 |
|
 |
 |
||||||||||||||
Phylum: Craniata (Chordata)
Class: Amphibia
Amphibians
were the first tetrapods to evolve from fish ancestors during the late
Devonian period, some 360-370 million years ago (Zug 1993). Only one
of the 3 major groups of ancient amphibians evolved into the modern
salamanders, frogs, toads, and caecilians. Today amphibians have invaded
almost every environment where fresh water is available for some period
of time. Some spadefoot toads survive in deserts in underground burrows
and only emerge during the occasional monsoon rains that fill temporary
ponds.
Amphibians have many unique characteristics that distinguish them from other vertebrate classes. Amphibian skin is glandular and lacks scales, feathers, or hairs. The skin is also permeable allowing for water and gas exchange. This restricts amphibian activity to wet and/or humid conditions, but this trait also allows amphibians to respire and absorb water through the skin from the surrounding environment. This is only one of the methods of amphibian breathing. Respiration is also accomplished by the use of lungs, gills, and buccal pumping, where the animals swallow air and force it into the lungs (Beebee 1996). As the name amphibian implies, most toads and salamanders, exhibit a two-phase life cycle, an aquatic larval stage and a terrestrial adult stage. This life-cycle trait is by no means dominant, however, as two-thirds of amphibians do not incorporate this strategy (Pough et al 1998). The amphibian egg is also a distinctive feature. It is shell-less and lacks many of the embryonic membranes of the amniotic egg of the other tetraopod classes, therefore development is restricted to a moist environment.
Amphibian Diversity
Three orders of amphibians exist today: the frogs and toads(Order Anura), the salamanders (Order Cuadata or Urodela) and the caecilians (Order Gymnophiona). There are over 4,600 species of amphibians encompassing vast differences in morphology, body size, ecology, and behavior (Pough et al 1998). Anurans (frogs and toads) are the largest group of amphibians having approximately 3,800 species (Zug 1993). Frogs and toads are also very ecologically diverse, inhabiting aboral, aquatic and terrestrial niches with great success. The salamanders consist of roughly 400 species which cluster into three evolutionary lineage: the sirens (eel-like aquatic salamanders), primitive salamanders, and advanced salamanders. Within the advanced salamanders, the Plethodintids or lungless salamanders are most prolific. A remarkable feat for an organism relying on its skin for gas exchange! In fact, Pough (1998) states the reduction of lungs seems to be a trend in the evolution of terrestrial salamanders. Salamanders, unlike anurans and caecilians, are the most diverse and abundant in temperate regions, particularly in the Appalachian mountain range. Little is known about the leggless, worm-like caecilians. There is 160 known species, all of which are restricted to the tropics and inhabit areas of abundant rainfall (Zug 1993).
Northeastern Wisconsin Amphibians
Northeast
Wisconsin is home to 18 species of amphibians; 7 species of salamander,
and 11 species of frogs and toads (Casper 1996). Certain species, such
as the eastern American toad (Bufo americanus americanus) and
the green frog (Rana clamitan melanota)are quite ubiquitous and
can be found throughout the area. Other species like the eastern newt
(Notophthalmus viridescens louisanenis) and the pickeral frog
(Rana palustris) are restricted to a particular habitat.Spring
breeding choruses of the 11 species of frogs, toads, and treefrogs can
also be heard in this area. Interestingly, the regional anurans follow
a chronological order of calling based upon tolerances of cold and water
temperature (Vogt 1981). Vogt describes this order beginning with the
western chorus frogs (Pseudarcris triseriata triseriata) in late
March or early April. This is followed by the wood frogs (Rana sylvatica)
and spring peepers (Pseudacris crusifer crusifer). A few weeks
later the Leopard Frogs (Rana pipiens), Pickeral Frogs (Rana
pulustris), green frogs, and the eastern American toad begin to vocalize.
Two treefrog species, the eastern gray treefrog (Hyla versicolor)
and the Cope's gray treefrog (Hyla chrysosclis), as well as the
Blanchard's cricket frog (Acris crepitans bairdi) begin to call.
Finally, mink frogs (Rana septentrionalis) and bullfrogs (Rana
catesbeina) begin their seasonal chorus by the middle of June. In
a prestine wetland or aquatic habitat it is possible to hear many of the
species through the spring and early summer. The Wisconsin
Frog and Toad Survey has been monitering anuran populations based
on vocalizations throughout Wisconsin. It is one of the first regional
amphibian community investigations of trends in population dynamics in
North America. A key to amphibian
tadpoles is also available online.
Amphibian Importance
Amphibians are a valuable part of the biotic community. Amphibians are important predators of insects, other invertebrates and vertebrates. A single Blanchard's cricket frog consumes approximately 4,800 insects per season (Johnson and Christiansen 1976). Other anurans such as the leopard frogs, wood frogs, and eastern american toads spent summer months foraging for insects in the terrestrial landscape. Larger frog species, like the bullfrog and green frog, incorporate insects, small amphibians and reptiles, and even young water fowl into their diet. In some northern hardwood forests, redback salamanders (Plethodon cinereus) can occur in such impressive numbers (2,950 individuals per hectare) that their biomass is twice that of breeding birds and equal to small mammals (Burton and Likens 1975). This impressive population is essential in controlling invertebrate populations. Amphibian larva are also efficient predators of algae, insects, and zooplankton in the aquatic environment (Vogt 1981). Amphibians provide sustiance for other organisms, as well. The tadpole provides food for fish, salamanders, snakes, turtles and shorebirds (Vogt 1981). Frogs and salamanders are consumed by foraging mammals. Amphibians are important as prey items because they convert invertebrate "energy" they consume to usable energy in higher tropic levels.
Amphibian Decline and Conservation
Since the 1970's, scientists have been noticing a decline in the global amphibian population. Many reasons are suspect for the decline, however human impacts such as landscape fragmentation, environmental degregation, and pollution have obviously reduced or eliminated many populations (Pechmann and Wilbur 1994). Other reasons linked with decline may include global climate change (Beebee 1995), road mortality to migrating populations (Fahrig et al 1995), increases ultraviolet light due to atmospheric ozone depletion (Blaustein and Wake 1995), timber harvesting (Petranka et al 1993), and disease. A Wisconsin species, the Northern Leopard Frog (Rana pipiens) has experienced catastrophic declines. Pechmann and Wilbur (1994) state that leopard frogs were observed at only 15 (5%) of 301 suitable breeding sites in Wisconsin, and an educated guess suggests that there has been a 50% decline in U.S. leopard frog populations during the past 10 years.
Amphibians decline issues must be put into perspective. Presently, the worldwide destruction of biodiversity is greater now then any other period in the history of earth. A decrease in amphibian population numbers may be similar to those of experienced by mammals, birds, fish, and reptiles (IUCN 1996). Out of the 4000 species of amphibians, no more than 1% have experienced global declines (Beebee 1995). More knowledge of long-term amphibian population dynamics are needed, in order to make assumptions of amphibian decline on a global level. However, it is clear that some amphibian species have experience considerable decline, and these declines can emphasize environmental problems with wide implications for other organsisms besides amphibians.
Many organizations, like the Great Lakes Declining Amphibian Working Group and the Declining Amphibian Population Task Force, are currently involved in issues and research with amphibian decline.
Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects
© 2001-2004 The Cofrin Center
for Biodiversity and the University of Wisconsin Green Bay,
All Rights Reserved
Last
updated on
July 6, 2006