Connectivity of green and blue infrastructures: living veins for biodiverse and healthy cities

Urban amphibians

Are there frogs in cities? Where are they?

In 2016-2017 I participated in a National Geographic Society expedition to find a rare—and, let’s face it, ugly—frog in southern Chile. I wrote about it here if you read Spanish, its for you, otherwise you can just enjoy my drawings and the photos of my colleague Andrés Charrier. Writing about the experience of finding frogs in rural valleys and mountains made me wonder, “Are there frogs in cities? Where are they?”

I grew up in a town, not a city, and behind our house there was a remnant woods and a small wetland. There were lots of amphibians there. In spring, there were always a few weeks when the lawn was full of tiny toads and frogs, about 1cm long, going somewhere. My brother and I captured a toad and two tree frogs, which we fed with sweep-netted insects in summer and mealworms and crickets in winter. The toad made a little hollow under a piece of bark, and the tree frogs sat next to each other in the upper corner of the terrarium. Watching them suddenly become rapid and riveted when a jar of leafhoppers and grasshoppers appeared was always a treat. Their tongues flip out, like a catapult. They use their front toes to stuff antennae into their mouths. Their eyes sink into their heads when they swallow.

The only time I ever saw a salamander, though, was in Denmark, near a wetland, in the countryside.

Amphibians are mostly metamorphic, and their eggs and larval stages (tadpoles) are aquatic. Usually they need gently flowing streams or ponds, with some overhanging vegetation or aquatic vegetation to protect them from predators, although some require full sun. Tadpoles are carnivorous, and so they also need little invertebrates in the water to eat. As this kind of habitat is not necessarily very common in cities, one might imagine that amphibians rarely breed in urban areas. Even if they can breed, the adults will usually disperse—which means they need other moist or aquatic habitats to disperse to, and suitable paths to get to them by. And one thing you learn when you look for amphibians, is that they spend most of the day taking refuge, like my toad, under pieces of bark, leaf litter, logs, and large rocks. Such things are relatively rare in cities where tree parts are generally not allowed to accumulate and slowly decompose. Urbanization de-links the different habitats that amphibians need at different times in their lives. It also creates dangerous habitat, including roads and sewers (amphibians fall in and drown or can’t get out again). Are there natural populations of amphibians in European cities?

Kazemarek et al. (2014) report on amphibians in Poznan, Poland, one of the cities in BIOVEINS. Amphibians are more abundant in Poland than in Western Europe. They report finding 9 species of amphibian (frogs, toads, and salamanders) in Poznan, with all but one principally living in remnant forests and urban parks. They found that, compared to a study from 20 years ago, some amphibian breeding sites had been destroyed, and more sites had lost species than had gained them. However, the mean number of species per site had not changed. The authors suggest that over time, increasing car traffic will slowly wipe out these urban populations.

Ficetola & De Bernardi (2004) studied the persistence of amphibians in wetlands of peri- and sub-urban Milan, in Northern Italy. They write, “The commonest species, the pool frog (Rana synklepton esculenta) and the Italian tree frog (Hyla intermedia), are able to move through the matrix using canals and hedgerows, and can maintain [populations] across the landscape;”-- as you will recall from the post on fragmentation, the “matrix” is inhospitable anthropogenic landscape—“the rarest species (newts and toads) are more sensitive to habitat alteration, and they are strongly affected by isolation effects. If human exploitation of the landscape continues, only few species, mobile and opportunistic, will persist in this landscape.”

In addition to cars, another major predator of amphibians, which increase with urbanization, are fish. In Australia, Hamer & Parris (2011) found that frog “species richness decreased at ponds surrounded by high densities of human residents and at ponds with high water [speed].., whereas species richness increased substantially at ponds surrounded by a high proportion of green open space… Urbanization had strong negative effects on [frog] species that were associated with well-vegetated, sunny, fish-free ponds.”

Rubbo & Kiesecker (2004) explain how fish are helped by urbanization. They looked at tadpoles along a rural-urban gradient in Pennsylvania, USA. They explain that, “[u]rbanized wetlands commonly [are].. modified …leading to a permanence of standing water. In accordance with an increase in the frequency of permanent water, there was an increase in the presence of fish—which are common predators of amphibians—in suburban and urban sites.” They point to the need to maintain a gradient of types of wetlands, managed in different ways—something mentioned by most papers on this topic--including allowing wetlands to dry out periodically. This seems a little bit counterintuitive, and certainly goes against the urban park and garden aesthetic. The water features in a garden are not supposed to dry up and disappear. Nature, though, is about change, cycles, ephemerality, patterns. One of the problems with cities is that we spend a lot of effort (usually) on maintaining them in a stable state (see the post on Ruins). This gives little toe-hold to the many species that thrive on the waxing and waning of dry and wet, shade and sun, hot and cold, exposed and covered, etc.

Rannap et al. (2009) describe successful restoration of pond landscapes in protected areas south of Tartu. They constructed hundreds of ponds, in clusters, with varying depths, slopes, sizes, and littoral widths. They also removed the fish. The ponds were soon (re-)colonized by endangered amphibians. Could something like this be done to connect urban, peri-urban and rural areas? In Switzerland, Schmidt & Zumbacht (2008) reviewed methods for either helping amphibians cross roads, or preventing them from doing so. These include temporary and permanent fences along roads, and road underpasses. However, there are a number of issues with these design technologies. For example, it is thought that newts may dry out while trying to cross underpasses. More research and design efforts to help amphibians cross human infrastructure would be valuable.

Amphibians are not one of the taxa included in BIOVEINS. I asked Dr. Francois Chiron, one of the researchers, why. He replied that although amphibians are a good study taxa for understanding urban connectivity, “finding good sites was complicated with the database we were using (Urban Atlas) which lacks the resolution to show a pond or a water body of medium size.” In other words, frogs, toads and salamanders need habitats that are generally smaller than a park. At the same time, they need a network of other habitat types too—but this fine scale embedded in an urban or peri-urban matrix is hard for us to detect when starting with mapped satellite data.

So, there are frogs, toads and salamanders in cities—but do we know how to find them? In a forest you go along, turning over logs and stones (be sure to turn them back!). Where might you find them in a city?

I know one place where I have seen frogs in Paris—the Ile Saint Germain park. I saw it one day in summer in the small pond near the entrance. A little boy was also looking at it and excitedly told me that when the frog wasn’t moving it was invisible!

I suggest that an interesting game to play in a city would be to calculate the average time it takes you to find a frog, from your house to actually seeing the frog (or toad or salamander). Captive amphibians, for example in a zoo, obviously don’t count. From this, we could make a map of mean urban frog visibility. Maybe next summer?

Note: Our skin is not good for amphibian skin, and if you touch or hold an amphibian, do not do so for very long. Skin infections are also a common cause of amphibian mortality. If you touch more than one frog, especially frogs from different places, disinfect your hands or change your gloves in between. Some amphibians are illegal to capture and keep without a licence.

The drawings are mine ((c) MR-B). The photo with caption is from Schmidt & Zumbacht (2008).

--Meredith Root-Bernstein