Vernal pools are small, sensitive wetland habitats that can easily be disturbed or
destroyed by human activities. The kinds of plants and animals that can use a wetland are
dictated by the environmental conditions. Important environmental factors include
seasonal temperatures, rainfall (timing and amount), ground water levels, and land
use around the pool. Several key threats to vernal pool habitats are discussed below
along with some conservation recommendations. Best management practices that minimize
threats to vernal pool habitats are discussed further in the Conservation and Management
Seasonal pools are often not identified as wetlands due to their temporary nature. Pools
that are not identified cannot be protected during development activities that would alter or
destroy them. Some estimates state that over 50% of the wetlands in the United States have
been lost since European settlement (Vileisis 1997), and likewise over half of all wetland
habitat in Pennsylvania occurring historically has been filled (Myers et al. 2000). The rate
at which seasonal pools have been lost is likely even greater considering that these areas are
often overlooked. Filling seasonal pools, deepening them into permanent fish ponds, and
disturbances to the vegetation and soil around pools from agriculture, building and road
construction, quarries, and logging operations lead to direct mortality of animals and
habitat destruction or degradation.
Vernal pool habitat extends beyond the pool basin. Young vernal pool animals start out living
in water but they turn into adults that live on land. They must leave the pool before it dries
up or else they will perish. Each new generation of adults leaves the pool and seeks a place to
live on land. During the many months that a vernal pool is dry, adult amphibians find food and
shelter in the surrounding upland forest. They live underground or near the surface under rocks,
rotting logs, and the moist layer of leaves on the forest floor.
The Environmental Protection Agency recommends managing a 1000 foot radius area beyond the
edge of a vernal pool basin as vernal pool upland habitat (Brown & Jung 2005). This distance
is based on scientific studies of animal movement. Vernal pool animals do best in upland
habitats that are relatively natural habitats such as forests and uncultivated fields. As
the uplands are turned into agricultural lands, developments, lawns, golf courses, roads,
and the like, the adults have fewer suitable places to live. The smaller the patch of natural
habitat around a pool, the less food and shelter is available. Back to top.
Fragmentation of the landscape by various human activities destroys or reduces the amount of habitat available
to vernal pool animals and reduces their ability to find and colonize new habitats. Roads are a common form of
habitat fragmentation, even in relatively undeveloped areas. And roads near seasonal pools present a physical
obstacle to animals moving from their upland feeding habitats to their seasonal pool breeding habitats.
Many animals are killed as they attempt these crossings. The presence of roads also provides opportunities
for undesirable elements to get into the pools. Roads bring invasive plant species, sediments, and contaminants
to pools through runoff. During the winter the application of road salt poses a problem for pools located near
roads. As the snow melts, salt-laden water flows into these pools and increases the salinity of the water,
making the water less hospitable to wildlife.
Development of extensive agricultural, urban and suburban areas also creates biological
“islands” of isolated natural areas. It can be difficult for animals to move long distances
across unsuitable habitat such as open lawns and parking lots and to navigate large obstacles
such as highways and fences. Some highly mobile animals are able to disperse over these
obstacles. Other more sessile animals are unable to move great distances from their preferred
habitat. For those species that can travel large distances, there is increased risk that
they will not be able to find suitable habitat at the end of their journey in a fragmented
landscape with diminished habitat.
Just as habitat fragmentation isolates a group of animals on an island of habitat; it also
isolates the gene pool collectively held by that group of animals. When animals cannot make
contact with other populations, inbreeding within one population takes place. This means
there is no gene flow between populations and genetic diversity is lost. As genetic diversity
is lost, the ability of that population to adapt to changes in the environment is reduced.
This increases the chances that the population will not be able to survive over the long-term.
Fragmentation around vernal pools can be lessened with land use planning. First, vernal
pools and their upland habitat need to be identified and mapped. Then corridors can be
designated to ensure long-term connectivity between vernal pools and other patches of
forest and wetlands. Back to top.
Semi-permanent and permanent ponds that infrequently or never dry up can support animals such as bull frogs whose tadpoles take at least two years to develop. Bull frogs are voracious predators that alter the predator-prey balance in a vernal pool environment. Credit: Jack Ray
Alterations to the natural water regime change many habitat parameters such as inundated area and depth,
length of inundation, temperature, dissolved oxygen levels, and types and amounts of vegetation in and around
the aquatic habitat. All of these factors are important in proper development of aquatic eggs and larvae.
There are several common causes of altered pool hydrology. Increased water demand from nearby developments
can lead to decreased water tables and decreased water retention in seasonal pools. Deepening of pools
to increase their water retention is a common practice in agricultural settings. This usually changes
a pool from seasonal to permanent aquatic habitats and changes the plant and animal species that can
live there. Logging of forests near pools can change the water table with the loss of trees and can
alter water flow patterns with the creation of logging roads and ruts. See the Conservation and
Management section for recommendations on how to minimize negative impacts during logging. Back to top.
Changing Water Chemistry
This pool receives run-off directly from a nearby dirt road. Credit: Betsy Leppo
Many human activities have degraded water quality in aquatic habitats. Agricultural runoff is a
major source of pollutants in Pennsylvania where there are abundant farmlands. Agricultural runoff
is generated as rainwater flows through fields that have been treated with fertilizers, herbicides,
and insecticides, and through livestock pastures and feedlots, washing the waste into local aquatic
Suburban and urban runoff comes from rainwater that flows off rooftops, down streets and parking
lots, through industrial zones and garbage disposal sites, and through lawns and golf courses treated
with various chemicals. This runoff contains sediments, garbage, road salt, oil, pesticides,
herbicides, and a whole mixed bag of other household and industrial chemicals.
Testing for a variety of synthetic chemicals and heavy metals that can be toxic to wildlife
is an important part of water quality monitoring. Other important variables used to monitor
water quality include temperature, pH, dissolved oxygen, sediment load, nutrient levels such as
nitrates and phosphates, and bacterial and algal levels. The types of plants and animals found
living at a site can also be used to evaluate water quality. Certain species are very tolerant
of poor water quality, while others will only be found in clean habitats. The types of plants
and animals living at a site can be monitored for change over time. If species intolerant of
pollution disappear from a site, and species tolerant of pollution increase, this is an
indication that there is a problem with water quality.
Seasonal pool animals are adapted to the relatively high acidity levels naturally found in
seasonal pools, but they have tolerance limits. Increased acidification of seasonal pools
takes place due to acid rain. In a review of literature, Hulse et al. (2001) describes one study that
showed that embryos living inside eggs of spotted and Jefferson salamanders experience higher
mortality at lower (more acidic) pH values. In a lab setting, no hatching occurred at pH
values below 4.5. Another study showed that natural pools that supported Jefferson salamanders
had significantly higher (less acidic) pH values than pools that did not. Other studies on
spotted salamanders found that increased acidity can deform egg membranes and developing
embryos and produce a decrease in hatching success and higher levels of larval mortality
(Hulse et al. 2001). Back to top.
The term substrate is used here to refer to the bottom of a seasonal pool. Substrate requirements
can be quite specific for seasonal pool animals. The amount and composition of materials such as
leaves, woody debris, and muck are important factors. The substrate can be altered by activities
such as dumping, dredging and filling. Driving machinery or off road recreational vehicles
through dry pool basins can easily damage soils and harm wildlife hiding under the leaf litter.
Gradual non-point effects from runoff can also change substrate. For example, surface runoff
from agricultural fields carries fine sediments. The silt can be deposited into nearby
seasonal pools and eventually fill the pool basin. There are many sediment and erosion
controls that can be implemented to reduce the amount of sediments that flow into waterbodies
from dirt roads, fields, and other areas where the earth has been disturbed. Back to top.
Changes in Vegetation
In-pool and upland vegetation are an important part of the seasonal pool habitat.
Removal or change in composition of vegetation in and around a pool affects which species
can use the pool. For species that lay their eggs in plant material, loss of vegetation
eliminates egg-laying sites. For species that lay their eggs in the water, removal of
vegetation reduces shade. This causes water temperatures and evaporation rates to
increase. These effects are particularly noticeable in small wetlands like seasonal
Changes in vegetation from one type to another happen naturally after forest fires and storms.
Most vernal pools in Pennsylvania are surrounded by forest, but some are located in an opening
of some type within the forest. Vernal pools that occur in woodlands with gaps in the tree
canopy, small forest clearings, shrublands, and old fields can support plants and animals that
require more light. Human intervention may be necessary to keep these habitats open.
People can create and maintain partial or complete openings around or near vernal pools
through carefully planned tree cutting and/or infrequent mowing. Changes to the vegetation
should be made with great care with respect to the vernal pool habitats and species found
there (see the Conservation and Management section for more information). Sites with multiple
vernal pools may be ideal for management in a variety of settings, from partially open to
heavily forested areas, to encourage diversity. Back to top.
This vernal pool was either created or expanded by earth removal. This photograph from 1978 shows disturbed soils and a lack of vegetation around the perimeter. Credit: Jack Ray
The same pool photographed 26 years later in 2004. Vegetation was allowed to grow up around the perimeter, which now provides shade to the pool. Credit: Betsy Leppo
A forest fire opened the canopy above this vernal pool, allowing marshy vegetation to grow in the pool basin. Credit: Betsy Leppo
Frequent mowing up to the edge of this marshy vernal pool (on the right) removes protective vegetation and woody debris where vernal pool animals can find food and shelter when they leave the pool. Credit: Betsy Leppo
Use of certain pesticides and herbicides in and around seasonal pools can disrupt the
balance that is necessary between predators and their prey. Larval and adult seasonal
pool animals feed on the smaller animals that share their environment such as mosquitoes,
midges, gnats, and other flies. Predators such as amphibian larvae and insects such as
dragonfly and diving beetle larvae and adults help control the insect species that are
considered pests. However, when homes encroach upon wetland habitats, municipalities
and homeowners often take measures to control mosquitoes and other nuisance insects.
The pesticides used to control these pests have many negative effects on non-target
species. Direct mortality of all insect species occurs when broad-based killing
agents are used. More specific killing agents are available that only harm black
flies or mosquitoes, but they indirectly affect the predators by decreasing the
availability of their food.
Encouraging natural predators such as birds and bats by installing nesting and roosting
boxes for them next to vernal pools is one environmentally friendly way to increase pest
control. Many homeowners overlook mosquito breeding sites around the yard that are most
likely to be the source of annoying bites. Cleaning rain gutters frequently, refreshing
bird baths, and keeping flower pots and other plastic containers free of standing water,
should be the first line of defense in controlling mosquitoes. Back to top.
All aspects of life in a seasonal pool, from amphibian migration to egg and larval
development to adult feeding, thermoregulation, and reproductive success, depend on
certain environmental cues and conditions. For example, the migration of salamanders
to seasonal pools is in response to environmental cues. One study showed that salamander
migrations started when evening rains occurred with moderate temperatures over 44.6 F
during the day and over 40 F at night (Hulse et al. 2001). In Pennsylvania, migrations can
start as early as the end of February and as late as the beginning of April depending
on local temperatures and rainfall.
While predictions of climate change do not agree on the details, evidence shows that
changes in average yearly temperatures, minimum and maximum temperatures, rainfall amounts,
and other environmental changes are occurring. These changes are having an effect on wildlife,
which respond more quickly to environmental changes than humans, who are able to create a
favorable stable environment inside their home.
There is already evidence that the geographic ranges of plants and animals have been
shifting in response to changes in climate. Climate change is of increasing concern for
species of special concern, especially in regard to species that are geographically restricted
(Westfall and May 1996). Back to top.