Hybrid frogs discovered: A hidden threat to an endangered frog

Genetic data has revealed that the endangered Booroolong Frog is breeding with a more common species, the Eastern Stony Creek Frog. Hybrids of these two species were hiding in plain sight, often showing few physical signs of being different to their parents. The frequency of hybridisation was surprising, and we must now consider the risks that hybrids may pose to our threatened species.

When we think about threats to wildlife, we often think of deforestation, climate change, disease, or the spread of invasive species. But there’s a less obvious threat that has the potential to undermine conservation progress for some of our most vulnerable animals: hybridisation.

Hybridisation occurs when different species breed and produce hybrid offspring. Most of us would be familiar with hybrid animals. You may have heard of ligers (lion-tiger crosses), mules (donkey-horse crosses), grolar bears (grizzly and polar bear crosses), or a plethora of other seemingly bizarre pairings. Sometimes, these hybrids are the result of human intervention – for example, by putting species together in captivity that don’t normally interact in nature – and sometimes they are infertile. However, hybridisation is a natural process that occurs relatively frequently in the wild between related species, and hybrids can often be fertile, breeding with each other and with the parent species.

In some cases, hybridisation is harmless or even beneficial, introducing new advantageous traits. Because of this, hybridisation can be an important evolutionary force. But it can become a serious problem if the hybrids are very successful – for example, if they consistently outcompete the parent species for food, space, or other resources. It can also be a problem if the hybrids are very unfit, meaning that the parent species waste valuable reproductive efforts producing offspring that don’t survive. This could lead to the decline or even extinction of the parent species. For species that are already rare, threatened or declining, hybridisation could push these species further towards extinction.

Hybridisation between the Booroolong Frog and the Eastern Stony Creek Frog

Recently, we detected hybridisation between the threatened Booroolong frog (Litoria booroolongensis) and the more abundant Eastern Stony Creek Frog (Litoria wilcoxii), two Australian stream frog species. This discovery has important consequences for how we manage Booroolong Frog populations.

While the Eastern Stony Creek Frog has been doing quite well and is often locally abundant across its eastern Australian distribution, the Booroolong frog has declined dramatically in recent decades. Today, the Booroolong Frog continues to face threats from habitat modification, disease (notably, the amphibian chytrid fungus) and predation by introduced fish, and now its genetic future could be further compromised by these interbreeding events.

Discovering Booroolong Frog and Eastern Stony Creek Frog hybrids was a surprise to us. Before our study, there was no official documentation of hybridisation between these species or what they might look like. We didn’t even know that hybridisation occurred between the species, so documenting it certainly wasn’t on our mind. But during regular field surveys, we noticed some slightly odd-looking individuals. They looked like Booroolong Frogs, but also like Eastern Stony Creek Frogs! We had trouble assigning these individuals to one species or the other. Suspecting that some of these individuals might have been hybrids, we dug deeper.

We analysed genetic data from more than 120 individuals of suspected Booroolong Frogs, Eastern Stony Creek Frogs or their hybrids in the Central Tablelands of New South Wales to figure out their true identity. The results were unexpected: almost 1 in 5 of these frogs were hybrids or introgressed individuals (the result of hybrids breeding with each other or with one of the parent species).

We detected these hybrids in all five of the sites where both species co-existed and, in some sites, more than 30% of the frogs were hybrids. These are relatively high rates of hybridisation, made more concerning by the fact that some hybrids weren’t necessarily obvious from their physical appearance and that more than 70% of the Booroolong Frog’s geographic range overlaps with the range of the Eastern Stony Creek Frog. This means that hybridisation could be affecting many Booroolong Frog populations without us realising.

The hidden threat of cryptic hybridisation

Our findings highlight the need to consider the threat of hybridisation to the already threatened Booroolong Frog. To do this, we need to accurately identify hybrids across the range of the species and monitor their impacts on local populations. Unfortunately, knowing whether an individual is a hybrid or not is not as simple as just looking at it.

While some hybrids of Booroolong Frogs and Eastern Stony Creek Frogs were obvious from their physical appearance, others looked almost identical to one of the parent species. In other words, they were cryptic; their physical appearance was not always a reflection of their true identity. In fact, only about 42% of hybrids were correctly identified as hybrids based on their appearance alone. This shows just how tricky it is to detect hybrids without genetic testing.

What Needs to Be Done?

By using genetic tools, we can ensure that hybridisation doesn’t continue to slip under the radar and unknowingly contribute to the decline of the Booroolong Frog or other threatened or range-restricted species. There could be many more undocumented cases of hybridisation among frogs, and it is important that we explore this possibility.

Dr Gracie Liu, Scientific Officer, Herpetology, Australian Museum Research Institute.