Forest Fire Smoke Reduces Bird Diversity

When forests burn, the direct consequences are obvious: houses go up in flames and habitats are destroyed. The damage caused by the smoke that extends far beyond the fire area is less visible. Sarah Meier, an environmental economist at ETH Zurich, investigated this in relation to avian populations in the US together with Eric Strobl from the University of Bern.

This involved the researchers factoring in data from the North American Breeding Bird Survey, a long-term programme running since 1966 to monitor bird populations in North America. Each year, trained volunteers count all the birds they see or hear along fixed routes, each around 40 kilometres long, at some 50 stops. Meier and Strobl analysed data from around 2,800 such routes in the US over a 15-year period, from 2008 to 2022, and combined it with high-resolution data on particulate matter pollution from wildfires.

In addition to the number of species, they also examined how evenly the birds were distributed across different species and how much these species differed from one another in evolutional terms.

Smoke leaves its marks into the subsequent year

The result: if an area had been more heavily affected by wildfire smoke the previous year, bird diversity there was lower in the following breeding season. If smoke levels rose from an average year to a significantly smokier one, the number of species dipped by around three percent. On an average survey route with 55 species, this equates to roughly one to two fewer species. At the most heavily affected monitoring sites, however, the number of species fell much more significantly.

"I wasn't sure whether we would see any effect at all across such a large and ecologically diverse area," ETH researcher Meier relates. This makes it all the more remarkable that the correlation is evident across the US as a whole.

"The effect isn't huge, but it is measurable. And we never know which species will disappear and which of these are particularly important for an ecosystem. Therefore, even a small decline can, in the worst-case scenario, cause an ecosystem to become unbalanced."

Is smoke really to blame?

Forest fires often occur during periods of heat and drought - conditions that can also have a direct impact on birds. Applying a method from econometrics, the researchers therefore isolated the effect of smoke from other factors. They compared the same areas over many years, taking into account factors such as weather, land use and differences in how the bird counts were conducted.

In addition, they examined two particularly obvious explanations. Firstly, the lower bird diversity could be a direct consequence of destroyed habitats. However, even on survey routes where no areas had burned within a five-kilometre radius, the effects of the smoke persisted. Secondly, birds might simply have moved to neighbouring areas to avoid the smoke. Even after the researchers had taken such local shifts into account in their analysis, reduced avian diversity was still evident.

The study does not reveal exactly what happens to the birds. Other research suggests that smoke can weaken birds, possibly causing loss of weight and respiratory damage. Their behaviour may also change during the breeding season.

What is the impact on the economy and ecosystems?

For Meier, the study findings highlight a consequence of forest fires that is decidedly difficult to capture in traditional damage assessments. Birds perform important functions in many ecosystems: they disperse seeds, pollinate plants and help to control pests.

A decline in avian diversity can therefore also affect other animal and plant species - and ultimately sectors such as agriculture and forestry.

The study intentionally does not quantify the extent of the economic damage caused by a decline in avian diversity. Meier focuses initially on demonstrating individual impacts as reliably as possible.

"In the case of burned-out buildings, we can relatively easily quantify the cost of the damage. With biodiversity, things are far more complex," says Meier. "There are other consequences of forest fires that we have barely taken into account to date, even though they have an impact on society."