Are ground beetle populations decreasing? It depends on where you look and what species you look at. A recent paper in Global Ecology and Biology, Habitat–trait interactions that control response to climate change: North American ground beetles, explores the connections between climate change, habitat types, and ground beetle traits. The study could help researchers model which ground beetle species may be at risk and the habitats needing protection to preserve them.
Are Ground Beetles Declining?
Ground beetles (Carabidae) are found all over the world in nearly every habitat and climate zone, from densely canopied forests to grassy plains and even deserts. Carabidae is one of the largest and most diverse families of beetles, with more than 40,000 known species globally and more than 2000 species documented in North America.
The Carabidae family shows astonishing diversity. Some are fliers, while some scuttle on the ground or crawl along tree branches. They range in size from the giant warrior beetles (Pasimachus elongatus) at 1.5 inches to the tiny 1/8-inch specimens of the Bembidion genus. Many of them are voracious predators that hunt caterpillars, aphids, spiders, slugs, and other invertebrates. Others are omnivorous or subsist on seeds, fungi, algae, or decaying plant material.
With such a broad species composition, it can be hard to generalize how Carabidae populations are responding to changes in habitat and climate. A large 2020 meta-analysis suggests that terrestrial insect populations are declining globally at a rate of about 1% per year. However, an analysis of insect abundance and diversity at Long-Term Ecological Research (LTER) sites in the U.S. did not show a similar decline.
For Dr. Tong Qiu, now an assistant professor at Penn State University, these discrepancies invited further research. He tackled the problem as a postdoctoral researcher in Dr. Jim Clark's lab at Duke University, with a project funded through a National Science Foundation (NSF) grant.
Qiu explains, "We started with ground beetles because we have these papers with contrasting findings. NEON has ground beetle data collected using the same sampling protocol across all sites. And it also has data on habitat variables, such as ground cover types and canopy structure. We wanted to incorporate these two things, the ground beetle abundance and diversity data and the habitat variables, to understand how ground beetle populations are changing over time in different habitats."
Qiu, Clark, and their coauthors hoped that looking at habitat variables alongside ground beetle data, including species traits, would shed some light on how different types of ground beetles might respond to changes in habitat. In turn, this could enable better predictions of which beetle populations might increase or decline in different areas.
Clark says, "The problem with monitoring insect populations is that the data are hugely noisy; there can be large fluctuations in populations from year to year based on things like annual precipitation or temperatures. Because of that, it makes it hard to pick out trends over time and determine, for example, if a particular species is increasing or decreasing. With the NEON data, we were looking for variations in ground beetle populations spatially and seeing if we could relate those patterns to habitat variables."
Looking at Habitat-Trait Interactions for North American Ground Beetles
So, how do ground beetle traits (e.g., flying vs. crawling or predator vs. omnivore) influence their ability to thrive in different habitat types? To dive into this question, the researchers used ground beetle data from pitfall traps collected at NEON terrestrial field sites in all climate zones.
Carabidae beetles were identified in the NEON data to the species or morphospecies (a group of species with similar traits). Qiu and Clark used the species identifications to generalize the diverse ground beetles by traits, including body size, body color, diet (carnivore, omnivore or other), activity (diurnal vs. nocturnal), movement (flight, burrowing, climbing, running), and preferred habitat (forest, grassland or other).
They then looked at habitat characteristics at the locations where the beetles were collected, using lidar and hyperspectral data from the NEON Airborne Observation Platform (AOP). Remote sensing data allowed them to classify sites by habitat variables such as canopy openness, understory density, presence of woody debris, topography (surface roughness), and canopy nitrogen content. They also used climate data (temperature and precipitation) from NASA's DAYMET products.
Their findings suggest that ground beetles with differing traits will have differing responses to changes in habitat. Additionally, some habitat characteristics may prove beneficial to beetles with certain traits, even in the face of climate change. Take, for example, canopy density. Habitats with closed canopies and dense understories tend to be favorable to large-bodied, burrowing omnivores. However, diurnal fliers tend to do well in open grasslands. Beetle traits may impact how well different species are able to respond to changes in those habitats. In particular, fliers may be more resilient due to their greater ability to relocate in response to habitat loss or climate change.
It appears that habitat variation contributes more to differences in beetle species abundance and community composition than does climate variation. Overall, canopy density and temperature are more highly correlated with differences in both beetle abundance and traits than other habitat variables, such as surface roughness, soil fertility, or canopy nitrogen.
Predicting the Future of Carabidae Beetles
Climate change is expected to have a big impact on the habitats where ground beetles live, which will influence which species will thrive in different areas. The researchers used climate scenarios from WorldClim to develop models of habitat change and changes in ground beetle populations over time.
The study highlights the complexity of trying to answer questions like "Are ground beetle populations declining?" or "How will climate change impact ground beetle populations?" The answers are likely to be highly dependent on the types of Carabidae beetles in question and their sensitivity to change.
Clark says, "Prediction is a difficult thing to do because there are a lot of different things that can affect these bugs. Agricultural impacts, the legacy effects of pesticides, land cover change, invasive species…there's just a lot going on. But if we understand that certain species like certain kinds of habitats and climates, we can say a lot about how habitat changes will affect these species."
Qiu and Clark are continuing their work at NEON field sites, this time with an NSF grant to look at seed production to quantify food supply in different habitats. Qiu is also working on scaling up the models using NASA remote sensing data to map habitat variables across the country. His lab is looking at biodiversity and habitat variables for various taxa, including birds and small mammals.
Clark's lab has developed a set of online tools and maps to explore habitat change and biodiversity for ground beetles, breeding birds, small mammals and trees. Using these tools, it is possible to model habitat suitability and species abundance across the continental U.S. under different climate scenarios for the years 2040-2069 and 2070-2099.
"As we think about how to preserve biodiversity in the face of climate change," says Clark, "we can now use these models to put together information about what kinds of habitats are suitable for different species and how those habitats are expected to be impacted by climate change in the future. That's important information for determining which kinds of habitats we want to preserve to protect biodiversity."