Types of Frogs: Endangered and Otherwise

picture of a bull frog

Nature stories covering the frog world often focus on large scale stories such as the discovery of Chytrid Fungus and its problematic effects on global frog populations.

Smaller scale local interest stories with a frog angle also tend to do well. Almost all conversations about the types of frogs that share the world with other living creatures begins with a presentation of some facts. Up first, the approximately 5,000 global frog and toad species fit into twenty-five families.

Closer to home, the ninety North American species span nine families. Approximately eighty per cent of those species fit into one of three families: true frogs (Ranidae); tree frogs (Tree Frogs) and true toads (Bufonidae). Representative species of frogs are presented below according to families to help with basic frog identification questions.

True Frogs

North America hosts twenty six true frog (Ranidae) species. The bullfrog (Rana catesbeiana), one tough and adaptable frog, exemplifies the Ranidae. Most accounts list it as an indigenous species east of the Rocky Mountains, with a late nineteenth and early twentieth century introduction in the West, in conjunction with an American taste for frog legs cuisine. Able to outlast the dietary craze, Bullfrogs multiplied in western ponds, lakes and wetlands, until they became the dominant amphibian in their territory.

Size and dietary habits explain their adaptive success. North America's largest frog species consume almost anything that moves in their territory such as fish, insects, other frog species, and other bullfrogs. Scientists express concern that the bullfrog's territorial dominance contributes to declining populations of other frog species such as the two native Red-legged frog species.

picture of a Northern Red-legged frog

Once abundant throughout the state, the California Red-legged frog population has decreased over 90% and has been listed on the Endangered Species list since 1996. The remaining population now survives primarily along California's coastal areas, where they still compete for territory with California's coastal loving human population.

Northern Red-legged frogs (Rana aurora) inhabit coastal and inland areas of the Pacific Northwest from northern California to British Columbia. They prefer slow moving or still, shallow water habitats for breeding. During non-breeding season their range extends to many of the nearby forest floors.

The name red-legged frog comes from the red color of the back of the legs and bodies.

While they are not listed as endangered, their populations have declined over the years due to habitat destruction and habitat competition with the larger and more aggressive Bullfrog.

picture of a Leopard Frog

Rana species with large green frogs with spots and dorsal ridges along the sides of the body generally get categorized by the name Leopard Frogs. They inhabit many water habitats across North America and Central America, accounting for their nicknames such as meadow frog and grass frog.

Once believed to be a single species, scientific research, especially DNA analysis, now suggests the existence of anywhere from twenty to thirty different leopard frog species. In areas where species overlap, hybridization can also occur. The Northern Leopard Frog (Rana pipiens), probably the most widespread of the species, inhabits areas across most of the northern United States and southern Canada. Rana pipiens also now serves as the benchmark for organizing scientific thinking on all leopard frogs, which now get grouped into what is called the Rana pipiens complex.

Amphibian populations across the United States continue to decline. Currently the United States Fish and Wildlife Service lists the actions it has taken with respect to various leopard frog populations, and their potential for endangered listings as follows:

  • Relict leopard Frog (Lithobates onca): Candidate. A species under consideration for official listing for which there is sufficient information to support listing.
  • Chiricahua leopard frog (Rana chiricahuensis): Threatened. A species "likely to become endangered within the foreseeable future throughout all or a significant portion of its range."
  • Vegas Valley leopard frog (Rana fisheri): Resolved Taxon. Species that have been petitioned for listing and for which a Not Warranted 12 month finding or Not Substantial 90-day finding has been published in the Federal Register. Also includes species that have been removed from the candidate list.
  • Ramsey Canyon leopard frog (Rana subaquavocalis): Resolved Taxon
  • Northern Leopard Frog (Rana pipiens): Under Review. Species that have been petitioned for listing and for which a 90 day finding has not been published or for which a 90 day substantial has been published but a 12 Month finding have not yet been published in the Federal Register.
  • Lowland leopard frog or San Felipe leopard frog (Rana yavapaiensis): Species of Concern. Species that have not been petitioned or been given E, T, or C status but have been identified as important to monitor.

Tree Frogs

picture of a Pacific tree frog

The Pacific Tree Frog (Hyla Regilla), a small frog found in ponds and other small watering holes throughout the West Coast, is well known for its vocalizations. Sometimes called the Pacific Chorus Frog, they can really croak up a symphony of sound during the night hours.

Much like the chameleon, Pacific Tree Frogs adopt a variety of colors depending on environmental factors such as background light. It's not unusual to see their skin change from green to brown to rust, and any combination, in a matter of hours. Scientists believe that the frog's color changing ability acts as a passive defense mechanism, protecting it from predators, such as garter snakes.

In 2007, Washington State designated the Pacific chorus frog its official state amphibian.

picture of a Canyon Tree Frog

The Canyon Treefrog (Hyla arenicolor) inhabits the rivers and stream banks of rocky areas in the desert Southwest. Skin color varies from location to location, and it generally serves as an effective camouflage against predators.

Their diet consists of the aquatic and terrestrial insects in their territory.

picture of a Cuban Tree Frog

The Cuban Tree Frog (Osteopilus septentrionalis), a non-native species, was introduced to Southern Florida in the early Twentieth Century. It has subsequently expanded its territory to include most of the Florida peninsula. Cuban Tree Frogs are also the largest tree frog species in the United States, and their presence in Florida continues to stress some native tree frog species.

They range in color from the light tan specimen in the top picture, to a darker shade of spotted green. Most practice nocturnal behavior, sleeping in trees and shrubs during the day and perching on walls and ceilings during the warn night hours.

Toads

picture of a Western Toad

Endangered Frogs

Around the world. disease and a changing climate also create stresses on frog populations. The Global Amphibian Assessment (GAA), an ongoing research project sponsored by the International Union for Conservation of Nature (IUCN) and Conservation International (CI), shows population declines in many of the world's amphibian species.

A first of its kind report, the June 2007, the GAA listed a total of 5,918 amphibian species in the world. Frogs and toads lead the list with 88% or 5,211 different species. Of those, 1,590 or 30.5% were listed as either vulnerable, endangered or critically endangered.

Salamanders and newts account for the second largest number of amphibian species with 535 or approximately 9% of the total. Of those, 249 or 46.5% were listed as either vulnerable, endangered or critically endangered.

Caecilians round out the list with 172 or a bit over 2% of the total. Of those, 5 or 2.9% were listed as either vulnerable, endangered or critically endangered.

The list is far from complete with little or no data on amphibian species entered for some states such as the Democratic Republic of Congo. Forthcoming data from other states such as Indian, Indonesia and Malaysia is anticipated to add to and alter the aggregate results. Therefore, current statistics need to be read as a first take comparative examination of global amphibian population trends.

Various reasons are given for population declines, with habitat loss cited as the primary causal factor. Amphibian population levels are exacerbated by two inter-related geographical factors. First amphibians are tied to both a specific geographical location and a specific habitat. Close to 90% of the world's amphibians live in a forest habitat. Global deforestation trends in the past couple of decades has contributed to declining amphibian populations.

Currently ten native frog and toad species are listed as either endangered or threatened under the terms of the Endangered Species Act. Habitat encroachment accounts for most of the stress on native frog populations. The introduction of non-native species accounts for other stresses. The bullfrog, for example, a nonnative, agressive species, was introduced into many ponds and lakes, where its subsequently chased off the native species. The Cane Toad in the video, the world's largest toad, continues to encroach on native species habitat in where ever it is introduced.

Frogs and Chytrid Fungus

Another stress on frog populations, an amphibian skin disease called chytridiomycosis, caused by a chytrid fungus called Batrachochytrium dendrobatidis, Bd for short, takes on a sense of urgency as world wide frog populations continue to plummet.

Bd thrives in higher elevation, cool, moist habitats. In the United States, its presence has been most problematic in the Mountainous West, where a handful of species, including the Mountain Yellow-legged Frog population, an endemic species of the Sierra Nevada Mountains, have been affected.

East Coast frog populations, generally found in lower elevation, higher temperature habitats, have not been hard hit. Elsewhere, Bd outbreaks have caused severe stress on frog populations in Australia and Central and South America.

Recently a few, small research studies have shown promise for treating the fungus.

Poison Frogs

No general discussion of frogs would be complete without mention of poison frogs. Often the term refers to two different groups of frogs:

  • Mantella species indigenous to Madagascar
  • Dendrobatidae: Dart-poison frogs or poison dart frogs

Dendrobatidae, new world, neotropical frogs, inhabit tropical forests of Central and South America. Small in size, the nickname name poison dart frog came from the fact that indigenous peoples were known to use their poison on blow gun darts for hunting.

In addition to their small size, many Mantella and Dendrobatidae species are colorful, a reminder to remind potential predators in their territory to stay away.

Partly because of their toxicity, they are active during the day, hunting insects, their food of choice. In fact, recent research from the National Academy of Sciences, Convergent evolution of chemical defense in poison frogs and arthropod prey between Madagascar and the Neotropics found that the frogs' poisonous alkaloids result directly from their diet. They consume poisonous insects, often ants and millipedes, and the poison accumulates in their skin.

Because the poisonous alkaloids are not naturally occurring, many poison frog species are raised as pets. After a time, feeding captive bred poison frogs non-poisonous insects, creates non-poisonous, poison frogs.

Currently the IUCN lists one hundred and fifty seven Dendrobatidae species arranged according to eleven genera. Close to thirty are listed as either endangered or critically endangered. However, that number could be substantially higher due to the lack of data available on all of the species.

Seven of the sixteen Mantella species are listed as either endangered or critically endangered.

© 2001-2016 Patricia A. Michaels