Build a Better Vernal Pool - Top 5 Reasons for Failed Projects

Natural vernal pool - photo from WarwickMA.org

Every year, I work with landowners, agencies, communities, and schools on wetland projects that they describe as "Our Vernal Pool Project."  What they envision is very typical of the Mid-Atlantic's historic landscape - spring ("vernus") flooded wetlands that dry out every summer, and are refilled in the winter by snowmelt or rain.  Biologists and environmental educators have spoken loud and proud about the importance of these habitats to migrating wetland birds and to a long list of amphibian species who are losing habitat up and down the east coast.  As a wetland restoration ecologist, I love that! 

These tadpoles will probably run out of water and
die before having the chance to metamorph
into adult toads - photo from Owlsland.com

However, the results over the years have been pretty disappointing.  Simply put, the projects aren't working the way vernal pools should. And if they are supposed to have "educational value," there are some additonal problems based on the fact that these wetland projects just aren't working, and in some cases, are having a negative impact on certain amphibian populations. Poorly designed or constructed habitats of any type can be a population "sink" rather than a "source," as reproducing adults have offspring in the low quality habitat.

Top Five Reasons that Vernal Pool Projects Fail

5.  Failed long term maintenance of  project site

4.  Incoherent or impossible project goals

3.  Habitat has poor structure

2.  Improper soil

1.  Improper hydrology

What does it take to make vernal pools really work?

1. Sufficient hydrology - For a vernal pool to be functional as amphibian habitat, it needs to provide habitat for the entire larval period of amphibians, plus provide habitat for prey items of the amphibian larvae.  The single biggest error in vernal pool design and construction is to build pools that are not sufficiently deep (or are too well drained) to stay flooded into late June, when the last amphibian larvae, notably toad and tree frog tadpoles, transform and move into drier habitats.  


During typical east coast winters and early springs, many areas appear to amphibians (and humans) to be wet enough to serve as wetland habitat.  The problem is that these pools often dry up before the majority of larvae can survive on land.  Even before the pool has dried up, smaller volumes of water mean high densities of amphibian larvae - which mean great hunting for raccoons and birds.  


While this is a natural phenomenon in existing low-lying areas and seasonal wetlands, it is unacceptable to "not be sure" if a project you're designing will be too shallow or evaporate/drain too quickly to  perform many of the critical functions of vernal pools.  Successfully designed vernal pools have deeper areas called "refugia" that will remain flooded (and free of fish) into the summer.

A successful vernal pool may look like this in July.  A failed (built) or false (natural) vernal pool may look like this in May.  The difference between the two is site selection, good design, and proper construction method.

False vernal pool in winter, courtesy of www.HikingwithChuck.com

2. The right soil type.  The highest functioning vernal pools are usually located on the most poorly drained soils.  High clay content (drainage class C and D soils) means that the pool will work as an isolated system, both hydrologically and microbiologically. It also gives the pool the greatest possible chance of remaining wet and functioning well into the growing season. If soils are well drained, some biologists will use a plastic liner or a clay "pan" to mimic the poor drainage.  Whether this is worthwhile depends on what the goals (and expected lifespan) of your project are.  Like most things in restoration ecology, you're best served by investigating existing (natural) vernal pools in the area and see how their soil stacks up (clay content, nutrients, acidity, carbon content).  If you're lucky, you can model to that benchmark and design toward it.

Nice vernal pool, photo courtesy of www.field-notebook.com

3. Habitat with real physical structure.  A vernal pool isn't just the wet hole in the ground.  It's the connection to groundwater, the connection to surface runoff, and the interaction between soil, plants, and water. A "vernal pool" built in the middle of a mowed grass yard or manicured school ground is not going to be good for amphibian populations.  The same project built along a timber road under tall trees, with tree roots dangling into the vernal pool and tall wetland vegetation growing along the sides, could be great for amphibians.

Created vernal pool with significant habitat structure.  Photo: Tom Biebighauser, USFS

4. Project goals that make sense and reflect the project site and budget - This is an idea that I try to spread far and wide - that you must begin with the end in mind - any earthmoving project should have discrete, reasonable goals before the first shovel of dirt has been moved.  Small wetland projects most often cannot be truly great habitat AND provide great pollution reduction AND provide high quality outdoor recreation for people.  And that's why it disturbs me when I read project descriptions that say, "this vernal pool will reduce 90% of nitrogen pollution and provide habitat for endangered salamanders, in a place where 6,000 students will visit on school trips twice a year to learn about wetlands."  Sorry......that's not going to happen. 

Sediment entering a restored wetland from this
pipe may eventually convert the wetland to an
upland if the sediment is not periodically
removed by volunteers or maintenance crews
(photo: Cacapon Institute)

5. Sustainability through maintenance and stewardship - this has been a dirty secret of the habitat "industry" for the last 20 or more years.  Sure, a restored or created wetland looks great when it is 1 or 2 years old.  What happens after 5 years? Or 30 years? Is it still there? Did somebody fill it in?  The working assumption of habitat ecologists has been that wetland restoration projects are "self maintaining."

Hint: It's not true.  The wetland you've designed and created is an ecological snapshot.  If you'd like it to remain the same, that "snapshot" will have to be maintained.  The physical components of the wetland - soil, hydrology, and vegetation - will have to be managed by someone.

Otherwise, nature will take its course.....remember - wetlands come and go over time.  Is a long term conversion of your wetland to an upland a positive outcome for your project? If not, consider long term maintenance.  Here's two ways to do that:


1. Engaging volunteers throughout the project
As the designer or ecologist, you won't want to do this.  You don't even have to tell me. It's complicated.  And it's difficult. But maximizing the number of people involved in the project is one of the best ways to make sure it gets taken care of (and not totally forgotten) over the long run.  Reality: the average job in the environmental field is between 2.9 and 5.2 years, depending on which study you believe.  You will move on, and when you do, you will probably leave your vernal pool projects behind.  Volunteers will remember it and occasionally check on it, though. And they may just hold somebody accountable!

2. Engaging the maintenance staff of the property throughout the project.
Gosh, this is a big one too.   It's great that a refuge manager or a school principal tells you, "Sure, we'll take care of those vernal pools you're building!"  But you need to talk to the men and ladies with the lawnmowers, backpack sprayers, and weedeaters.  Will they leave the project in place? Watch over it? Your project will be well-served if you take the time to educate maintenance staff about how the vernal pool is supposed to look, and what they (staff) are supposed to do to take care of it (or leave it alone).

Yes, O Mighty Wetland Designer, these guys will be in charge of your project
after you kill the budget and wrap up the project.
Better be nice to them and tell them how to take care of it!

I hope this gives you something to think about as you are planning to build a vernal pool on your property or someone else's.  Vernal pools are amazing projects that can be restored or created without tens of thousands of dollars of work, and so I hope no one finds this post discouraging.  Just proceed forward consciously (not even cautiously) and be very aware and open about what your project goals and limitations really are.  Good luck!

Creating a small vernal pool in Maryland. Photo: Wayne Davis, EPA