The ideal amphibian pond

By definition, a pond is a standing water body with a surface area of less than 5 hectares. These are estimated to make up more than 30 per cent of the world's inland waters. In recent years, they have suffered particularly from water shortages. Across Europe, they are at historic lows and many are drying up permanently. For amphibians that live on land and in water, they are small oases. "However, water scarcity, increasing pressure on the surrounding landscape and climate change with its extreme weather events are severely affecting these ecosystems and the amphibians that depend on them", explained IGB Director Professor Luc De Meester, co-author of the study.

Amphibians are the most threatened group of vertebrates. In Europe, around a quarter of the species are classified as threatened (i.e. categorised as ‘vulnerable’, ‘endangered’ or ‘critically endangered’) according to the Red List of the International Union for Conservation of Nature (IUCN). “We therefore need to better understand which factors we can use to positively influence amphibian populations“, said IGB scientist Dr Thomas Mehner, co-author of the study.

These amphibians were found in the ponds studied

Thirty different amphibian species were found in all of the ponds studied. The average number of species per pond was 3 species, with Spain having the highest average local richness with around 5 species and the United Kingdom the lowest, with around 2 species. Some species were restricted to a single country, with Spain having the highest number of unique species (9), followed by Turkey (5), Switzerland (1) and Germany (1).

The most common species was the common newt (Lissotriton vulgaris), which was found in 41.8 per cent of all ponds in the countries surveyed, followed by the northern crested newt (Triturus cristatus, 30.4 %), the common toad (Bufo bufo, 27.9 %) and the European common frog (Rana temporaria, 25.4 %).

It depends on the latitude, but local factors that can be influenced also play a role

It is known from other studies that the species richness of amphibians is related to latitude, as climatic factors such as water availability and temperature are important factors limiting the distribution range of these cold-blooded and highly moisture-dependent animals. The current study also shows this relationship to climatic factors. “Apart from that, however, we show that local characteristics of ponds play just as important a role as climatic factors. And these can of course be better influenced“, saif Thomas Mehner.

The researchers found the highest diversity of amphibian species in ponds with few nutrients, no fish, medium size, shallow water and well-developed shoreline vegetation. Ponds located in protected areas had a slightly higher amphibian species richness. The following measures can therefore be used to create a species-rich amphibian pond by taking local factors into account:

Reduce nutrient pollution

The most important indicator to explain the variation in amphibian species richness in this study was the chlorophyll-a concentration. The chlorophyll-a concentration indicates the algae biomass and is therefore also an indicator of nutrient pollution. This is because algae can grow particularly well when there are a lot of nutrients in the water. Nutrient pollution in ponds can reduce the species richness of amphibians by reducing the survival rate of eggs and larvae, reducing reproductive success and increasing susceptibility to disease. “Nutrient pollution is a common problem in ponds that are located near agricultural land and are exposed to increased amounts of artificial and natural fertilisers. Therefore, management options to increase amphibian species richness in ponds should focus on the reduction of nutrient pollution and on livestock management that restricts cattle access to the respective ponds“, said Thomas Mehner. This applies in particular to ponds that have previously only been exposed to low levels of nutrient pollution: At low concentrations, even a small increase in chlorophyll-a levels has a strong negative impact on amphibian abundance, according to one finding.

The optimum pond surface area and depth

Pond area and depth are also important factors for amphibian richness. The results show that the highest species richness was observed in medium-sized (200-2,500 m2) and shallow (less than 1.5 m deep) ponds. This pattern could be due to the fact that smaller and shallower ponds are more likely to dry out, while fish are more likely to live or survive in larger and deeper ponds. “These mechanisms may lead to amphibian abundance peaking at intermediate values of pond size and depth“, said Thomas Mehner.

Balancing amphibian and fish conservation

The number of fish species was found to be a significant factor in the decline of amphibian diversity, especially when three or more fish species were present in a pond. Predatory fish are generally considered to be the cause of the decline in amphibian diversity. Amphibian eggs, tadpoles and adults are susceptible to predation by various fish species found in the study (e.g. the inasive species prussian carp and sunfish, or the native species pike). Additionally, fish can compete with amphibians for food resources and fish introduced into ponds can transmit pathogenic fungi, further increasing egg mortality. “In terms of management, amphibian ponds without natural fish stocks should also be kept fish-free. However, it is important to remember that some European fish species - such as the crucian carp - are themselves endangered, and it is important to take this into account when making management decisions“, said Thomas Mehner.

According to this study, pond vegetation and land use play a subordinate role

Although pond vegetation is important for the amphibian population, for example because it provides shelter, protection and food, it explained only a very small part of the variation in amphibian species richness in this study. This observation is in contrast to other studies that show that vegetation is a reliable determinant of amphibian abundance. Even the direct land use factors within a radius of 100 metres, such as an urban environment, agriculture, roads or protected areas, did not have as strong an influence on amphibian species richness as the other factors.

“The study shows that species-rich amphibian waters can actually occur anywhere - even in an urban context.  This study can feed directly into the development of Europe-wide initiatives such as the European Pond Conservation Network and provides important information for pond projects at a local level, enabling decision-makers to take better-informed amphibian conservation measures“,  said Thomas Mehner.

Methodology

The research team included 201 small waterbodies in seven European countries in the study and defined certain characteristics for each pond. The species diversity of the amphibians was determined with the help of eDNA from water samples. eDNA stands for environmental DNA. This makes it possible to determine the genetic fingerprints that organisms leave behind in their habitat. The relative influence of climate, local abiotic and biotic factors and land use variables on the variation in amphibian species richness in the ponds was quantified using boosted regression trees. Ecologists use this statistical model to explain relationships between different environmental factors and to make predictions. It is flexible enough to account for typical characteristics of their data such as non-linearities and interactions.

What the study did not take into account

The study did consider current climatic drivers, but not historical climate variables. Previous studies have shown that palaeoclimatic processes have a major influence on the current gradient of amphibian species richness. Future studies incorporating these data could therefore provide an even more comprehensive understanding of how historical and contemporary processes interact in shaping amphibian diversity.

Furthermore, not all influencing factors were considered, such as food availability (density of large invertebrates), shading or connectivity between ponds, the presence of environmental chemicals and their effects. Field studies areable to establish clear cause-and-effect relationships only to a limited extent. Nevertheless, the use of boosted regression trees in this context of complex data structure is reliable and the results provide a clear pattern of how multiple pond characteristics play a role in explaining amphibian species richness of pond ecosystems at a large geographic scale.