MERCED NATIONAL WILDLIFE REFUGE -- The endangered California tiger salamander, tadpole shrimp and vernal pool fairy shrimp went homeless last year in the Snobird unit of the Merced National Wildlife Refuge.
Dry conditions didn't let their sensitive habitat of vernal pools fill with the rainwater they needed.
This year, however, the rain did come.
The invertebrates that depend on this special type of wetland were again able to swim, breed and complete their life cycle. Bright yellow and purple wildflowers sprang up in halo-shaped rings around the pools.
But with a dry March, pools are evaporating fast. Earlier wetter conditions made the wildflowers -- goldfields, tidy tips and lupin -- flourish. But unless more rain falls this spring, they will disappear, along with the water bugs that live within the pools.
At least, until the next good water year comes.
Vernal pools are naturally occurring wetlands made by a depression in the landscape with a hard-pan clay bottom, said Jack Sparks, outdoor recreation planner for the wildlife refuge.
They fill with rainwater in fall, winter and early spring, and dry up when that water evaporates. "Vernal pools are completely controlled by nature -- that defines them," he said.
And they are unusual, said Alice Poulson, an ecology and sustainability graduate student from California State University, Stanislaus. She is helping the refuge with its comprehensive conservation plan, and she accompanied wildlife managers to tour the vernal pools last week.
"It looks like a mud puddle, but it is so much more than that," she said next to a pool. "It's such endangered habitat."
Other wetlands in the refuge are filled artificially. Wildlife managers mimic what nature would do if rivers flowed nearby, filling and draining the wetlands according to the season.
One reason vernal pools are so important is that their diverse water chemistry and soils cater to certain sensitive species, said Richard Albers, refuge manager. The California tiger salamander and invertebrates, such as tadpole shrimp, vernal pool fairy shrimp and conservancy fairy shrimp, are among these.
Important native plants, such as bunch grass, thrive around vernal pools. But unfortunately for a healthy ecosystem, the area is about 95 percent taken over by invasive species.
This habitat is so sensitive that the half-mile-wide, 3-mile-long Snobird unit isn't even open to the public. Neither is the nearby Arena Plains unit, which also contains vernal pools. Private tours occasionally are offered, and sometimes public tours during healthy water years.
Wildlife managers are conducting a study to determine what types of invertebrates, animals without backbones, are using specific vernal pools.
"Do they have a determining factor?" Albers asked. "Size, depth, water quality, soil chemistry, the life cycle of all these species that utilize them. Get a better idea of what's driving this."
Scientists also are researching what effects certain types of management -- such as prescribed burns, razing or leaving the area alone -- have on the habitat.
They wanted to start the study last year, but it was too dry. This is the first year of the study, but it likely will end within the next month or so as the pools dry up.
The project requires three years' worth of data, Albers said. So research will start again next year if conditions are right.
The information is important to gather as the habitat changes over time. "Groundwater runoff, the health of the pools ... species disappear," he said. "We need to know what's changing. Are we managing properly?"
Albers knelt down to examine a tuft of butter and eggs, a collection of tiny yellow and orange wildflowers related to alfalfa. Patches of goldfields, golden-yellow wildflowers, carpeted the grasses that spanned either side of him.
But the 38 vernal pools that feed their surrounding array of flowers are 60 percent to 70 percent dried up.
The pools often stay full into March, April and even May before they evaporate. Wildflowers bloom as the moisture begins to recede in the spring. They popped up last year, although the pools never filled, but the flowers weren't as showy, Sparks said.