Citizen science reveals the breeding seasons of frogs is longer in the city

Human activity is causing frogs to breed earlier and for longer. What does this mean for the future of frogs?

For humans, buildings, traffic noise and streetlights are part of everyday life. But the changes we make to the environment can have serious consequences for wildlife. As habitat is cleared to make way for cities or farms, animals living in these environments need to adapt or risk decline. We used data from the Australian Museum’s citizen science project FrogID, to reveal that frog breeding seasons are beginning earlier and are almost 3 weeks longer in areas that are highly modified by humans, including our cities. Could this be a sign that they’re adapting to urban life? It’s possible, but it’s a little more complicated than that.

Frogs, like most animals, breed during specific times of the year. When frogs breed, it’s hard to miss because there’s often a raucous chorus of croaks, clicks, cackles and bleats from male frogs trying desperately to woo a female. For many frog species, this breeding period is during the warm spring and summer months, but for some, it’s during winter, and for others, it’s all year-round. Ultimately, all of these species have a common goal: to breed at a time that maximises their offspring’s chances of survival. Usually, this is a time when food is plentiful, when there’s water around, and the weather isn’t too harsh.

Timing is everything. If frogs try to breed too early or too late in the season, it may be too hot, too cold, or too dry for successful breeding. Their eggs may dry out, or their offspring might starve and never make it to adulthood.

So how do frogs know when to breed? Frogs tune into their environment, which holds important clues as to when the conditions are suitable for breeding. The right temperature, moisture levels and abundant food will trigger the start of breeding. And when the food drops off, when it gets too cold (or too hot), or their breeding pond or substrate dries out, their attempts to breed (and their calling) stops. But if frogs are relying on their environment to tell them when to breed, what happens if the environment changes?

Cities and suburban gardens are drastically different to frogs’ natural habitats, which range from forests to grasslands. They’re often warmer, noisier, artificially brighter (from street and building lights), and more polluted. Could frogs adapt their breeding behaviours to cope with these conditions?

To answer this question, we need a huge amount of data. Specifically, we need repeat records of frogs over time (throughout the year) from many different areas to establish when breeding is occurring in different habitats exposed to various levels of human disturbance, from cities to suburbia, from farms to forest. Gathering this data is challenging in itself and is likely why no one has explored this question – until now.

Citizen science data, data gathered by members of the public, is a game changer that allows us to investigate large-scale ecological questions like this for the first time.

We analysed patterns of calling across more than 226,000 frog records submitted to the Australia-wide citizen science project, FrogID. This data spanned 42 species (the number of species with enough records) and surprisingly, we found that all 42 species had longer breeding species along a human modification gradient. In other words, species inhabiting highly modified habitats (such as cities) bred for longer than species inhabiting unmodified natural habitat. This difference was about 23 days, or just over 3 weeks. This was almost always a result of frogs in modified areas starting their breeding seasons earlier in the year (as opposed to ending their breeding seasons later in year).

Three weeks might not seem like a long time but producing these advertisement calls is incredibly taxing. It requires a lot of energy, and it alerts potential predators to their location. Given the risks, are the benefits worth it?

In the best-case scenario, changes in their breeding seasons could be providing frogs with an adaptive advantage in modified areas. Perhaps they’re taking advantage of warmer temperatures in cities to breed more. Perhaps there’s more food in urban areas to support their breeding efforts. Perhaps they’re capitalising on garden and urban ponds that are full year-round. These can be reliable breeding habitats for pond-breeding frogs as the risk of their eggs or tadpoles drying out is low.

But there’s also the more concerning possibility these changes aren’t adaptive at all, that frogs are, in fact, just wasting their efforts. Males might be calling more to females in modified habitats, but these calls might fall on deaf ears. Or males might make it past the first stage and secure a mate, but their offspring might succumb to the pressures of urban life and fail to survive to adulthood.

If frogs are wasting their energy carrying out costly breeding behaviours in human-modified habitats, this could be a huge blow to frog populations, which have already been hit hard by habitat loss and degradation, disease, climate change, fires, droughts, and floods.

Our research is the first step to understanding how frogs are responding to human-imposed pressures. Now we know that frogs are responding by breeding earlier and for longer, we need to examine the consequences. Are these changes allowing frogs to successfully adapt to human-modified landscapes? Are they mating more? Are they laying more eggs? Are their offspring surviving? These are important questions to answer and, with the help of citizen scientists, will go a long way in helping us better understand the consequences of humans on frogs and in informing appropriate management actions.

Gracie Liu, Research Assistant and FrogID validator, Herpetology, Australian Museum Research Institute; PhD Candidate, UNSW Sydney.