by TFH Magazine on April 19, 2013 at 7:26 am
Springtime is heralded by the sound of croaking frogs coming from the garden pond. In London, where nowadays winters are relatively mild, the European common frogs, Rana temporaria, somewhat similar in appearance and life history to American leopard and green frogs, Rana pipiens complex and Rana clamitans can appear in the pond as early as mid-February.
Male frogs tend to arrive at the pond before the gravid females. A frenzy of activity ensues when the females arrive as males try to grab a female partner, mounting her and enveloping her in a spawning clasp (amplexus). A large female lays a few thousand eggs, which are fertilized by the male while on her back. The eggs initially sink but rise again as their protective jelly coating absorbs water. When spawning is complete, the adult frogs play no further part in the development of their young, and they are left to the elements.
From Embryos to Tadpoles
The black eggs develop into long, black, sliver-like embryos that at hatching have the head hardly discernible from the body. These tadpoles feed primarily on algae but also take any decaying matter such as is found in all ponds. The tadpoles breathe through external gills to start with, but as they grow the gills are covered by a fleshy operculum and become internal.
Frog tadpoles develop their hind legs first, and sometimes weeks after front legs appear. The tail is slowly absorbed and shortens as the tadpole starts changing color and configuration of its mouth form and takes the form of a tiny frog. Froglets have to leave the water as their diet changes to small insects and other tiny prey.
The Frog-Friendly Pond
The ideal pond for frogs should be as large as possible, since larger bodies of water are more likely to develop into ecologically balanced, stable systems. An important feature of the pond is that it is stepped at the edges so the pond margins are as little as 6 to 12 inches in depth, which the middle is at least 3 feet deep.
The edges of the pond should be planted with both marginal plans and submersed plants. Suitable marginal plants at this latitude include Mentha aquatic (water mint), Caltha palustris (marsh marigold) and Typha minima (dwarf cattail). Submersed plants are also very important in these shallow areas and could include flora such as Myosotis scorpioides (water forget-me-not) water starworts (Callitriche spp.) and Canadian pondweed (Elodea canadensis).
General cover could also be provided by floating plants such as Hydrocharis (aptly named frogbit) and Azolla caroliniana (fairy moss) as well as water lilies (miniature forms such as Nymphaea odorata minor and N. pygmaea alba or larger types such as N. odorata alba).
The pond, depending on size, could house a few goldfish, though generally fishes will eat or bother frog eggs and tadpoles. The important thing is not to overstock the pond with fish.
The shallow planted edges of the pond are important initially as they provide calling and egg-laying sites for the frogs. Later tadpoles develop into froglets, they tend to remain in shallow water for a time before emerging from the water to the land, often all at once. During this vulnerable period before they leave the water, a well-planted shallow area provides cover from predators such as birds. Another important function of the shallow area is to enable to froglets to leave the pond easily.
Frogs spend the majority of their six-year natural lifespan out of water, though they remain near the home pond. Hence pond surroundings are almost as important as the pond itself for the survival of frogs in the garden. Part of the pond border should have fairly dense vegetation and perhaps a compost heap nearby to provide damp cover for the frogs and their prey. Here the frogs will be able to find their natural foods such as insects, pillbugs, spiders, and slugs. You should consider maintaining this part of the garden organically, without the use of insecticides and herbicides. Another way of encouraging frogs to stay in the garden is to provide areas nearby with piles of old wood or stones under which frogs can hibernate during winter.
Frogs in Danger
In Britain, Rana temporaria (as well as other frogs and toads) is disappearing from the countryside because of loss of habitat. As small farms surrounded by hedgerows and the customary ponds give way to huge monoculture farms devoid of ditches, wetland areas, and small ponds, all the small wild animals lose habitat and often disappear. Use of chemicals to maintain these new farming methods have proved equally destructive. Moreover, effluents from farms running into ponds, streams, and rivers have often killed both fishes and other aquatic creatures.
Frogs tend to return to the same breeding pond year after year, usually where they were born. Froglets leave the water and can move some distance in the two or more years they take to reach sexual maturity before they feel the urge to return to “their” pond to spawn. The frog’s journey to the pond can be a perilous trip these days. Apart from natural predators that frogs have to avoid, including birds such as herons, owls, and magpies plus grass snakes (Natrix natrix) in warmer weather, the frogs might have to run the gauntlet to cross roads, where they are run over by cars.
As people have become more interested in the plight of frogs, conservation groups are able to instigate the laying of culvers under roads that are used by a large number of frogs and toads on their yearly migration from hibernation places to spawning sites.
The suburban garden pond is one way of ensuring the survival of the common frog in Britain. But even here the frog is under attack, although mainly from natural predation, starting from the egg all the way to the adult. Eggs are at risk from the time they are laid. For a start, all the eggs might not have been fertilized and will therefore not develop at all and will start to fungus. The fungus might spread to fertilized eggs, killing them. The eggs are to a large extent reasonably safe from fishes because in early spring fishes are not fully active.
Once the eggs hatch, the tadpoles are open to predation by fishes as well as by other pond inhabitants such as dragonfly nymphs and water beetles, but sufficient numbers of tadpoles should survive each year, provided the pond is reasonably well planted and not overcrowded with fish. The tadpoles develop into froglets that have to leave the water to spend the next couple of years on land. If the pond is steep-sided, this could provide an impossible task, resulting in the froglets drowning.
Once on land, froglets make a tasty snack for European blackbirds and grass snakes. They are also in danger from manmade hazards in the form of poisons such as insecticides and slug pellets used to control garden pests, which in fact are food to frogs; when contaminated prey end up in their stomachs the result is a slow painful death.
Froglets tend to be out and about at night but might still be about in the morning in the wet grass, where they are in danger not only from cats and dogs but also from enthusiastic gardeners with a lawn mower. Today a variety of predators from owls to herons and badgers to foxes have also moved to suburbia, where they will happily take a frog it they catch one. This is the natural order of things and is beneficial in the long term as it usually removes weak or sickly animals. However, if your garden is frog-friendly, sufficient numbers should survive each year to start a new generation each spring.
Frogs add a lot of interest to the garden pond. The whole family is able to watch the natural cycle of metamorphosis spawning to egg to tadpole to frog. Frogs in the garden pond are a sure sign that you have a well balanced natural habitat in your garden. Usually there is no need to introduce frogs to a new garden pond – frogs from surrounding areas should make their way to the new pond themselves, provided the pond offers a suitable habitat and the garden is not surrounded by impenetrable fences. However, you could introduce a small amount of frog spawn to the pond in spring. (Beware local and national laws that may prohibit collecting frog eggs or moving them about from one locality to another; never introduce a foreign frog into a pond).
Only a very small proportion of tadpoles and froglets survive each year, the vast majority ending up as an important food source for all the varied inhabitants that go to make up a lively, interesting garden. By having a healthy, frog-friendly pond in your garden, you will be doing your small part to ensure the survival of the frog.