Drought Impact and Vernal Pools

What will happen to the vernal pool critters in a drought?

This is a question many people have asked me and my pals at Splash this year.

“How will the animals that live in vernal pools survive a year with so little rain?”   It’s a great question and the answer might surprise you. “Oh, they will be fine,” I say right off.  “We’re the ones who are in trouble!” 


I say this because I know that the animals that inhabit vernal pools have lived on this planet for a long time.  Many of the little crustaceans we see in the wet phase started their evolution millions of years ago.  Fairy Shrimp have been around for 500 million years – 25 million years before fish! 


When you imagine what kinds of extreme conditions have influenced the genetic make-up of vernal pool organisms over millions of years, a drought of a few years is hardly worth mentioning. Try a 100-year drought on for size!  People been in the Sacramento Valley for 14,000 years; some vernal pools were already 200,000 years old when they arrived!  Left the way nature made them, vernal pool critters would easily outlast humans. 


However, our vernal pools are no longer the way nature made them; the impacts of humans on the ecosystem have been huge.  Less than 10% of the vernal pools still exist around Sacramento. Those that remain are being invaded by species of plants that have been carried across the globe by people, accidentally and on purpose.  These plants threaten the very existence of the ecosystem because they change the habitat itself.  Plants create habitat for animals, and the animals cannot adapt quickly enough to the rate of change that is occurring.


As recently as 15 years ago, it was unusual to see annual grasses growing in vernal pools at Mather Field.  Now, many of the shallow vernal pools at Mather Field fill with grass if the pool doesn’t stay full of water.  The grasses push out the vernal pool plants and change the habitat. This invasion seems to advance faster in warm, dry years when winter rains do not keep the pools filled.  I find myself wishing for long, wet winters but we seem to be getting warm, dry winters instead.



So, back to the question:  I’m not so worried about this drought.  It’s the expanding invasion by non-native plants that concerns me and this weather seems to encourage that invasion.   The pools will weather this drought, even if it goes on for years, but I’m not convinced the pools will survive another decade if we do not find a way to combat the weedy invaders that are changing the very nature of vernal pools.