INVASIVE FISH SPECIES CONTRIBUTE TO THE DECLINE OF THE COMMON TOAD
Most amphibians rely on ponds for reproduction, which makes these habitats crucial to their life cycle. This is also true for the common toad, a species that was once widespread, but whose population sizes are now shrinking drastically. In this study (conducted in collaboration with INBO), we examined the extent to which aquatic communities are associated with the status (declining or stable) of common toad (Bufo bufo) populations. We found that communities rich in fish, especially those containing invasive species, place additonal pressure on toad populations.
Some background
The common toad is experiencing a general decline across Europe. While the exact causes remain unknown, it is believed that multiple stressors are involved. In this study, we investigated whether aquatic community composition can contribute to the decline of this otherwise common species. Therefore, we combined fieldwork with an experiment designed to assess the impact of aquatic communities on the common toad.
Predation experiment
In our experiment, we placed common toad egg strings and tadpoles into containers. We then introduced various fish species, three invasive and three native, to these containers (one fish species per container) to observe the extent to which each species preyed upon the early life stages of the common toad.
We found that:
The survival rate of eggs and tadpoles to be significantly lower when kept in a container with an invasive fish species. In other words, the predation pressure is much higher.
Native species do not effect toad egg string survial, but the ten-spined stickleback does reduce the survival rate of larvae.
Environmental DNA in ponds
We also collected water samples from 20 ponds, half with stable toad populations and half with declining populations. These samples allowed us to conduct environmental DNA (eDNA) analyses, identifying the species present in the water based on the DNA found. Since this method cannot detect crayfish, which can also significantly impact toads, we subsequently set traps to determine whether they were present.
We found that:
Declining toad populations are associated with communities rich in fish.
No direct link between invasive species and declining populations, but nearly all invasive species were detected in ponds where toad populations were in decline.
Implications for nature conservation
These findings contribute to the management of toad breeding ponds. They demonstrate that predation pressure from (invasive) fish should not be underestimated. The impact on the survival of egg strings or tadpoles varies depending on the fish species and the complexity of the pond environment. Despite these variations, it is crucial that the management of toad ponds considers not only water quality but also the composition of the aquatic community. Controlling fish populations, especially invasive ones, can be an effective strategy for maintaining stable toad populations.
Want to learn more?
You can read the full scientific study here. This research part of the OSSOBufo project, supported by the Special Research Fund of Ghent University (BOF23/GOA/008), investigating causes of decline of the Common Toad (Bufo bufo). If you want to learn more about this project, you can visit the project page on our website.
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