Vernal Pools

Vernal Pool,ephemeral pool, vernal pond – Literally, an area of pooled water associated with the spring months that begin with the vernal equinox (equal night ~ 21 March) and end with the summer solstice (sun standing [still] ~ 21 June).

Vernal pools are seasonal wetlands that result from the accumulation of snowmelt and spring rains in low lying areas that provide a unique habitat for many plants and animals, notably amphibians. In general, the aquatic habitat is ephemeral or short-lived, as the heat and reduced precipitation of summer promote evaporation.  Eventually, only the cracked mud and silt remains, a condition  that can persist through the autumnal months. As the annual progression of seasons continues due to earth’s tilted-axis solar orbit, cooler and wetter conditions refill the pool. Many species have adapted to this cycle, living and procreating as the aqueous environment allows … estivating, desiccating, or migrating in the dryness of the summer months. Depending on local environmental factors and weather patterns, some pools may go through several cycles of filling and draining in a single year. When overall precipitation is above normal, they may persist through an entire year; areas with rock or clay substrates reduce drainage losses. Conversely, during extended droughts, vernal pools may fail to fill, drying out for the duration. [1]

The raison d’être for the coevolution of many species to a precarious reliance on an unreliable habitat like a vernal pool is the absence of fish predators. Life in the aquatic habitats of oceans, seas, lakes, and streams is a hierarchy of predation that is the epitome of survival of the fittest. At the top of the food pyramid are the streamlined, killing-machine sharks with rows of dagger teeth that are fearful even to humans, the most predatory of all animals. That fish eat other fish caught the eye of Benjamin Franklin as he travelled by ship, noting that when cod were hauled on board “I saw smaller fish taken out of their stomachs.” His on and off vegetarian regimen inspired by the works of Thomas Tryon thereafter included fish, since “ … if you eat one another, I don’t see why we mayn’t eat you.” [2]  The oceans abound with clever tactics to find food but avoid becoming food that range from the secretive lives of octopodes with eight suckered arms surrounding a beaked mouth that escape in a cloud of opaque black ink to the blue whale, a literal leviathan that consumes upwards of one ton of small crustaceans called krill every day. The cleverest of all of  the aquatic animals were those that moved from the littoral onto the shore to escape the fish carnage as amphibians during the Devonian Period about 400 million years ago.  The vernal pools are their palladium for the same reason.

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Wood frogs are the most visible, audible, and frequently encountered denizens of vernal pools.  The reason is that they are obligate … they require an aqueous habitat for egg deposition and tadpole maturation. The sexual positioning of the male and female frogs depicted above is called amplexus (Latin for embrace) and can persist for days.  Eggs are fertilized individually by male spermatozoa activated by contact with the water so they gradually float up and around the embracing couple. The protective and nutritive eggs are round sacs of yolk surrounding the frog embryo protected by a surface coat. They coalesce into softball size globules each with about a thousand eggs that float just beneath the pool surface like billowing clouds. The eggs, once adrift,  are entirely on their own. [3]  After several weeks, they hatch into tadpoles (Middle English for “toad head” as that is what they seemed to be). With a “beat the clock” mandate, tadpoles must mature to adulthood, develop lungs and venture ashore before seasonal heat turns the vernal pool into a slough of despond.

Obligate vernal pool fauna can be subdivided into four basic categories according to the timing and duration of habitat occupation. Those that arrive in the spring, mate and move on are non-wintering spring migrants, like wood frogs. The other three categories all involve over-wintering, surviving the dry period in the form of eggs or cysts that are drought resistant or as larvae or juvenile adults capable of burrowing to residual moisture. Some are year round as perpetual denizens and some are either spring or summer recruits, leaving the pool area during a portion of the annual cycle. [4] And then there are the facultative animals that use the vernal pools as a matter of choice but are not obligated to do so.  They are drawn to the vernal pools for the same reasons that animals are drawn to water generally. Some come to drink the essential elixir of life. Herbivores come to feed on the plants growing where there is water and carnivores come to prey on anything looking for food or water. This group would include the primarily aquatic green and bull frogs that will eat anything smaller than their considerable bulk, and the primarily terrestrial American toads that use their extended sticky tongues to zap the insects that swarm there. Adult eastern, red-spotted newts are the most notable of this category, swimming through the water to consume the eggs of other amphibians after wandering about the forest as juvenile red efts looking for a new home.

Vernal pools, like any ecological niche habitat, operate according to nature’s law of supply and demand where everything has its place in the food chain. In this the invertebrate world, the amphibian vertebrates are the apex species that must be eluded whenever possible. Bipedal terrestrial humans are myopic megafauna, oblivious to the teeming masses of the miniscule that exist sight unseen. Adult and larval insects of all sizes and shapes, crustaceans  such as fairy shrimp and tiny copepods, and a variety of freshwater hydrozoa coexist by feeding on each other, and in many cases on amphibian eggs, prolific for that reason. How they manage to survive and replicate is exemplified by fairy shrimp, vernal pond obligates whose eggs are transported unwittingly by migrating waterfowl like mallards who eat gravid females in one pond and then fly to another. Their eggs are called cysts as they form a protective sac that can survive for decades even after being subjected to extremes of temperature ranging from near absolute zero to boiling. [5]  Dragonfly, damselfly, and caddisfly larvae occupy the middle ground as food for the adult frogs and toads and as predators of amphibian eggs and smaller invertebrates and crustaceans. [6] The real survivors are the hydrozoan hydras that can regenerate incised body parts and appear to be immortal, a matter of some interest to the study of human senescence and longevity. Since they reproduce solely by asexual budding, their stem cells must be continually renewed to sustain unmutated genetic function. [7]  The vernal pool may then also be the fountain of (eternal) youth.

Wetlands in general and vernal pools in particular are genetic repositories, a fact that has elevated their importance as habitats in need of protection. As is the case with many environmental concerns, wetlands are vulnerable to the tragedy of the commons … any resource  used by everyone and owned by everyone in common will become despoiled and no longer usable absent some usage rules. In the case of water, tragedy may not be a strong enough word as it is fundamental to almost all living things. The only recourse is government regulation to prevent their witting or unwitting annihilation, an intrusion seen by some as encroaching on the private property rights of landowners. The generic term wetlands came into use in the mid 1970s to definitize a panoply of colloquialisms such as bog, fen, swamp, and marsh. According to the Federal Geographic Data Committee standard FGDC–STD-004-2013, wetlands are “lands transitional between terrestrial and aquatic systems where the  water table is usually at or near the surface or the land is covered by shallow water.”  However, there is no absolute line of demarcation from either the physical or the ecological perspective between wet and dry areas as there is a continuous gradation from one to the other. This is particularly true of vernal pools that are on the fringe by their very nature; any mud puddle on a rutted dirt road could qualify … occasionally tadpoles can be seen squirming in them. The issue of what is or is not a wetland is generally moot until a change to the land is proposed. Not infrequently this involves developments like housing tracts or roads to access them. The generally accepted though far from ideal work around is called mitigation: wetlands can be destroyed as long as new wetlands are artificially created to compensate for the loss of habitat. [8]

The destruction of wetlands due to urban sprawl is one of the more persistent aspects of the human footprint that defines the Anthropocene.  The extent of the problem is at best a rough estimate due to wetland diversity, the lack of historical record, and inadequate survey data due in no small part to their ephemeral nature. Historical soil data in the Central Valley of California indicate that vernal pools occupied over 4 million acres a century ago. Less than one fourth of that amount remains ― one third of obligate crustacean species are thought to have become extinct. [9] The problem is most evident in the northeast and mid-Atlantic states where land is largely owned by  private parties of expanding populations. As a case in point, the town of Orono hosts the University of Maine and must account for housing needed for students, faculty, and support staff. A fifty seven home project on a forty acre tract on undeveloped land that had been approved by the town planning board ran afoul of the Army Corps of Engineers ruling that a vernal pool  could not be filled in.  The reduction in the allowable number of houses rendered the project uneconomical and it was cancelled. While this was a victory for the environment, it surely did not go over all that well with the Orono town council nor with the local chamber of commerce. [10]

The counterpoint to the tragedy of the commons is the erosion of democracy as citizens become frustrated with regulations they perceive as intransigent and arbitrary. Mitigation seeks to balance the commonweal of natural resources with the shared benefits to society of economic prosperity. But for the mitigation replacement of wetlands to count, it must be with an alternative that sustains the unique plants and animals that live there. An excavated depression filled with rocks and surrounded by berms that sometimes passes as ersatz wetland is not a habitat for wildlife but an opportunity for invasive species. The United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) established the Wetland Mitigation Banking Program (WMBP) to incentivize a better tradeoff. Using the concept of credits like that implemented to reduce acid rain, the system works by allowing the purchase of offsets in the form of wetland restoration, enhancement, or creation by any commercial or personal entity that seeks to fill one in somewhere else. Restoring or enhancing extant wetlands are the easiest to implement, mostly by establishing perpetual conservation easements. Creating a new wetland is a formidable task; engineering the ecology that nature does naturally requires planning and attention to detail. The plan must include the hydrology of water flow  management and  the establishment of soil type and consistency to ensure that fundamental qualities of a wetland habitat are retained. Flora and fauna considerations follow with the planting or  promotion of wetland vegetation to grow and attract amphibians, crustaceans, and hydras to the proposed oasis. [11] Joyce Kilmer poetically observed that only God can make a tree. This applies equally to aboriginal wetlands but mitigation is better than nothing.

References:

1. https://www.epa.gov/wetlands/vernal-pools    

2. Stuart, T. The Bloodless Revolution, A Cultural History of Vegetarianism form 1600 to Modern Times, W. W. Norton and Company, New York, 2006, pp 243-244.

3. Book, R. The Frog, Its Reproduction and Development, The Blakiston Company. Chapter 3 https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Frog_Its_Reproduction_and_Development_3    

4. https://mnfi.anr.msu.edu/abstracts/ecology/vernal_pool.pdf   

5. Brennan, D. “Vernal pools: Rains bring to life mini-ecosystem of button celery, Otay Mesa mint and fairy shrimp” The San Diego Union-Tribune, 1 April 2019.

6. Johnson, S. Vernal Pools, Documenting Life in Temporary Ponds, North American Nature Photography Association, 2021, pp 46-55.

7. Boehm, A. “FoxO is a critical regulator of stem cell maintenance in immortal Hydra” Proceedings of the National Academy of Sciences Volume 109 Number 48, 27 November 2012 pp 19697 – 19702. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3511741/  

8. https://www.fws.gov/wetlands/documents/Classification-of-Wetlands-and-Deepwater-Habitats-of-the-United-States-2013.pdf   

9. Brown K. “Vanishing Pools take Species with them” Science  31 Jul 1998: Vol. 281, Issue 5377, p 626.

10. Adams J. “Pooling Resources” Science  2 Oct 2015: Vol. 350, Issue 6256, pp. 26-28

11. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_025863.pdf