6
AMPHIBIANS
Few amphibians have been able to survive the urban environment. Unlike plants, whose seeds are blown in the wind or deposited in bird droppings to grow wherever they land, or fish, which complete their entire life cycle in a pond, most amphibians are not well suited to living in densely populated human environments. Many salamanders live on land much of the year and then migrate to water to lay their eggs; most frogs spend part of their lives in the water and another part on land. Urban and suburban environments do not offer habitats suitable to this kind of movement. Although there are ponds and woods, the two are not always adjacent, forcing amphibians to be exposed part of the time. This exposure substantially increases their risk of predation, not only by animals but also by humans, who are among the most dangerous predators for small, harmless, fascinating vertebrates. For example, people who wouldn’t dream of bringing a water bug home will eagerly catch a spotted salamander. A few people, over a period of several years, can extirpate an entire population of salamanders just by going out a few nights each spring when the salamanders gather to lay their eggs. Greed is one of the motivating factors in the extirpation of amphibians, since pet shops will buy anything that will sell. Most of the animals die in captivity without ever reproducing in the wild again.
Some amphibians, however, can still be found in urban areas, and these examples are discussed in this chapter. The ones that do best in towns tend to be those with the simplest life histories: species that don’t migrate across roads, so they don’t get run over; species that don’t pass through an aquatic stage, so polluted waters are not an issue; and species that spend much of their lives underground, so people don’t pick them up. These are the animals that often do well in suburbs and cities. Other species have preadaptations that allow them to survive around people. Some toads will lay their eggs in just about any pool of standing water; since the tadpoles metamorphose so rapidly, the young toads often reproduce in an area where few other amphibians would have a chance. And frogs that spend almost all their lives in or next to the water, never venturing far from the safety of their pond, can escape danger and do just fine in an urban park.
SPOTTED SALAMANDER
The spotted salamander (Ambystoma maculatum) ranges throughout most of the eastern United States from Maine to northern Florida. These amphibians are heavy-bodied salamanders that are usually about 6 inches (15 cm) long, though specimens as long as 9 inches (almost 23 cm) have been recorded. They are generally jet black above, with a number of yellow-orange spots. Some specimens have very few spots, but 25 is about average. The venter, or belly, can be gray to almost black. In between the front and the hind legs, along each side, are from 11 to 13 costal grooves.
You will not see these large, attractive salamanders, though, unless you know when and where to look for them. The adults spend most of their lives moving about in underground tunnels and burrows where they feed on soft invertebrates that are narrow enough to fit into their relatively small mouths. In the early spring, they come out of the burrows to mate and lay their eggs in small freshwater pools and shallow, slow-water areas in rivers, ponds, and lakes. In the North, spotted salamanders appear when the spring thaw sets in, the ground becomes saturated, and the vernal ponds are full.
Natural selection has favored spotted salamanders that breed in temporary ponds, the small pools that dry up before the summer ends. Because fish do not live in these ponds, the larvae avoid these potent predators. But the larvae must hatch, develop, and leave before the ponds dry up, and this can create a race for time, since many of these ponds are gone by July. The spotted salamander is among the more highly adaptive amphibians in this respect. It has developed a rapid reproductive cycle that tends to accomplish all the necessary tasks before midsummer. It migrates during the first warm nights of early spring, when the temperatures are still too cold for most other amphibians. Movement starts after sundown when few bird and mammal predators are active.
This journey may last one to several nights, depending on the distance, terrain, and weather. Some salamanders travel as far as a mile (1.6 km) at a top speed of about 2 yards (1.82 m) per minute. When the temperature falls below freezing, they usually seek shelter until the weather warms up. A few salamanders arrive at the ponds during these cold nights, but warm rains are more suitable for mass migrations.
The females deposit masses of 15 to 250 eggs on submerged twigs and other plant material. Each egg is surrounded by gelatinous material that immediately swells when it hits the water. This substance attaches all the eggs to one another, as well as to the twig or plant material to which they are attached. The clump of eggs will remain in position for four to six weeks, at which time the partially developed larvae wriggle out of their protective sheaths.
As they grow, the larvae feed on small aquatic invertebrates ranging in size from daphnia to fairy shrimp to larger water bugs. Most spotted salamander larvae grow for three to four months, by which time they have resorbed their gills and begin to show faint markings where their yellow spots will appear. In time these young assume the more terrestrial appearance of the adults. The transformed juveniles leave the pond and water for the woods, where they will complete their maturation while feeding in the leaf litter and abandoned rodent tunnels. To avoid desiccation, an individual may spend the remaining summer months in a single tunnel, moving little more than six feet (1.8 m) a day.
While spotted salamanders still thrive in a vast portion of the country, the increased numbers of people have decimated many populations. When pet stores began to sell spotted salamanders, collectors in search of quick money reduced and even eliminated many of these amphibians’ breeding populations. This practice is still going on, posing a serious threat to salamander populations.
It is important to spotted salamanders that large woodlots be left intact, without being crisscrossed by roads or disturbed by people. Suburbs and spotted salamanders can easily coexist, and do, as long as large tracts of valuable habitat are left intact. Some spotted salamanders still hang on in New York and other cities. While small patches of their habitat may be protected, in most urban areas they could still be wiped out as a result of relentless collecting.
Spotted salamanders are identified by their yellow spots, which may serve as a warning that there are toxins in their skin. Raccoons and opossums are known to prey on spotted salamanders. Some larger animals have learned to swallow the salamanders whole, rather than to chew on the unpleasant-tasting skin, which might make them sick. As far as I know, no one has ever adequately investigated the purpose and effectiveness of the spotted salamander’s black and yellow markings. Other amphibians on other continents, for example, the European fire salamander (Salamandra salamandra) have evolved a similar pattern so the markings are presumed to have some value.
RED-BACKED SALAMANDER
The red-backed salamander (Plethodon cinereus) ranges from southern Quebec and Nova Scotia south to North Carolina, west to Minnesota, and through the Great Lakes region, Indiana, Ohio, West Virginia, and Virginia. These salamanders vary in appearance from one individual to another. The two most common variants are called the red-backed and lead-backed phases. Salamanders in the red-backed phase have a reddish brown dorsal stripe extending down their back, from their head practically to the tip of their tail. In the lead-backed phase, the salamanders are uniformly grayish black all over. These phases may be related to the localities in which the salamanders live. Though the adults reach lengths of about 5 inches (12.7 cm), they are usually only 3 to 4 inches (7.6 to 10.1 cm) long, and because they are so thin, they look quite small. They are rarely seen out in the open, unless you go out at night with a bright light and look harder than is usually worth it. Instead, just lifting rocks and logs will be sufficient to turn up some red-backs if they’re in the area, although if the soil is dried out, they may have gone down deeper. When looking for salamanders, it is important to return the stones and logs to their original position to avoid disrupting the microhabitat.
In many cities and suburbs, where much of the vertebrate fauna has been altered significantly and few reptiles and amphibians survive, the red-backed salamander is one of the last to persist. These amphibians need woodlands, but beyond that, their requirements are few. Unlike many other salamander species, the red-backed salamander lays eggs on land. These hatch directly into small terrestrial salamanders without gills, which means that their habitat requirements are less complex. In many suburbs, red-backed salamanders can live in small woodlots that remain relatively undisturbed.
Red-backs don’t migrate to breeding pools in the spring, so they don’t expose themselves to predators or collectors by gathering together each year. And the young aren’t exposed to the risk of insufficient rainfall, which dries out many vernal ponds before salamander larvae of other species are able to leave the water. The red-backed salamanders spend their entire lives in the leaf litter and underground. In some healthy woodlands, calculations have shown that the total weight of the red-backed salamanders exceeds the total combined weight of all other vertebrates living in the same area. That’s how numerous these animals are in undisturbed areas.
Once you understand how many red-backed salamanders can live in a healthy forest, you can’t help but wonder what the effect is when salamanders no longer live there. It appears that woodlands in busy urban areas get trampled until the top layers of earth are so compressed that rain runs off instead of percolating through. The runoff causes erosion, and the compaction kills off species that normally burrow through the humus and lower levels. This process is accelerated by off-road vehicular traffic. Over the years, the wheels of lawnmowers and brush cutters can do considerable damage to an area. For most species, soil structure is far more important than many people realize.
COMMON TOAD, WOODHOUSE’S TOAD, AND FOWLER’S TOAD
The Fowler’s toad is one of several subspecies of the common toad (Bufo woodhousei) that is distributed across the United States. Bufo woodhousei fowleri extends from southern New Hampshire west to southeastern Iowa, south to the Gulf Coast, and east through northern Georgia and South Carolina to the North Carolina coast. In these regions, it is probably the most common of the toads that are able to survive in city parks and in the more wild areas in the suburbs.
Although Fowler’s toads previously lived in many coastal areas, including most islands near the mainland, insecticides—particularly DDT, which was sprayed from the late 1940s until the 1960s—decimated total populations of these animals. Often the insecticides were sprayed in an effort to control mosquitoes, but the toads died, too. In those days, all it took was a few suburbanites complaining to their local elected officials about mosquito bites; in no time a truck was dispatched and deadly chemicals were sprayed everywhere.
Attitudes toward insecticides have changed, and government workers are now less inclined to respond to complaints by spraying indiscriminately. Officials have learned that other taxpayers will strenuously object to chemicals being used to kill mosquitoes, which are a valuable asset to many ecosystems. Besides, no one ever told people to move in near marshes where mosquitoes are known to be numerous. An interesting twist to this problem took place in Breezy Point, a coastal community in Brooklyn. The residents wanted to do something about their mosquitoes, so Fowler’s toads were introduced as a form of biological control. Now Breezy Point has both, lots of mosquitoes and lots of toads.
Fowler’s toad
The common toad breeds opportunistically in vernal, freshwater pools and marshes any time from spring to fall. The tadpoles rapidly metamorphose into small toads, which leave before the pools dry up; otherwise the tadpoles die. The dark tadpole coloration helps them blend in with the bottom, where they sit while filter-feeding shortly after hatching. When larger, they are often seen in clusters in shallow water near shore. This may be to avoid fish that can’t pursue them into such shallow water, and it may also be because the shallow water is warmer. Another reason the tadpoles often swarm together is to stir up the bottom sediments, increasing the food in suspension that can be consumed by these filter-feeders. Another possible explanation for why they have such dark coloration may be to absorb the sun’s radiation when in shallow water; this could help elevate their body temperature so they can feed and metamorphose as rapidly as possible.
Some of the larger toad tadpoles eat the smaller ones. Carnivory helps stagger the emergence of the metamorphosed toads. Those that consume the greatest percentage of the resources are the ones that grow the fastest and are the first to metamorphose and leave the pools. Unlike most frogs, these toads metamorphose and leave their breeding pools when they are still very small, which is another adaptation that helps get them out of the water before it dries up.
Adult toads have dry, warty skin, which helps conserve body fluids by reducing evaporative loss. This is valuable in coastal and arid environments where fresh water is in short supply much of the year and where the desiccating effects of the sun, salt, and breeze can prove deadly, especially to a terrestrial amphibian.
The large bumps behind the eyes are parotid glands, and are filled with a thick, milky, and poisonous solution. On common or Woodhouse’s toads, these glands are about twice as long as they are wide; they just touch the cranial crests, which are the bony ridges on the head. Most common toads also have a light line going down the middle of their back.
The southern toad (Bufo terrestris), which is closely related to the common toad, is an ecological equivalent that replaces the Fowler’s toad along the coastal plain in the South. Three subspecies of the common toad are distributed in the Midwest and in the West. Other common toads, such as the American toad (Bufo americanus), still persist in many suburban areas, but they don’t seem to last in more built-up areas.
MARINE TOAD
The marine toad (Bufo marinus) is a tropical and semitropical species native to South America and Central America, as well as to the southernmost regions of the United States. This species has become established in many warm areas around the world where it was introduced to control snails, insects, and rats. It is now found in southern Texas, Jamaica, Puerto Rico and many other Caribbean islands, Bermuda, Hawaii, most of the larger islands of Micronesia, New Guinea, Fiji, Tonga, the Solomon Islands, the Philippines, and Australia.
These are the largest toads found in North America; they are usually 4 to 6 inches (10.1 to 15.2 cm) long, though they occasionally grow as long as 9 inches (22.8 cm). Besides their size, they are also identified by their enormous, pocked, parotid glands that extend back from their cranial crests to a level equal with their mouth. The poison from their parotid glands is purported to be able to blind a dog, though it almost never does.
Bufo marinus is remarkably well suited to breeding in temporary pools that form in suburban and urban areas any time of the year. Like other species, the toads have learned to take advantage of the “night light” that exists in urban and suburban areas. Street, building, and porch lights attract moths and other insects, which the toads have come to rely on as a food source.
AFRICAN CLAWED FROG
The African clawed frog (Xenopus laevis) was brought to this country for a variety of reasons. For many years it has been a useful laboratory animal because it does well in captivity and is easy to breed. During the 1960s this frog was used for pregnancy tests. Later on, some of them showed up in pet shops, and by 1974 African clawed frogs were found breeding in a reservoir in San Diego. In time they were found in several reservoirs in San Diego County. From there they moved into permanent ponds, and by 1980 they had spread throughout the county.
These frogs have voracious appetites and will eat almost anything that moves. Now turning up in backyard ponds, they are even eating goldfish. So far they are found only in Los Angeles and San Diego counties. Whether they will spread cannot be predicted. Since these cities are surrounded by arid areas, it will be difficult or perhaps impossible for the frogs to disperse on their own. However, humans have repeatedly shown that they are effective vectors of a wide range of species that otherwise would have never dispersed beyond their restricted range.
Of all the frogs now living in North America, this has to be one of the more peculiar looking. Its body is flattened, the head is much narrower than the rest of the body—not unlike the narrow-mouthed toad (family, Microhylidae). The little, round eyes, on the top of the head, have a staring gaze because they lack eyelids. The small forelimbs stick out from the front of the body, and the larger hindlimbs project from the back of the body, all in the same flattened plane. Their dorsal coloration ranges from a light to a dark olive gray, and there are dark blotches spread about at random. The venter, or belly, is white with a creamy or yellowish tinge on the arms and legs. The largest specimens reach a length of only about 4 inches (10.1 cm), while many of the smaller ones are about 1 inch (2.5 cm) in length.
CUBAN TREEFROG
The Cuban treefrog (Osteopilus septentrionalis), which is native to the Caribbean, has been introduced to northeastern Puerto Rico, St. Croix, and the Florida keys, as well as the U.S. mainland from Collier County to Highlands and Palm Beach counties.
It has been rapidly expanding its range in the urban areas of southern Florida, where it does well near swimming pools, in gardens, around exotic trees such as Ficus and Casuarina, and near rundown buildings, railroad trestles, and bridges. This frog seems to be associated with a wide range of vegetation growing on disturbed areas.
For a while it was feared that this frog was seriously decimating the native treefrog populations in southern Florida, but actually it hasn’t been having such an effect. Although the Cuban treefrog does eat other frogs, even members of the same species, it doesn’t seem to be totally displacing the native species. The green treefrog (Hyla cinerea) and the squirrel treefrog (Hyla squirella), two closely related frogs, still occur in adjacent natural areas, as well as in suburban areas. Most people in southern Florida who have been observing the expanding numbers of the Cuban treefrog feel that there is a chance it could eventually displace the native species, but at this point it still occurs primarily in disturbed areas where the native species are usually reduced in numbers anyway.
For identification purposes it should be helpful to know that the Cuban treefrog is by far the largest treefrog north of Mexico, some reaching a length of about 5 inches (12.7 cm). All the true treefrogs have an expanded pad at the tip of each toe that is used for climbing. The dorsal surface of the Cuban treefrog is all green or brown, or it may be irregularly mottled. The skin on the head is fused to the skull, giving this frog an unusual appearance.
BULLFROG
The bullfrog, the largest North American frog, ranges from southern Canada to central Florida, and west through Wisconsin, Nebraska, most of Texas, and northeastern Mexico. There are also many isolated populations farther west all the way to the coast, where they are widespread throughout western California. Many bullfrog populations in Mexico, Canada, and in the western states were introduced to those areas as an edible species.
Of all the frogs found in the United States, this one probably has had the most success in surviving the effects of human encroachment. The bullfrog (Rana catesbeiana) has gone right on living in aquatic habitats in many of the largest, most populated urban areas despite everything that’s going on. In many cities, bullfrogs appear to be doing even better than their rural counterparts. This isn’t to say that they can live anywhere, because they can’t, but they are very tolerant and well adapted to some of the conditions that prevail in urban areas.
The bullfrog gets as large as 8 inches (20.3 cm). It’s usually greenish or a greenish brown on the back, with distinct bands on the hind legs. The underparts are generally lighter, with some blotching. The throat of the male is yellowish, like that of the green frog. A ridge extends from behind the eye, back over the tympanum, the frog’s equivalent of an eardrum. This ridge, unlike that of the green frog, does not extend the rest of the way along the back. The longest toe on the bullfrog’s hind feet extends beyond the webbing. This is in contrast to the pig frog’s (Rana grylio).
The bullfrog tadpole takes longer than most other species to metamorphose. Two years seems to be typical, which means this species will not become established in areas where ponds regularly dry out completely. The extended larval stage allows the tadpoles to reach lengths of almost 4 inches (10.1 cm) before metamorphosing and emerging from an underwater, gilled lifestyle. This may be important for efficient utilization of the available resources in city ponds, and it may also be an important factor leading to the large size attained by the adults.
Bullfrog
These frogs are abundant in relatively rich, shallow ponds that have gently sloping banks and never dry up. City ponds are almost always rich in nutrients, the result of erosion from runoff caused by soil compaction and exposed soils along paths where the turf gets trampled around urban ponds. In addition, fertilizers that are applied to lawns are often washed into the ponds. Resident waterfowl constantly stir the nutrients from the bottom of the pond, as well as adding their excrement to the water’s richness. Sometimes effluents from local leaky sewers also drain into these bodies of water. Together, these sources of nutrients make the ponds very rich environments. The tadpoles eat the algae that thrive under these conditions. Because the water is often quite murky, avian predators have trouble preying on the amphibians, and the dense fish populations have poor species diversity, which leads to high numbers of stunted fish. As a result, the larger tadpoles and frogs are relatively immune from the predator pressures that they might normally face elsewhere. These factors all create an environment where the bullfrog thrives.
GREEN FROG
The green frog (Rana clamitans) closely resembles the bullfrog, but it is only half as big. It can also be identified by the distinguishing ridges that extend from its head back about two-thirds the length of its body before they reach the legs.
Two subspecies of green frogs have been described. The northern form is known as the green frog (Rana clamitans melanota). It ranges from southern Canada to North Carolina, west to Wisconsin, south to Oklahoma. The southern form is the bronze frog (Rana clamitans clamitans ), which ranges from eastern Texas to southern North Carolina, and south to northern Florida.
Green frog
Courtship among these frogs extends through the warmer months. In the northern part of their range they lay their eggs during May, June, and July. This species is closely associated with permanent water and is never as terrestrial as either the pickerel frog (Rana palustris) or the wood frog (Rana sylvatica). It appears that frogs requiring standing water in the warmer months do well in urban areas where ponds almost never dry up. Frogs that leave the ponds during part of the summer are more vulnerable in densely populated areas.
In rural areas, bullfrogs and green frogs often coexist in the same ponds. But bullfrogs seem to win out in most urban ponds. I am not suggesting that this has anything to do with competition between the two. It may be that green frog tadpoles are less tolerant of rich, murky water. In the summer, when the water warms up and visibility through it is severely restricted, it is common to observe bullfrog tadpoles surfacing for what appear to be gulps of air, probably in response to the oxygen deficit on the bottom of the pond. Perhaps some green frog tadpoles cannot survive these stressful conditions.
Nevertheless, green frogs are widespread in urban environments, though they seem to choose habitats that differ slightly from the areas where bullfrogs do best.