Ecology of the Forest Floor: Ground Beetles and Habitat Health

by James Darling and Shan Leung

Ever notice those little black beetles scurrying across the ground? They’re called ground beetles (Carabidae family) and believe it or not, they are very important little insects.

Before earthworms were introduced to North America via horticultural trade from Europe in the 18th century, ground beetles were the primary soil-churners responsible for mixing nutrients and contributing to floral diversity. Even today, there are parts of North America (generally toward 60° latitude) where earthworms have not reached, so the soil is still churned by beetles. Some ground beetles act as biological indicators: their presence or absence provides insight into the health of an ecosystem.

A man holds an uprooted plant outside while another looks on.

We—Environmental Monitoring Programme students James Darling (left) and Shan Leung (right)—have been surveying ground beetles on the Canadian Museum of Nature’s 76 hectare wetland property in the Aylmer sector of Gatineau, Quebec.
Image: Noel Alfonso © Canadian Museum of Nature

Our surveying involves collecting using five pitfall traps, which consist of an acrylic glass strip laid on its side and a plastic container embedded in the ground on either end. Once collected, beetles are pinned, identified and stored in the museum’s collections.

Our overall objective is to document which beetle species live in the various habitats that are found on the museum grounds: forest, wetlands, meadows, etc. We also want to monitor what changes in the beetle species we can observe as those habitats naturally evolve—meadows becoming forests, for example.

Several insects float in a small tub sunk into the ground.

Our traps work by guiding beetles along Plexiglas and into pitfalls/cups filled with propylene glycol (which kills and preserves the beetles). This particular trap contains several bronze carabids. Image: James Darling © Canadian Museum of Nature

Carabus nemoralis: The Bronze Carabid

Early on, we noticed that one species in particular was filling our traps disproportionately. This species is the bronze carabid (Carabus nemoralis), a European ground beetle that was first recorded in North America approximately 150 years ago.

Since its accidental introduction to North America (again in the 18th century via European settlers), the bronze carabid has spread as far north as Quebec and as far south as California, U.S.A. There are species in the same genus that are native to North America, seven of which can be found in Ontario and Quebec.

Illustration of a bronze carabid.

The bronze carabid is completely flightless; this beetle spends literally all of its time on the ground. Image: François Génier © Canadian Museum of Nature

Despite dozens and dozens of bronze carabid specimens collected over the past summer, only one specimen of a native Carabus species was found. Could it be that the bronze carabid is displacing native species by outcompeting them for food or other resources?

The bronze carabid’s success in North America might be attributed to its skills as a hunter. Members of genus Carabus prey on slugs and are specially adapted for detecting mucus trails. Some slugs are agricultural pests that consume large portions of crops, but with the help of the bronze carabid, we may be able to control slug infestations. So despite its invasive nature, the bronze carabid could be an effective biological control for keeping our crops slug-free.

Highlights from Summer 2015

Three men pause in the water while holding a net.

The Environmental Monitoring Programme team in 2015: left: Shan Leung; centre: Noel Alfonso, who runs the programme; right: James Darling. Image: Brian Coad © Canadian Museum of Nature

Bronze carabid numbers throughout May, June and August were high. Quite often we would collect as many as a dozen specimens from a single trap!

Interestingly, we collected hardly any bronze carabids throughout July. This is because July represents the time between the death of the adult beetle and the complete metamorphosis of their larvae.

Bronze carabids overwinter as adults and become active again in April or May to reproduce. These adults produce larvae in June and by July are all dead. Larvae emerge as adults in August, which explains the sudden reappearance that we observed.

It is worth noting that our pitfall traps were not selective: we have never exclusively found ground beetles in our traps. Usually it’s other ground-dwelling insects that find their way into our traps, but every now and then we come across mammalian or amphibian by-catch.

Over the course of the summer, we have accidentally caught three shrews, four voles, five wood frogs, six American toads and one blue-spotted salamander. Next summer, we can prevent by-catch by covering the pitfalls with a wire mesh: this will limit the size of what can fall into our traps.

But should we prevent by-catch? Of course, one could argue for the sake of limiting our impact on local biodiversity, but by-catch from the beetle traps is our only source of vertebrate specimens. It is because of our beetle traps that we found a vole species that had not been recorded previously on the property!

Overall, we had a successful summer of beetle collecting, having added more than 1000 specimens to the existing Environmental Monitoring Program beetle collection. Approximately 400 of these specimens are ground beetles, representing twenty one species. Once all the data has been organized and all the numbers have been crunched, we will have a clearer picture of the local biodiversity and the shape it’s in.

A man walks in a forest.

James Darling on a weekly excursion. Image: Shan Leung © Canadian Museum of Nature

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