The Three Phases of the Vernal Pool Ecosystem

The Wet Phase: Winter in the Vernal Pools

As soon as the winter rains begin to puddle in the vernal pools, tiny creatures called bacteria and protozoa spring to life. Many of them feed on detritus, bits of dead plants and animals that lie on the bottom of the pool. These detritus feeders are, in turn, eaten by many other tiny animals. Microscopic (very tiny) green plants called algae are the next to appear. They are like tiny floating food factories, providing the energy that powers most of the other species in a vernal pool.

Now that the winter pools are full of water and are teeming with bacteria, algae and protozoa, many more aquatic species begin to appear. The water signals to the resting spores, eggs, and cysts of aquatic life that it is time to hatch and grow. Within a few weeks, dozens of species of invertebrates (small animals without backbones) will be living in the pools. Each aquatic species must hurry and complete its life cycle before the pool dries out in the spring.

When vernal pools are full of aquatic life, it’s like putting a meal on the table. Frogs, snakes, birds, and mammals come to the vernal pools for dinner. The food web connects all the species in the vernal pool grassland ecosystem. An ecosystem is a community of plants and animals that depend on one another and their environment for survival.

12b.jpgFact of Life: Not every individual of a species will survive long enough to reproduce. Most will become food for another creature. In an ecosystem the survival of the individual does not matter. The survival of the species is what is important. As long as some individuals reproduce, the species will continue.

We know very little about this ecosystem and the species that call it home. There is so much left to discover. However, we do know one thing for sure: vernal pool creatures need clean water. Clean water is the key to abundant life.


This baby Water Flea is surrounded by microscopic diatoms.  
Protozoa, bacteria, diatoms, and algae are all microscopic,  
meaning we need a microscope to see them at all.


An aquatic beetle larva known as a “Water Tiger” eats a Water Boatman.  
Like many invertebrates, they are macroscopic. Although you can see them  
with your naked eye, you need a magnifying lens to see their parts.

The Flower Phase: Spring in the Vernal Pools

Spring is a beautiful time to visit a vernal pool grassland. As the pools dry down in March, the seeds of vernal pool plants grow in the muddy soil. Over 200 plant species can grow in vernal pools. However, over 60 of these species are endemic to vernal pools, meaning they can grow only in vernal pools and nowhere else. If not for vernal pools, they would become extinct.

poolx.jpgThe plants grow quickly along the shrinking edge of the water. Within four weeks they cover the entire bottom of each vernal pool. As the first plants bloom, their flowers can make colorful rings around the outside of the pools. During the month of April, the display of wildflowers in the pools changes from week to week. By late April, yellow, white, pink, and purple flowers carpet the pools with splashes of color.

Vernal pools are like snowflakes in that no two are exactly alike. While each pool usually has 15 to 20 different species of wildflowers in it, the mix of species can be different in every pool. Plants select their host pools based on the growing conditions of each vernal pool. Some plants prefer shallow pools that dry quickly or the higher, drier areas within deeper pools. Other plants grow where the water is deeper and lasts longer into the spring. Most of the habits of vernal pool plants are still a mystery to scientists.

These pink monkeyflowers grow well  
on bare mounds of soil heaped up by  
pocket gophers that tunnel under the  
vernal pools searching for Brodiaea bulbs.   

12c_0.jpgThese plants bloom late in spring. The bulbs of the Brodiaea 
(larger flower) are a favorite food of Botta’s Pocket Gophers. 
Few vernal pools host Douglas’s Beardstyle; it needs very special conditions to grow.

The Dry Phase: Summer and Fall in the Vernal Pools


Scientists have found Fairy Shrimp eggs (called cysts)  over 100 years old. When the cysts were put in water, they hatched.  This cyst is seen with a special microscope that makes it look  100 times larger than normal. The Fairy Shrimp in the photo is  5 times larger than normal size.     

By the beginning of summer, the soil in the bottom of the vernal pools cracks and dries. Temperatures in the Central Valley reach over 100 degrees Fahrenheit. Most of the vernal pool plants dry up and turn brown. All the aquatic life dies or leaves the pools.

Although the critters are gone, they leave behind the eggs, cysts, spores and seeds that will carry the genes of their species through the long, hot summer. Each species has a way to survive the next eight months of drought. It has taken millions of years for them to adapt to live in this ecosystem. We have yet to discover the special adaptations that allow most vernal pool species to survive in this harsh environment.

Although it can look dry and barren, the grasslands and vernal pools support many species through the summer and fall. The seeds left by spring plants provide food for insects, birds, and rodents. Pocket gophers travel underground through a shallow system of burrows. They search for plant roots and bulbs, while hiding from the watchful eyes of hungry hawks and coyotes. Toads and frogs seek shelter from predators and the drying sun in these rodent burrows. Snakes slither through the dry grass poking their heads into the burrows, looking for dinner. The food web connects many species, even when this wetland isn’t wet.


Waiting for Rain

In the dryness of fall, it is hard to imagine that in a few weeks the winter rains will return. These bone-dry pools will fill up with runoff and the life cycle of hundreds of species will begin again. Vernal pools will add another year to their long history, giving us another year to investigate their many secrets.