By Tibor Kovács
The directorate of Danube-Drava National Park in Hungary initiated a long-term monitoring programme on the Hungarian side of the Drava River. Besides the amphibians and reptiles, the programme involves some 25 animal and plant groups that supposedly indicate environmental changes. I was focusing on the breeding sites of the amphibians rather than finding them after the mating season. The ultimate reason for the programme is to reveal the biodiversity before a water power station is built on the Croatian stretch of the river. Similar works were carried out on the local herpetofauna in Szigetköz (Gubányi and Creemers 1994) and Kis-Balaton (Kovács 1996) in western Hungary.
Three areas were involved in the monitoring: Őrtilos (46°17.0´; 16°53.5´), Lankóc Wood (46°13.5´; 17°03.5´) and Vízvár (46°06.0´; 17°13.5´). The breeding sites included abandoned gravel pits, fishponds, oxbow lakes, backwaters and alder swamp forests. Altogether 18 water bodies were surveyed of which 6 were selected for regular monitoring. The survey methods partly complied with the National Biodiversity Monitoring System (Kiss 2001) but also considered the specific features of each locality.
During the last two years I have found 12 amphibian and 3 wetland reptile species in the surveyed lakes and ponds: The Danube crested newt (Triturus dobrogicus), smooth newt (T. vulgaris), fire-bellied toad (Bombina bombina), common toad (Bufo bufo), green toad (B. viridis), tree frog (Hyla arborea), spadefoot toad (Pelobates fuscus), moor frog (Rana arvalis), agile frog (R. dalmatina), edible frog (R. kl. esculenta), pool frog (R. lessonae), marsh frog (R. ridibunda) terrapin (Emys orbicularis), grass snake (Natrix natrix) and dice snake (N. tessalata).
Moor frogs and agile frogs appeared in the same ponds roughly at the same time and probably competed for the better breeding sites. Moor frogs formed compact choruses and built egg-mats unlike the agile frogs, which dispersed their eggs. When the water level was low the moor frog sites became insufficient for choruses and laying eggs. So, while 2000 was definitely a rich year in Lankóc Wood, the drier 2001 season resulted in poor recruitment of moor frogs.
Green toads were not found in the breeding ponds in 2000-01. This was surprising since considerable numbers were killed on the roads of the surrounding villages. It seems that they are unwilling to compete with other species for the best breeding sites, and instead seek ephemeral puddles.
Large newts were identified as Danube crested newts by using the Wolterstorff index (forelimb length/distance between legs). This species occupies the Carpathian Basin (Griffiths 1995) and not the related northern crested newt (T. cristatus) from which it was separated. Danube crested newts were always found together with smooth newts, whereas the latter species occupied some ponds where the first was not present. The relative density of smooth newts was always higher than that of Danube crested newts.
Marsh frogs (from the green frog complex) are a rare species in the region. Even in the larger fishponds or lakes I found pool frog/edible frog communities or pure edible frog populations indicating disturbed habitats.
One of the most crucial factors affecting the breeding sites and amphibian breeding success is the water level of the Drava river, which changes dramatically depending on the schedule of the Dubrava power station in Croatia. Fast daily changes of water level make several inlets of the river insufficient for breeding amphibians. Another important point is the annual variation in water regime. In 2000 the water level in the river Drava did not show striking variation so the most optimal aquatic plant community developed in those ponds located close to the river. Due to favourable breeding conditions, the number of mating amphibians reached the available maximum. 2001 started with very high water levels in the sample sites at Őrtilos and Vízvár, so the native shallow water vegetation could not develop properly. However, in Lankóc Wood, 5 km away from the river, the level of the river did not influence pond wildlife.
Literature Cited