The muskrat population in Canada’s Peace-Athabasca Delta has decreased alongside the drying of its habitat, research has shown. This semiaquatic rodent relies on ephemeral water bodies but the extent of these has shrunk by roughly a third since the 1970s. Not only have muskrat numbers declined but the population density has also fallen.
One of the largest inland deltas in North America, the Peace-Athabasca Delta has been inhabited for millennia. It’s economically and culturally important to local Indigenous communities, a UNESCO World Heritage Site and an internationally important wetland. But increasing river regulation since the mid-20th century has exacerbated drying that began in the late 1800s.
To quantify these changes in the flooding regime, Ellen Ward and Steven Gorelick at Stanford University, US looked at satellite images captured every summer between 1972 and 2017. In each image, individual pixels were classified as representing either water or dry land, allowing the researchers to build a 46-year inundation record of the region.
Because muskrat feed and overwinter on ephemeral water bodies and at the margins of lakes and streams, the researchers ruled out pixels that showed either water or dry land for the entire period. They considered the remaining intermittently flooded region to be potentially suitable for muskrat. Although the extent of inundation varied greatly from year to year, Ward and Gorelick found an unmistakable downward trend in habitable area, decreasing by 60% between 1973–1977 and 2013–2017.
To assess muskrat population density, the researchers used an indirect measure based on counts of muskrat houses going back to the 1970s. Muskrat nest in houses that protrude from the water; it’s best to survey them by snowmobile when lakes and ponds are frozen. Ward and Gorelick matched the counts made each winter to the habitat area measured for the preceding summer.
Every 100 square km of habitat lost was associated with a decrease of 5–6 individual muskrat per square kilometre, and as the habitat area fluctuated, so too did the population.
Taking a small lake to represent the whole delta, Ward and Gorelick also measured the habitat persistence over five-year windows centred on three peaks in muskrat population density. All the population booms were followed by steep falls, and the speed of collapse seemed to be greater each time.
“We suggest that declines in critical habitat reliability could be the reason why the population is dropping more precipitously from peak values,” says Ward. “Together with our findings, concurrent observations of widespread loss of wetland and aquatic habitat at sites across North America suggest that critical habitat loss could be responsible for the ongoing decline of muskrat across its native range.”
Climate change, which will affect high latitudes more severely, is expected to bring further drying to the Peace-Athabasca Delta, and planned hydropower projects will restrict flooding even more. For muskrat and the other species that share the delta, the situation will only get worse.
Ellen Ward and Steven Gorelick reported their findings in Environmental Research Letters (ERL).
