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Effects of Pesticides on Bombina bombina in Natural Pond Ecosystems

1 Introduction

• 1.1 Background and aims
• 1.2 Biology of Bombina bombina
• 1.3 Criteria for selecting Bombina bombina

1.1 Background and aims

Pesticides are an important part of modern agriculture and are a potential threat to plants and animals in the margin of the fields. The influence of pesticides on world-wide amphibian decline is reviewed by Bishop (1992) and Carey & Bryant (1995). It was concluded that co-ordinated field and laboratory studies are needed to establish, whether causal relations exist between use of pesticides and the decline of amphibians.

This investigation is the field investigation part of such an attempt.

The aim of this field investigation was to clarify the impacts of pesticides in and near the habitats of an endangered species, Bombina bombina. Various lifestages of B. bombina were investigated under field conditions in the attempt to find effects of pesticides. Effects of pesticides on algae and invertebrates affect amphibians indirectly, as they are an important food source to tadpoles and adults respectively.

Hypothesis

It was the intention of the investigation to test the following hypothesis:

• Peak concentrations of pesticides decrease with increasing width of buffer zone around ponds. This lessens the worst case accidents of pesticide contamination of ponds.
• Pesticides affect survival and growth of Bombina bombina eggs and tadpoles under natural conditions.
• Pesticides affect the diversity and abundance of algae, bacteria and protozoa, which are important food items for tadpoles, and thereby have an indirect effect on growth of tadpoles.
• Modern agriculture affects the yearly survival rate of adult B. bombina.

1.2 Biology of Bombina bombina

Distribution

Bombina bombina is a small frog. The adults reach a maximal length of 55 mm. The back is brown, or nearly black, often with two light green spots on the neck, resembling duckweed. As a sharp contrast the belly has a very distinctive orange red pattern on a black background (figure 1.1). The pattern is highly specific for each individual which makes it possible to recognize individual frogs in a population year by year. B. bombina is found in 8 populations in the eastern and southeastern part of Denmark, and its existence in Denmark has always been limited to the islands east of Jutland. It requires a continental warm climate and is therefore on its northern and western border of its distribution in Denmark. B. bombina has a wide distribution in the North European lowland in Eastern Germany (former DDR), Poland, Lithuania and Belarus. It is also distributed in the lowlands of Russia (west of Moscow), Ukraine, Hungary, Czech, Romania, Bulgaria and former Yugoslavia.

Figure 1.1
The highly specific belly pattern of Bombina bombina can be used to recognize individuals in a population.

Klokkefrøens specifikke bugmønster kan bruges til at genkende individer i en bestand.

Ideal biotopes

Ideal biotopes for Bombina bombina are extensively grazed fringes and meadows with numerous more or less permanent ponds. B. bombina is often found in ponds surrounded by cultivated land, but requires at least that the surrounding area is fragmented by scrubs, quickset hedges, stone fences or some fallow land (Briggs 1993). In many parts of Europe the main part of the populations live in agricultural areas, for example in East Germany (Schneeweiss & Schneeweiss 1997) and in Poland (Briggs pers. obs.). In Denmark, adult B. bombina often only migrates 200-300 m between breeding and foraging ponds as well as to winter biotopes (Briggs 1993). Therefore their terrestrial habitat must be in the near vicinity of the ponds.

Prefers shallow temporary ponds for breeding

Bombina bombina prefers shallow temporary breeding ponds (30-60 cm) that do not dry up until the end of the summer. These ponds only exist in areas where the water table has not been lowered by drainage, and therefore such biotopes have become increasingly rare during the last 50 years in Denmark. The breeding ponds must be fully exposed to the sun and preferably sheltered from the wind because the tadpoles require warm water (>20 °C ) to complete their development. There must be densely vegetated areas where the frogs and tadpoles can hide from predators as well as zones with open water. B. bombina tadpoles graze upon the surface of macrophyte stems, feeding on algae, bacteria and protozoa (Mossin 1988, Andersen 1992).

Foraging ponds

After the breeding season the frogs often migrate to forage in other ponds. Foraging ponds are often eutrophic, and partly overgrown. Normally Bombina bombina enters the terrestrial habitat in the late summer where it hibernates from October to April.

In summary their yearly activity cycle is like this:

Yearly activity cycle

Exceptions from this scheme occur, and for example some individuals stay in the same pond from April to September. The newly metamorphosed frogs usually appear in August and, after an aquatic foraging period, they migrate towards the hibernation sites. The following 1-2 years are a foraging and hibernation period. At the age of 2-3 years they start breeding. The migrations observed in the studied habitat on Avernakø are of a length of 100-500 m (Briggs 1993).

Figure 1.2
Example of a good Bombina bombina pond (pond P3).

Eksempel på godt klokkefrø vandhul (vandhul P3).

1.3 Criteria for selecting Bombina bombina

The following describes why amphibians generally, and Bombina bombina in particular, are well suited for studies of the impact of pesticides and modern farming in general on pond ecosystems and the surrounding areas.

Amphibians in general

• Amphibians are dependent on both an aquatic and a terrestrial habitat. They form an important food-chain link between invertebrate prey and predatory vertebrates.
• Amphibians are thought to be sensitive to environmental pollutants because the pollutants can be absorbed both through the skin and larval gill membranes and taken in together with contaminated prey.

Bombina bombina was selected for this investigation according to the following criteria:

• Bombina bombina in particular
• It is widespread in the northern and central European lowland agricultural areas.
• It is threatened and in rapid decline in Germany, Poland, Czech republic and Hungary, and presumably also threatened in the rest of the European area of distribution.
• Its habitat is mainly aquatic in the season of intense spraying of pesticides (April-June).
• The adult frogs seem very vulnerable to the effects of intensive farming and the status of B. bombina in a pond therefore reflects whether the local farming practice of the surrounding fields is sustainable.
• The presence of B. bombina indicates a relatively large diversity of amphibians, algae, bacteria and presumably also invertebrates (Andersen, 1992).
• The feeding biology of the tadpoles of B. bombina has been studied thoroughly in Denmark (Mossin 1988; Andersen 1992). Breeding success has been monitored on a long term basis in Denmark (Briggs 1993).
• Juveniles and adults have an individual and easily recognisable belly pattern. Migrations and survival of individual B. bombina have been followed in Denmark from 1987 to 1993.
• The closely related species Bombina orientalis and Bombina variegata have proven to be very useful in toxicological tests.

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Version 1.0 October 2004, © Danish Environmental Protection Agency