Vernal pool with clay hardpan bottom, Vina Plains Nature Conservancy Preserve, California, United States
Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe development of natal amphibian and insect species unable to withstand competition or predation by fish. Certain tropical fish lineages (such as killifishes) have however adapted to this habitat specifically.
Vernal pools are a type of wetland. They can be surrounded by many communities/species including deciduous forest, grassland, lodgepole pine forest, blue oak woodland, sagebrush steppe, succulent coastal scrub and prairie. These pools are characteristic of Mediterranean climates, but occur in many other ecosystems.
Generation and annual development
An inundated rock vernal pool on Enchanted Rock. Note the one inhabited by cacti in the background.
During most years, a vernal pool basin will experience inundation from rain/precipitation, followed by desiccation from evapotranspiration. These conditions are commonly associated with Mediterranean climate such as the Central Valley of California.[1] Vernal pool basins are often characteristics of Mediterranean climates, but occur in many other ecosystems, such as forested areas of the Canadian Shield, where they are difficult to identify because of the forest canopy.[2] Most pools are dry for at least part of the year, and fill with the winter rains, spring snowmelts and rising water tables. Some pools may remain at least partially filled with water over the course of a year or more, but all vernal pools dry up periodically. Typically, though, a vernal pool has three phases each year: it is inundated in the winter (inundated phase) with the vernal pool holding onto the water from 10–65 days, it dries slowly during the spring (flowering phase), and it dries completely during the summer (dry phase). Vernal pools favor native species because many non-native species cannot tolerate the extreme seasonal changes in environmental conditions.
Some vernal pools have an underlying impermeable clay layer (also known as a hardpan) that reduces water percolation. The impermeable layer is hydrophobic and it prevents water from draining into lower soil layers, allowing vernal pools to become inundated for a very long period of time. This feature of vernal pools means that the water is allowed to slowly evaporate instead of draining. This is a key factor in the development of vernal pool plant communities as it keeps the soil at the waters edge just wet enough for vernal plant communities to flourish while those closer to the center of the pool are more inundated, leading to zonation of plant communities as the water level recedes. This clay layer also allows pools to exist long enough to prevent upland species from developing, while existing for just enough time to prevent aquatic plant species from taking over.[3]
Some authorities restrict the definition of vernal pools to exclude seasonal wetlands that have defined inlet and outlet channels. The justification is that such seasonal wetlands tend to be qualitatively different from isolated vernal pools; this is because they are fed by larger drainage basins so that firstly, inflow contributes higher concentrations of dissolved minerals. Secondly, flow patterns increase the periodic scouring and silting effect of flows through or simply into the wetland. Thirdly, longer distance inflow and outflow make for less strictly endemic populations and plants. Low dissolved mineral concentrations of smaller vernal pool basins may be characterized as oligotrophic, and poorly buffered with rapid pH shifts due to carbon dioxide uptake during photosynthesis.[4]
Vernal pools are so called because they are often, though not necessarily, at their maximum depth in the spring ("vernal" meaning of, relating to, or occurring in the spring). There are many local names for such pools, depending upon the part of the world in which they occur. Vernal pools may form in forest, but they are more typically associated with grassland and rocky plains or basins. While many vernal pools are only a few meters in width, playas and prairie potholes are usually much larger, but still are otherwise similar in many respects, with high water in wet periods, followed by dry conditions.[5] Some exclude desert playas from the definition of vernal pools because their larger closed drainage basins in areas with high evaporation rates produce higher concentrations of dissolved minerals, with salinity and alkalinity favoring different species. Playas may be inundated less frequently than vernal pools, and inundation typically coincides with colder weather unfavorable for plant growth.[6]
Ecology
Despite being dry at times, vernal pools teem with life when filled, serving as critical breeding grounds for many amphibian and invertebrate species. The most obvious inhabitants are various species of breeding frogs and toads. Some salamanders also utilize vernal pools for reproduction, but the adults may visit the pool only briefly. Other notable inhabitants are Daphnia and fairy shrimp, the latter often used as an indicator species to decisively define a vernal pool. Other indicator species, at least in New England, are the wood frog, the spadefoot toad, and some species of mole salamanders.
While vernal pools can be devoid of fish, in some habitats such as African Savannah vernal pools, killifish may, due to their ability of their eggs to survive desiccation, co-exist with their branchiopodcrustacean prey which utilize the same strategy to survive dry periods. Some killifish mature in only three to six weeks in order to make the most use of temporary pools before they disappear.[7]
Some of the species within vernal pools are endangered. Fairy shrimp are crustaceans in the family Branchinectidae. It takes about 30 hours for them to start to hatch in water and it takes 50 days for them to mature. In springtime, the eggs hatch and they can go dormant. There are different types of fairy shrimp in different vernal pools because the pools can act like islands because they are so isolated.[citation needed]
Certain plant species are also associated with vernal pools, although the particular species depend upon the ecological region. The flora of South African vernal pools, for example, are different from those of Californian vernal pools, and they have characteristic Anostraca, such as various Branchipodopsis species. In some northern areas, tadpole shrimp are more common. Some vernal pool inhabitants are becoming threatened due to habitat loss. One of these inhabitants is the California Tiger Salamander.[8]
Vernal pools harbor a distinct assemblage of flora and fauna that, in some cases, aren't found anywhere else on the planet. Despite this fact, about 90% of vernal pool ecosystems in California have been destroyed. Disturbingly, much of this destruction has occurred in recent years, with about 13% of remaining vernal pools being lost in the short interval from 1995–2005.[9] The major threats to vernal pool habitats in the Central Valley are agriculture, urbanization, changes in hydrology, climate change, and improperly managed grazing by livestock. They are sensitive to climate and land-use change.[citation needed]
Restoration
Vernal pools are prime habitats to be targeted for restoration work due to their value as hotspots of biodiversity as well as recent history of extensive destruction and degradation. However, there have been varying rates of success attributed to various restoration efforts. Several hypotheses have attempted to explain this:
Hypothesis 1: Constructed pools are too deep.
Hypothesis 2: Edges of constructed pools narrower than natural ones.
Hypothesis 3: Constructed pools have steeper slopes than natural ones.
Results: Research suggest that the last two details (Hypothesis 2 & 3) are crucial in determining the habitat value of man-made vernal pools. In general, most constructed pools were too steep and did not have wide enough edges.
Mitigation
There has been a fair amount of controversy surrounding the practice of mitigation, which is the destruction of protected or endangered species and habitats, such as vernal pools, on the condition that whatever entity (business, land manager, etc.) is destroying the habitat will undertake the construction of a replacement habitat to "mitigate" their impacts. This concept is difficult to apply to vernal pools, which represent a tremendous habitat value - but are difficult to successfully replicate using construction methods (as mentioned above). Thus, it has been very controversial to apply mitigation strategies to vernal pool systems due to the obvious risks inherent in trying to reconstruct this kind of habitat. Some agencies, however, are now requiring two replacements for every vernal pool that is destroyed to compensate for the lower quality of human-made habitat.
Soils
Vernal pools can form anywhere that a depression fills with rainwater, leading to low nutrients and low levels of dissolved salts. They are underlain with an impermeable layer of claypan, hardpan, or volcanic rock allowing for water retention. In many instances they contain grasslands that form over a variety of soil types containing silts and clays often covered by a layer of interwoven fibrous roots and dead leaves. The soil types present tend to relate to the local soil types and hydrology of the pool. Finer soils such as clay, silt, and muck are more common in perched situations, whereas pools which are more connected to the water table have more coarse soils like sand or gravel. Soils in vernal pools often reflect their inundated conditions, leading to low chroma horizons, mottling, and anoxic decay. They can develop hydric soils which are typical of flooded areas, including accumulations of organic matter, but this may not happen in drier areas. In some cases there is a hard pan layer which causes the retention of water in the pools.[10] The hardpan clay basin accumulates water due to the small particle size and therefore reduced porosity. This permits flooding and development of vernal pools.[citation needed]
Flora
Vernal pool flowers, with different species occurring in zones related to soil moisture and temperature gradients formed as the pool dries out. Sacramento National Wildlife Refuge, Calif.
In vernal pools, flowering occurs simultaneously because of the seasonality of favorable conditions. Vernal pool ecosystems may include both cosmopolitan species and endemic species adapted to unique environmental conditions. These conditions include moisture gradients, salinity gradients, and reduced levels of competition.[5] Microtopographical gradients also contribute to species distribution in vernal pool communities, where plants that flower sooner in the season are more likely to be found at slightly higher elevations than later flowering species. Many vernal pool plants have buried seeds which accumulate in the soil. Different species are suited to different moisture levels, and as water evaporates from the edges of a pool, distinctive zonation of species can be seen. Most pools receive annual deposition of tree leaves, which are critical to maintaining local life due to leaf detritus.
Many upland perennial plants are unable to withstand the period of flooding. Many wetland plants are unable to withstand the period of desiccation. Therefore, vernal pools are a distinctive habitat that provides a refuge from both terrestrial and fully aquatic plants. When dissolved carbon dioxide is depleted by daytime photosynthesis, vernal pool species like Howell's quillwort (Isoetes howellii) and pygmyweed (Crassula aquatica) collect carbon dioxide nocturnally using Crassulacean acid metabolism. Vernal pool basin habitats favor annual plants with some uniquely adapted perennials which suffer extensive mortality resembling annual reproduction. Annuals comprise approximately 80 percent of vernal pool flora. Listed below are some genera of the approximately one hundred vascular plant species associated with California vernal pool habitats. A typical pool will include only 15 to 25 species.[11]
Vernal pools are often threatened by development in the same way that other wetlands are. As a result, most pools have been converted into residential zones, roads, and industrial parks. That is why most extant pools occur on protected or private land such as national parks, and ranches.
Some other species, notably Anostraca, fairy shrimp and their relatives, lay eggs capable of entering a state of cryptobiosis. They hatch when rains replenish the water of the pool, and no stage of the animals' life cycle leaves the pool, except when eggs are accidentally transported by animal phoresis, wind, or rarely, by flood. Such animal populations may be very old indeed, when the conditions for seasonal vernal waters are stable enough. As an extreme example, Branchipodopsis relictus on the main island of the Socotra archipelago, which is exceedingly remote for what it is, a continental fragment of Gondwana, is believed to have been isolated since the Miocene. Branchipodopsis relictus is correspondingly isolated genetically as well as geographically.[13]
Vernal pools can serve as a temporary habitat for migrating birds, especially in California. The rich invertebrate population in these pools provide food for ducks, herons, egrets, plovers and many other species.[14]
Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca, Notostraca, the Devonian Lepidocaris and possibly the Cambrian Rehbachiella. They are mostly small, freshwater animals that feed on plankton and detritus.
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently for years or decades or seasonally for a shorter periods. Flooding results in oxygen-free anoxic processes prevailing, especially in the soils. The primary factor that distinguishes wetlands from terrestrial land forms or water bodies is the characteristic vegetation of aquatic plants, adapted to the unique anoxic hydric soils. Wetlands are considered among the most biologically diverse of all ecosystems, serving as home to a wide range of plant and animal species. Methods for assessing wetland functions, wetland ecological health, and general wetland condition have been developed for many regions of the world. These methods have contributed to wetland conservation partly by raising public awareness of the functions some wetlands provide. Constructed wetlands are designed and built to treat municipal and industrial wastewater as well as to divert stormwater runoff. Constructed wetlands may also play a role in water-sensitive urban design.
A marsh is — according to ecological definitions — a wetland that is dominated by herbaceous rather than woody plant species. More in general, the word can be used for any low-lying and seasonally waterlogged terrain. In Europe and in agricultural literature low-lying meadows that require draining and embanked polderlands are also referred to as marshes or marshland.
The tiger salamander is a species of mole salamander and one of the largest terrestrial salamanders in North America.
The spotted salamander or yellow-spotted salamander is a mole salamander common in eastern United States and Canada. It is the state amphibian of Ohio and South Carolina. The species ranges from Nova Scotia, to Lake Superior, to southern Georgia and Texas. Its embryos have been found to have symbiotic algae living in and around them, the only known example of vertebrate cells hosting an endosymbiont microbe.
The California tiger salamander is a vulnerable amphibian native to California. It is a mole salamander. Previously considered to be a subspecies of the tiger salamander, the California tiger salamander was recently designated a separate species again. The California tiger salamander distinct population segment (DPS) in Sonoma County and the Santa Barbara County DPS are listed as federally endangered, while the Central California DPS is listed as federally threatened. The Sonoma County, south San Joaquin, and the Santa Barbara County DPS have diverged from the rest of the California tiger salamander populations for over one million years, since the Pleistocene and they may warrant status as separate species.
An aquatic ecosystem is an ecosystem found in and around a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms—aquatic life—that are dependent on each other and on their environment. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. Freshwater ecosystems may be lentic ; lotic ; and wetlands.
The frosted flatwoods salamander is an endangered salamander species native to the Southeastern United States.
A garden pond is a water feature constructed in a water garden or designed landscape, normally for aesthetic purposes, to provide wildlife habitat, or for swimming.
The Laguna de Santa Rosa is a 22-mile-long (35 km) wetland complex that drains a 254-square-mile (660 km2) watershed encompassing most of the Santa Rosa Plain in Sonoma County, California, United States.
Linderiella occidentalis is a species of fairy shrimp native to California. It is a small crustacean in the family Chirocephalidae. It has a delicate elongated body, large stalked compound eyes, no carapace, and eleven pairs of swimming legs. It glides gracefully upside down, swimming by beating its legs in a complex, wavelike movement that passes from front to back. Like other fairy shrimp, L. occidentalis feeds on algae, bacteria, protozoa, rotifers and detritus.
The conservancy fairy shrimp is an endangered small crustacean in the family Branchinectidae. It ranges in size from about 1.25 centimetres (0.49 in) to 2.5 centimetres (0.98 in) long. This species is endemic to California in the United States.
The midvalley fairy shrimp, Branchinecta mesovallensis, is a small freshwater crustacean in the Branchinectidae family endemic to shallow ephemeral pools near the middle of California's Central Valley. These vernal pool ecosystems are home to other unique organisms adapted to the ephemeral nature of the water cycle in the pools in California's mediterranean climate.
The Yolo Bypass Wildlife Area, located within the Yolo Bypass in Yolo County, California, is managed by the California Department of Fish and Wildlife with the intent of restoring and managing a variety of wildlife habitats in the Yolo Basin, a natural basin in the north part of the Sacramento-San Joaquin River Delta. The creation of the wildlife area was spearheaded by the Yolo Basin Foundation. The California Department of Fish and Wildlife and Yolo Basin Foundation are the core partners in the operation of this resource located at 38.550515°N 121.626291°W
Great Valley Grasslands State Park is a state park of California, United States, preserving a parcel of remnant native grassland in the San Joaquin Valley. Such a temperate grasslands, savannas, and shrublands biome was once widespread throughout the whole Central Valley. The 2,826-acre (1,144 ha) park was established in 1982. Largely undeveloped, it was formed by combining two former state park units: San Luis Island and Fremont Ford State Recreation Area. Its chief attractions for visitors are spring wildflowers, fishing, and wildlife watching.
A pond is a small, still, land-based body of water formed by pooling inside a depression, either naturally or artificially. A pond is smaller than a lake and there are no official criteria distinguishing the two, although defining a pond to be less than 5 hectares in area, less than 5 metres (16 ft) in depth and with less than 30% with emergent vegetation helps in distinguishing the ecology of ponds from those of lakes and wetlands. Ponds can be created by a wide variety of natural processes, or they can simply be isolated depressions filled by runoff, groundwater, or precipitation, or all three of these. They can be further divided into four zones: vegetation zone, open water, bottom mud and surface film. The size and depth of ponds often varies greatly with the time of year; many ponds are produced by spring flooding from rivers. Ponds are usually freshwater but may be brackish in nature. Saltwater pools, with a direct connection to the sea to maintain full salinity, may sometimes be called 'ponds' but these are normally regarded as part of the marine environment. They do not support fresh or brackish water-based organisms, and are rather tidal pools or lagoons.
The San Luis National Wildlife Refuge Complex is located in the northern San Joaquin Valley, within Merced County and Stanislaus County of California. The complex, with four federal National Wildlife Refuges, is managed by the U.S Fish & Wildlife Service.
The Merced National Wildlife Refuge encompasses 10,262 acres (41.53 km2) of wetlands, native grasslands, vernal pools, and riparian areas in California. It was established in 1951 under the Lea Act to attract wintering waterfowl from adjacent farmland where their foraging was causing crop damage. In the last few decades, changes in agricultural practices and refuge management have reduced these wildlife/crop issues.
A drought refuge is a site that provides permanent fresh water or moist conditions for plants and animals, acting as a refuge habitat when surrounding areas are affected by drought and allowing ecosystems and core species populations to survive until the drought breaks. Drought refuges are important for conserving ecosystems in places where the effects of climatic variability are exacerbated by human activities.
The Phoenix Vernal Pools are located in Fair Oaks, California, a suburb of Sacramento city around 20 miles east of the city of Sacramento and north of highway 50. This land consists of seasonally inundated wetlands that form after winter rains. The climate type of Phoenix Vernal Pools is classified as Mediterranean, receiving 24 in (610 mm) of rain per year. The rainwater percolates into the soil until it reaches an impermeable hardpan that causes an elevated water table, forming the vernal pools. The Phoenix Vernal Pool ecosystem is relatively unique as is supports many species of fauna and flora endemic to vernal pools.
↑ Bauder, Ellen T.; Belk, Denton; Ferrer, Wayne T. Jr. (1998). Witham, Carol W. (ed.). Ecology, Conservation, and Management of Vernal Pool Ecosystems. California Native Plant Society. p.1. ISBN0-9434-6037-9.
↑ Bauder, Ellen T.; Belk, Denton; Ferrer, Wayne T. Jr. (1998). Witham, Carol W. (ed.). Ecology, Conservation, and Management of Vernal Pool Ecosystems. California Native Plant Society. pp.10–11. ISBN0-9434-6037-9.
↑ Bauder, Ellen T.; Belk, Denton; Ferrer, Wayne T. Jr. (1998). Witham, Carol W. (ed.). Ecology, Conservation, and Management of Vernal Pool Ecosystems. California Native Plant Society. pp.2–3, 5. ISBN0-9434-6037-9.
↑ Silveira, Joseph G. "Avian uses of vernal pools and implications for conservation practice." In Ecology, Conservation and Management of Vernal Pool Ecosystems. Proceedings from a 1996 Conference. Sacramento, CA: California Native Plant Society. pp.92-94. 1998.
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