Vernal Pools: Islands of Life
By Jon Aull
Vernal pools are islands of life in the immense sea of the Central Valley of California.
Just a few hundred years ago, the valley was a vast grassland with herds of thousands of elk and pronghorn antelope roaming freely. Today, the grasslands cover less than one percent of their historical range. Most have now been converted to some of the richest agricultural land in the world. We're lucky to have a small remnant of these original grasslands in Bidwell Park and around Chico, where we can find native grasses like Deer Grass and Purple Needle Grass, as well as a spectacular display of native wildflowers.
Some of the most dazzling displays are in those little islands called vernal pools.
Like islands, they host a variety of wildlife that isn’t found anywhere else. Vernal pools are seasonal wetlands that are dry for 8 months of the year. They are low points in the grasslands, underlain by impermeable hardpan or clay, which acts like the bottom of a bathtub. In Bidwell Park, the underlying hardpan is part of the Tuscan Formation, a series of volcanic mudflows, or lahars, that were deposited from about 2-4 million years ago and solidified into rock. This is what you see on the North Rim Trail and through the Upper Park along the creek downstream from Bear Hole. Because the pools get more water than the surrounding grasslands, the life in the pools can be quite different.
In California, we have a Mediterranean climate, with basically two seasons, a dry season and a rainy season.
When the winter rains come, the thin soil topping the hardpan can only absorb so much of the water, and the rest runs off on the surface. Some water can permeate deeper into the ground through cracks and filter farther into the ground. The saturated part of the ground is called the aquifer, and it is where we get our drinking water in Chico. The water that isn’t absorbed by the ground runs to the lowest point, which in Bidwell Park would be Big Chico Creek. Some water is trapped in these bathtubs called vernal pools. Their thin soil is dry and cracked in late summer and fall, but when they fill up with rainwater, they start to come to life.
Vernal Pool Tadpole Shrimp
In this wet winter phase, tiny organisms like bacteria, protozoa, algae, and invertebrates come to life and feed on detritus, rotting vegetation from last year's growing season.
Larger animals stay in suspended animation, just waiting for the brief respite from the parching summer heat. Fairy shrimp are about an inch long, have long bodies, big eyes, and 11 pairs of legs. (The locally occurring Conservancy Fairy Shrimp is an endangered species.) Shrimp swim by doing the wave with their legs, swimming in circles and loops in a graceful ballet. Some species of fairy shrimp are 50 million years old. Tadpole shrimp (like the also endangered Vernal Pool Tadpole Shrimp) are some of the most ancient creatures on earth. They look like tiny two –inch horseshoe crabs with 35 pairs of legs. Both types of shrimp make cysts, a sort of an egg with a fully developed embryo inside. Some say these cysts can survive for 100 years in suspended animation before rain comes to free them, but no one really knows how long they can survive. Since they are fully developed embryos, they are able to hatch within a day or two of a rain and get a head start on their short life cycle. Brine shrimp, sold as “sea monkeys” have a similar reproductive strategy.
Animals flock to these pools to enjoy the bounty.
Western Spadefoot toads, salamanders, birds and small mammals come to feed on the aquatic critters. Spadefoots are about two-inch long green to grey colored toads with vertical pupils. They don’t have the prominent poison glands behind the eyes that our more common Western Toads have, but when handled they can release a mild poison that is reported to cause sneezing and to smell like roasted peanuts. They spend eight to ten months of the year underground, immobile, living off of fat reserves, and emerging with the winter rains. They feed in vernal pools on invertebrates like shrimp and insects. These toads can lay their eggs in vernal pools in relative safety, since these seasonal pools don’t host predatory fish. The eggs are laid in February or March, before the toads go back into their hot weather hibernation, or aestivation. The eggs hatch in as little as two days, and the young must develop quickly in the drying pools. If they are to survive, they must find enough food and store enough fat to make it through the long dry season aestivation. The Spadefoot is a federal and state listed species of concern under the Endangered Species Act.
Vernal is Latin for spring, and these pools get their name from their impressive wildflowers.
Masses of yellow Goldfields, purple Downingia, and white Meadowfoam, Navarretia, Popcorn-flowers, and Wooly Marbles make rings around the evaporating pools. No two vernal pools are alike. Over 200 species of plants grow in vernal pools, and 60 of them are endemic, growing nowhere else. If not for vernal pools, they would become extinct. One of these endemic plants is Butte County Meadowfoam, another endangered species found only in Butte County vernal pools. While valuable in its own right, it also contains oil that is a very good lubricant under high temperature and pressure. This oil is a suitable replacement for sperm whale oil, made from blubber and used for the same purposes of lubricating fine machinery. Thus, meadowfoam oil can save the lives of the many whales killed each year to supply this need.
Meadowfoam, like most vernal pool flowers, is pollinated by native solitary bees.
Many vernal pool flowers are pollinated by only one species of bee. These are relationships that have developed over thousands of years. Without their specific pollinators, these flowers would disappear completely.
When we think of bees, we most often think of the hive building social European Honeybee, a non-native species.
But nearly all species of bees live alone, like our native solitary bees. They are smaller and usually black or grey, looking much like flies on first glance. Rather than building hives, they dig nests to lay eggs. The female digs a main tunnel with brood chambers off to the side lined with wax. She makes a ball of collected pollen and nectar and lays one egg on it. The brood chamber containing the pollen and egg ball is then sealed with wax. A mother may make 10 of these brood chambers. The bee spends most of its life in the chamber, spending the summer eating the pollen ball as a larva and metamorphosing into an adult in the fall. Most bee species reproduce in this way. Only honey bees, bumble bees, and stingless bees are social and live in hives. Vernal pool solitary bees spend the winter underground, only emerging when their host plant is in bloom. Researchers at UC Davis have discovered that these bees can remain underground for up to four years, avoiding drought years and waiting for the ideal conditions when their host flowers are blooming. How the bee knows the perfect time to emerge is a mystery.
As springtime temperatures in the valley soar to 100 degrees, the soil dries and cracks, leaving huge fissures.
The aquatic life in the vernal pools is the first to die. They leave their eggs or cysts in the drying mud for the next generation. The amphibians go into their underground aestivation. Next, the plants turn brown and die, leaving seeds for next season.
One way to see vernal pools in the Park is to take a walk around the periphery of Wildwood Park in spring.
There are interpretive signs that point them out. As the wildflower display changes weekly, it’s like looking back in time 100,000 years, when the valley was a much different place and these pools were common at the base of the foothills. This annual renewal is a reminder of how much we depend on the life-giving winter rains, and how much we would be impoverished without the ever-changing tapestry of Butte County and Bidwell Park’s vernal pools.