Mixed odds for wildlife in severe drought

08/04/2021

Humans aren’t the only creatures affected by our drought. Last winter’s historically low rainfall is reshaping the habits and habitats of West Marin’s wildlife—helping some, hurting others.

“Wherever there is water, animals are pulling in closer to it,” naturalist and tracker Richard Vacha said. 

While fog helps keep the region’s coastline moist, softening the drought’s blow, our more interior landscapes are unseasonably dry. Instead of mid-summer, it feels like early autumn.

“Wetlands that usually remain wet until August and September were dried by June or July this year,” said Megan Isadore of the River Otter Ecology Project. 

Shrinking waterholes and drying vegetation affect wildlife in different ways, biologists say. Creatures that can’t travel must adapt or die. More mobile animals are wandering much further for a drink. Prey, such as rodents and deer, are more vulnerable; predators, such as coyotes and raptors, feast on easy meals.

When hungry and thirsty, animals are more likely to wander into our yards to eat from gardens, drink from birdbaths and rummage for insects on irrigated lawns, according to Patricia Kruger, a threatened and endangered species coordinator for the United States Forest Service in Mendocino County. 

They’re also more likely to venture onto the edges of roads to eat plants in roadside ditches or discarded trash, risking car collision. Their carcasses attract scavengers, who are also in harm’s way.

During droughts, “many animals are coming out near the roadways where they wouldn’t normally be,” Ms. Kruger said. As they wander outside their normal range, “you tend to see far more animals killed by cars.”

In general, fish are the first to be affected by a drought. Salmon in particular require streams to remain cool, oxygenated and flowing, Ms. Isadore said. When streams shrink, young salmon get caught in the disconnected puddles and are easily picked off by predators. Adults miss the cues to come upstream, and there’s not enough water for them reach spawning grounds.

This spring, there was an early die-off of aquatic insects like the stonefly, caddis fly and mayfly, said Stuart Weiss, chief scientist at the Creekside Center for Earth Observation.  Insects that feed on moist plants also struggled. In contrast, some butterflies did fine, as our midwinter warm spell accelerated their development. Migrating butterflies are helped by low rainfall because it gives them more time to fly and lay eggs, Mr. Weiss said. They’ll face greater challenges as the year progresses and nectar becomes scarcer.

Turtles simply estivate—a form of summertime hibernation. Tiger salamanders take refuge in cool ground squirrel burrows, “just hunkering down,” Mr. Weiss said. 

Frogs, toads, salamanders and other amphibians have to work a little harder to find mud, Ms. Kruger said. “They’ll be okay,” she said. “They’ll just bury themselves and can stay there for years. When there is enough rainfall, they’ll emerge.”

Birds that eat insects are more vulnerable than birds that rely on seeds, such as quail, said Katie LaBarbera of the San Francisco Bay Bird Observatory. Droughts are most deadly during birds’ breeding season, because chicks need insects; by now, most nestlings have fledged. 

As wildlands dry out, bird species that are normally shy and reclusive fly closer to our homes, seeking water. 

Otters can relocate if their river shrinks and aquatic insects dwindle, Ms. Isadore said. “However, moving is hard on them, as it is on any of us. They must continue to find new sources of food and resting spots, care for their young, compete for food, and protect themselves from predators…Otters are adaptable, but not impervious.”

Up on dry hillsides, where grasses are sparse, voles are dispersing. Rabbits and other rodents also are seeing their food supplies dwindle. But gophers, which eat bulbs, seem to be thriving, Mr. Vacha said.

If rodents get scarce, coyotes can switch to large insects, he said. Foxes are big fruit and berry lovers, so they get moisture from munching on late-summer crops.

But the drought has indirect effects, too. Animals are sharing watering holes, which concentrates their populations and increases the risk of competition, conflict and the spread of illnesses such as hemorrhagic disease in rabbits, fungus in frogs and parasitic whirling disease in fish.

Along the muddy edges of shrinking Nicasio Reservoir, “there’s just a blizzard of animal and bird tracks. The lake is a draw for animals from all around those hills,” said Mr. Vacha. “Other animals are forced into the dwindling damp zones and creek bottoms.” And because there’s less tall grass to shield them, it’s harder for them to hide. This concentration of prey offers a quick meal for predators like coyotes, owls, raptors and mountain lions. Fawns and other young animals, less resilient and unable to flee, are easy targets.

Predicting precisely which species win and lose during extreme weather events remains a major challenge for biology. After the last major drought, a detailed survey of 423 species in the semi-arid grassland of California’s Carrizo Plain National Monument offered a glimpse: smaller animals suffer the most immediate impact, while animals at the top of the food chain are losers when a drought wears on.

But droughts may help boost biodiversity by inducing periodic die-offs of dominant species and creating opportunities for more rare, fast-growing species, according to research by the California Department of Fish and Wildlife, the University of California, Berkeley and others.

This year, just as in previous droughts, animals will adjust, predicted Mr. Vacha.

“Over millions of years, all of our local animals have thoroughly adapted. There’s a lot of flexibility built into the system,” he said. “But right now, everybody’s put to the test.”