Reintroduced foxes don’t take completely to badlands

Swift foxes reintroduced to the badlands in South Dakota are struggling to adapt to some landscape features, though studies show healthy genetic diversity.

“It’s very clear that the rugged terrain of the Badlands National Park, is not very good terrain for swift foxes,” said Indrani Sasmal, a postdoctoral researcher at North Carolina State University and lead author of a recent study published in the Wildlife Society Bulletin.

Sasmal, a member of The Wildlife Society who conducted the research as part of her Ph.D. work at South Dakota State University, observed swift fox (Vulpes velox) pup rearing in the field in the summer of 2009 to find out what kind of habitat they favored as well as analyzed data gathered from captured animals from 2003 to 2009.

Swift foxes are thought to have been extirpated in the area around the turn of the 20^th century due to eating poisoned bait intended for coyotes at the time. The national parks translocated around 150 foxes from Wyoming and Colorado from 2003 to 2006, and Sasmal has conducted a number of studies to assess the health of the population in the park since then.

In the recent study, she and her coauthors found that initially, swift foxes did very well after reintroduction. This could be due to the low number of coyotes (Canis latrans) in the area, who sometimes depredate swift foxes. Coyotes were suffering from a disease that didn’t affect foxes.

But in the last years of the study data, juvenile (aged from 1 to 2 years old) and adults (older than two years) had low survival rates in the wild.

©Tony Ifland/USFWS

“The survival of the pups was around 47 percent while the survival of the juveniles and adults was only around 27 percent,” Sasmal said.

Potential reasons for this low survival rate included competition from other swift foxes, red foxes (Vulpes vulpes), and depredation from an increasing coyote population.

Black-tailed prairie dogs (Cynomys ludovicianus) — swift fox prey — also had relatively high numbers in the badlands area in the early years of swift fox reintroduction but later suffered from plague. Swift foxes are not affected by plague but can carry it and pass it on to prairie dogs, so researchers didn’t rerelease captured foxes whose blood tests showed plague.

Precipitation levels in the area were generally pretty low in the first years of reintroduction, which ensured lower vegetation and favors foxes’ hunting visibility.

A bad land for foxes?

But the bigger issue may be that the badlands aren’t that suitable for foxes in the first place. A previous study Sasmal coauthored in Conservation Genetics found that the foxes in badlands showed a good level of connection between a neighboring population in Fall River, South Dakota — good news, according to her. But this dispersal also showed that the area wasn’t that suitable for swift foxes.

Swift foxes, listed as threatened in South Dakota, live approximately six or seven years in the wild, and around 150 animals were reintroduced to the badlands. Sasmal estimated that by 2009 there were around 240 individuals in the area.

Sasmal says that adults and juveniles may show low survival due to the area reaching its carrying capacity. Her work on the species shows that the badlands themselves may not be as good for swift foxes as the surrounding grasslands.

This finding has implications for management, she said, since conserving grasslands in the national park might be more important for the foxes than the badlands themselves. Specific strategies that may benefit grasslands habitat include burning down higher vegetation while reintroducing prairie dogs into areas where they were decimated by the plague — a move that could benefit foxes as well.