery of aquatic insects, which are commonly used to assess water quality and the biological efficacy of stream restoration.