When I was an undergraduate, my Zoology course placed amphibians among the "lower vertebrates". At the time, this seemed reasonable. Biology had taught me to look at life as a progression of adaptation toward mammals as the penultimate vertebrates. After all, what could be more advanced than endothermy, dessication resistant skin, a four-chambered heart, and amniotic eggs and placental fetuses. It would be nearly a decade before I would fully appreciate how wrong that perspective was.
Amphibians represent an alternative suite of strategies to solving life's challenges. To understand that alternative strategy, you have to appreciate the spectacular nature of amphibian skin. Most people are casually aware that amphibians exchange gasses through their skin. The ability to take up oxygen and expell carbon dioxide through the skin is no minor adaptation. It contributes greatly to the low energy demands of amphibians. But amphibian skin is so much more. Amphibian skin contains poison and mucuous glands, promotes the growth of bacteria that resist fungal infections, has cells that produce and change colors, aids in processing of waste, has glands involved in courtship and mating, is used by some species to nurture their young, is a sensory organ for aquatic species, and has been modified to form structures such as "spades" for digging or "claws" for clinging to rocks in fast flowing water. Amphibian skin is argulably one of the most remarkable and versatile tissues in the animal kingdom, but is not without its costs. To function in gas exchange and other ways, amphibian skin must remain thin and moist. As a result, amphibians are sensitive to dehydration. In many ways, water is as much a currency as energy when it comes to governing amphibian ecology.
Amphibians have been tremendously successful. As of 2019, there were 8044 known amphibian species and that number has grown steadily. Amphibians have succeeded in radiating to occupy most terrestrial and freshwater environments that occur on earth. In some cases amphibians occupy harsh environments where no birds or mammals can persist. Because amphibians have such low energetic demands, they realize body sizes far smaller than endothermic animals and they can exploit very small and highly variable prey. Where they occur, amphibians often achieve abundances and biomass that dwarf other vertebrates, making them influential in the functioning of ecosystems. Amphibians can affect the abundances of invertebrates, are important prey for other animals, are important repositories for nutrients within ecosystems, and important conduits for the movement of energy and nutrients between terrestrial and aquatic ecosystems. By studying and attempting to see the world from the perspective of these organisms, I have gained a better understanding of how evolution shaped the diversity of life on earth, and how organisms function as components of ecosystems.
Despite their success, amphibians are now emblamatic of the challenges many organisms face in progressively human-modified landscapes. Many amphibian populations have declined leading to the imperliment of many species. Amphibians are identified as one of the most imperlied groups of vertebrates. Causes of declines range from the spread of emerging infectiious diseases and invsavie species linked to increasing global trade, habitat loss and degradation, and shifting climates. Amphibians have shown remarkable resilience to some dramatic historic changes in climate and habitat loss; however, these changes were often not as rapid or occurred as isolated threats. Today, amphibians face an onslaught of concurrent threats that create challenges to their conservation and management.
© John Maerz, 2015