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Rithet's Bog LogoThe Frogs of the Bog

BARA Bugle (Broadmead Area Residents Association Newsletter),
Spring 2004 - Sharon Hartwell

It’s a great time for evening walks now that the frogs are in full chorus at Rithet’s Bog. Most people are familiar with the sound of this springtime serenade, but few of us know much about the frogs themselves. Here is a quick introduction to the two frog species found at Rithet’s Bog.

Pacific Treefrog

These are the frogs you hear singing every night. They are by far the most common frog at Rithet’s Bog, and also the cutest: a mere 5 centimetres in length (2 inches for those who don’t do metric), they vary from bright emerald green to pale tan in colour, and have a dark mask running from the tip of the nose, across the eyes and down to the shoulders. Their toes are not webbed, but end in large, fleshy pads, which gives them a somewhat tropical appearance.

Tree frog habitat varies with the time of year. Outside the breeding season, they disperse far from water and can be found anywhere in the park or the adjoining neighbourhood. Those sticky toe pads assist them in clambering over plants and yes, up trees. They hunt spiders and insects as they prowl the vegetation, and on warm, rainy days, individual males may be heard calling from their perches. The winter call is a simple, but amazingly loud and deep ‘crr-i-k’, emitted singly and at long, irregular intervals.

You are far more apt to hear one of these winter-calling Pacific Treefrogs than to see one. The males call loudly and vigorously, but quickly fall silent if they sense danger approaching. To add to the difficulties of frog stalking, they are able to project their voices, making it hard to figure out just where these mini ventriloquists are situated. As if that weren’t enough, they can also rapidly change colour, although this seems to be a response to temperature or humidity, rather than background colour.

With the approach of spring, Pacific Treefrogs travel to the nearest body of water in order to breed and lay eggs. The males also sing a new tune: the single ‘crr-i-k’ of winter gives way to the rapidly repeated ‘crek-ek’ or ‘ribbet’ of the breeding season. Many males group together, calling in unison to produce the familiar breeding chorus we hear each evening. This vocalizing serves to attract females, and it occurs at night, in the absence of the predators that hunt the frogs during the day – mainly herons, kingfishers, garter snakes and larger frogs.

Pacific Treefrogs are able to breed in shallow, temporary bodies of water which dry up later in the summer – ‘ephemeral wetlands’, such as those along Chatterton Way. This gives them an advantage over other frogs that spend their entire life in water, and must have deep, permanent water bodies in order to breed. It also means fewer predators, since there are no large fish to eat the treefrogs or their tadpoles.

Breeding and ‘metamorphosis’ (the emergence of tadpoles and their development into froglets) must occur rapidly, before the shallow wetland dries in the summer. The females lay small, irregular clusters of 10 to 70 eggs, and attach them to bits of vegetation in the water. Minute tadpoles emerge 2 to 3 weeks later, graze on algae and plant matter, and develop into tiny, 1 centimetre long frogs within 8 weeks. As the water area diminishes in size, the froglets make their way to the surrounding shrubs and forest, and the cycle is complete.

Red-legged Frog

The Red-legged Frog is much larger, quieter and far less common than the Pacific Treefrog. A drab, freckled brown in colour, it is approximately 7 to 10 centimetres in body length (3 to 4 inches), and the females are larger than the males (the norm in the frog world). It has a prominent ridge or ‘dorsolateral fold’ running from behind each eye and down the sides of the back. The long, thin hind legs are flushed a translucent red colour on the inner surface, almost as if they are bruised. The toes of the hind legs are heavily webbed, unlike those of the Pacific Treefrog.

Red-legged Frogs are found in cool, moist, shady areas such as the central pine forest of Rithet’s Bog, or the shadier portions of the perimeter ditch. They make use of shallow, slow-moving bodies of water during their very early and short breeding season, which begins in January or February and is probably already over for this year. Hopefully, you will never hear a Red-legged Frog calling! The males vocalize, like other frogs, but they do so while underwater, at depths up to one meter below the surface.

After breeding, the female frogs produce large, jelly-like globs of eggs, which are attached to vegetation and float below the water surface. Since breeding takes place in permanent water bodies, the metamorphosis process is much slower than that of the Pacific Treefrog: the eggs do not hatch for a month, and the tadpoles take up to 5 months to develop into frogs. Outside the breeding season, adult frogs spend most of their time away from water, but stick to moist, shady areas that have plenty of cover, such as fallen trees and logs.

Red-legged Frogs are one of the many species of amphibian whose numbers are declining. Habitat destruction has reduced their range, and they are included on the provincial ‘Blue List’ of species at risk. They are also one of many native species suffering from predation by the introduced American Bullfrog.

American Bullfrog – An Unwanted Intruder

So far, we have no records of Bullfrogs at Rithet’s Bog. This is fortunate, as here in western North America these large, predatory frogs are the ‘bad guys’ of the amphibian world. Originally native to eastern and southern North America, they were widely introduced in the west in the 1940s as entrepreneurs attempted to raise them for the frog leg market. When the frog farms failed (such as one at Elk Lake) the frogs escaped and became naturalized in the surrounding wetlands. Bullfrogs are now an invasive species posing serious ecological problems. They displace and prey on native amphibians, and also eat the chicks of small waterfowl such as grebes, rails and coots.

Bullfrogs are huge and unmistakeable. Eight to 20 centimetres in body length, they have long, muscular legs and can weigh up to half a kilogram. They need deep, permanent water for breeding (a fact which has helped to keep them out of Rithet’s Bog) but are capable of hopping long distances on land. When in the water, they float at the surface with their large eyes exposed. Their call is a loud, bass ‘brummm’ which can travel for up to a kilometre. Tadpoles are also large (the size of a child’s fist) and spend 2 years in the water before metamorphosing into frogs.

If you see or hear a Bullfrog, please report it to Uvic researcher Purnima Govindarajulu, Phone:250-472-4684 or 250-383-6262, e-mail:purnimap@uvic.ca. Your information will assist in monitoring and controlling this invasive frog.

Further Information

If you want to learn more about frogs, toads and amphibians in general, here are three excellent web sites:

http://wlapwww.gov.bc.ca/wld/frogwatch, home of the BC Frogwatch Program.

http://web.uvic.ca/bullfrogs/index.htm, home of the Bullfrog Monitoring Program. Hear sound clips of all the frogs mentioned in this article, and learn more about the problems caused by the spread of Bullfrogs in BC.

http://www.carcnet.ca/, home of the Canadian Amphibian and Reptile Conservation Network. Treats all amphibians and reptiles found in Canada, not just frogs and toads.

Happy Frog Watching!

Sharon Hartwell

RBCS

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