Earth Science Life Science

Vernal Pools: Ephemeral Ecologies

Those of us that enjoy taking a wander into nature are well-acquainted with the habitats around us. We regularly observe the wildlife that calls our forests, grasslands, and coastal areas home – however, it is easy to forget that there is a whole other world that resides just under the surface of the water, hiding just out of our view.

Our freshwater surroundings provide vital habitats for thousands of species, both aquatic and terrestrial, that we rarely delve deep enough to witness. Freshwater makes up only three percent of all the world’s water – yet it provides refuge for over 100,000 species of plants and animals.

The diverse range of freshwater habitats includes lakes, streams, rivers, creeks, ponds, and wetlands, all supporting different communities of wildlife. In this article, we will focus on a type of wetland called vernal pools.

A small cluster of vernal pools between mima mounds near San Diego, California.
A small networked cluster of vernal pools between mima mounds near San Diego, California.
Photo by USFWS | Public Domain

What Are Vernal Pools?

Vernal pools are a unique type of wetland habitat, defined as “episaturated seasonal wetlands that are characterized by a unique assemblage of vegetation and soils”. They are essentially small, seasonal pools that have no permanent inlets or outlets, unlike most ponds and lakes.

This lack of water flow means that they rely almost entirely on precipitation, so are filled up with rain and snowmelt in the winter and spring, but completely dry up for most of summer and fall. They exhibit four stages that cycle with the seasons: a wetting phase, an aquatic or inundation phase, a waterlogged terrestrial phase, and a drought phase.

The pools are typically small and shallow, up to 1m deep, ranging from the size of a puddle to a small lake, and are usually found on sloping grassland plains. In some cases, they are connected by small drainages known as vernal swathes, forming larger complexes of multiple pools. Due to the specific conditions needed, they are often found in clusters.

The soil beneath the pools contains higher amounts of fine silt and clay and has low permeability which helps retain the water in the pond. Water levels are also influenced by the vegetation around the pond: plants create shade which slows evaporation and moderates pool temperatures, as well as taking in water through transpiration. Additionally, plant matter falls into the pool, building up a layer of organic matter in the basin which helps retain water even longer.

Vernal pools are relatively localized – occurring across North America, but typically found on the West Coast, particularly in California and Southern Oregon. They can also be found in glaciated areas of the northeastern and midwestern states – areas that have a Mediterranean climate with dramatic seasonal changes. These distinguishing features, along with the unique assemblage of life that they support, differentiates these pools from other types of wetlands and make them incredibly important to conserve.

Park ranger and biologist look for endangered fairy shrimp in a vernal pool near San Diego.
Park Ranger Lisa Cox and Biologist John Martin look for endangered San Diego Fairy Shrimp in vernal pools. Notice the encroaching housing developments in the background. Expanding human settlement has destroyed approximately 90% of California’s vernal pool habitat.
Photo by Joanna Gilkeson/USFWS | Public Domain

Why Are Vernal Pools Important?

The dynamic nature of the pools means that they look very different throughout the year. During the spring, the ponds will be alive with life and surrounded by wildflowers in bloom; yet by the summer, the cracked clay underneath is barren and brown, appearing completely devoid of life. During a single season, pools may fill up and dry out multiple times, or in years with little rain, some pools may not fill up at all.

However, appearances are deceiving, as even in the summer they provide important functions. The unique cycling of these pools allows many rare plants and animals that require this temporary habitat to survive even in harsh conditions. The organisms associated with vernal pools fall into two categories: obligate species that depend on them entirely for certain phases of their lifecycle, and facultative species that are found in and around the pool but are not completely dependent on them, as they can utilize other wetland habitats.

A large vernal pool on the Santa Rosa Plateau
A large vernal pool on the Santa Rosa Plateau.
Photo by Joanna Gilkeson/USFWS | Public Domain

In addition to providing homes for wildlife, vernal pools also offer us many ecosystem services. A variety of pollutants are washed into water bodies from urban and agricultural land, including soil particles, fertilizers, pesticides, grease, and oil. These toxic chemicals are often attached to soil sediment which, once in the water, is trapped by vegetation, removing the toxins from surface waters. The pools also help remove the chemicals themselves, as vegetation takes up excess nutrients such as nitrogen, and other chemicals can be converted into less harmful forms through processes such as exposure to sunlight.

As some pools are connected to other pools and streams that flow seasonally or with rainfall, the cleaner water often leaves the pools and travels downstream, thereby cleaning other water sources in the process. This mechanism also means that vernal pools help to regulate the water flow of downstream areas and alter their physical characteristics, even changing the course of the waterways. Due to their capacity to hold relatively large amounts of water, they also help to store and slow floodwaters and improve the flood resilience of the area.

Which Species Rely on Vernal Pools?

Obligate species are very sensitive to the duration and timing of the four stages of the pool cycle. These are often amphibians, insects, or other aquatic species that spend the dry season as seeds or eggs, requiring the dry land to germinate, hatch, grow, and reproduce before the water returns. Due to the nature of the pools, fish cannot survive, giving these species a chance to thrive. Some of them are pool-specific, meaning they must return to the pond in which they were born to breed, which is difficult if the pond does not return. Pool-specific animals are incredibly localized: one study found that 17 out of 67 species surveyed were found in only one pond.

Footage of a vernal pool near San Diego captured by USFWS biologist David Zoutendyk. It has been stabilized and annotated by The Common Naturalist.
This derivative work is released under Creative Commons license CC-BY-SA by The Common Naturalist.

Vernal pools provide essential breeding habitat for many amphibians, such as the marbled salamander (Ambystoma opacum) that relies on eastern ponds, the spotted and blue-spotted salamanders (A. maculatum and A. laterale) that rely on pools in Maine, as well as Jefferson salamanders (A. jeffersoniamum) and wood frogs (Rana sylvatica).

Fairy shrimp (order: Anostraca) are the archetypal temporary pond species. They lay their eggs on the dry ground during the summer months, as they can survive desiccation during the drought period. They lie dormant until the Fall rain refills the ponds, and within 48 hours they hatch out and can begin to breed within 3 weeks. Speed is essential: they must grow and breed before their ponds dry out again. Otherwise, their lifecycle is cut short, and they are not able to reproduce. In some cases, eggs can lie dormant for many years, acting as an ‘egg bank’ in case the ponding period does not last long enough for the current adults to breed.

Close-up photo of a male San Diego Fairy shrimp, a species that lives only in southern California’s vernal pools.
Photo by Joanna Gilkeson/USFWS | Public Domain

As well as animals, there are some plant species that are well adapted to this instability: most pools will include between fifteen and twenty-five different types of plants. In Pennsylvania, there are five species of special concern that are known to utilize seasonal pool habitats: small beggar-ticks (Bidens discoidea); lance-leaved loosestrife (Lysimachia hybrida); Oakes’ pondweed (Potamogeton oakesianus); spotted pondweed (Potamogeton pulcher); and northeastern bulrush (Scirpus ancistrochaetus), a federally endangered plant found almost exclusively in Pennsylvanian seasonal pools.

Image of a Spreading navarretia in a dried up vernal pool in California.
Spreading navarretia is a vernal pool plant that comes alive after the vernal pool dries up. Spreading navarretia generally range along the California coast from the border of Mexico to San Luis Obispo County.
Photo by Joanna Gilkeson/USFWS | Public Domain

While obligate species cannot survive without vernal pools, facultative species can make use of other areas but take advantage of the resources that the pools provide. They often have physical or behavioural adaptations that allow them to utilize the habitat. This includes non-pool specific amphibians that can breed in other aquatic areas, such as the red-spotted newt (Notophthalmus viridescens), northern spring peeper (Pseudacris crucifer), and American toad (Bufo americanus).

Numerous insect larvae thrive in wetland areas, such as caddisfly larvae (order: Tricoptera). Caddisflies construct protective cases from stems, leaves, and woody material, and although they live in many freshwater habitats, they are an excellent addition to the pools and an indicator of good water quality.

The amphibians in the pools provide important food sources for some small carnivores, that are also classed as facultative species. This includes wood turtles (Glyptemys insculpta) and spotted turtles (Clemmys guttata), and the eastern garter snake (Thamnophis sirtalis) that feeds on tadpoles and salamander larvae. Shorebirds and waterfowl such as egrets, ducks, and hawks also make use of the seasonal food and water sources.

A western spadefoot toad tadpole in a vernal pool in southern California.
A western spadefoot toad tadpole in a vernal pool in southern California.
Photo by Joanna Gilkeson/USFWS | Public Domain
Spotted salamander (Ambystoma maculatum) which is known to occur near vernal pools.
Spotted salamander (Ambystoma maculatum)
Photo by Peter Paplanus

Verbal Pool Conservation

The ecological importance of these pools is clear, but unfortunately due to their size and nature, vernal pools are very sensitive to environmental conditions. In the US, it is estimated that 50% of pools have disappeared since European settlement, and in California, around 90% of pools have been destroyed. This is largely due to climate change, habitat loss, and fragmentation, pollution and changes in water chemistry, changes in hydrology, and the introduction of invasive species.

This loss means that many of the species that rely on the pools, particularly the obligate species, are also lost. This may lead to local extinctions, a decrease in biodiversity, and a decline in food availability for facultative species which affects the entire ecosystem.

Next time you are out and come across a small pool, make sure you take a peek under the water to discover what is living there and see a snapshot of the dynamic lifecycle of these incredible ecosystems – they may not be there for much longer!

Author Credits

This post was written by guest author Carla Broom for The Common Naturalist.

References and Further Reading

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