Discussion: Winners and losers of the two conservation strategies

the effect of conservation management for the target species corncrake on the diversity of plant- and leafhoppers. On the other hand, the consequences of process conservation on the corncrake population as well as on the Anura and Auchenorrhyncha communities can be assessed.

Currently, conservation management for the corncrake follows a very pragmatic ap-proach. No management plan officially regulating the mowing activity in the national park is in use. Nesting sites of the corncrake are mapped every year by voluntary local ornithologists. Based on this survey, instructions for mowing are given to the farmers because the present mowing practice, i.e. first cut at the beginning of June, would lead to the destruction of eggs or the death of freshly hatched chicks. Thus, small areas around the identified nesting sites are left uncut. In the long run it is intended to alter the mowing practice. The first cut is planned to be not before 15^th August and larger patches of grassland will be left uncut each year within the mown meadows (IUS 1998).

The comparison of the vegetation structure in meadows and fallows revealed that

“cover of dead phytomass” was the most correlating variable for the differentiation between suitable and unsuitable nesting sites for corncrakes. By mowing, phytomass is taken out of the meadows, thereby reducing its accumulation. Thus, mowing seems to be an appropriate management practice to preserve suitable habitats for corncrakes.

Green et al. (1997) states that the amount of dead phytomass in grassland is also reduced by winter floods. These findings could not be confirmed by the study of corncrakes as both fallows and mown grassland are regularly subjected to long winter floods, but clearly differed in suitability for corncrakes.

The study on Anura did not take the factor mowing into account but Blab (1996) and Beebee (1996) state that reducing land use has a positive effect on amphibian communities.

The study on plant- and leafhoppers showed that the current mowing practice does not affect numbers of species and individuals but significantly reduces the number of spe-cialists. As many specialists are listed in the Red Data Book (Nickel and Remane 2002), mowing has a negative effect on the Auchenorrhyncha species assemblage from the point of view of nature conservation. One might argue that the planned change in mowing practice might have a positive effect on Auchenorrhyncha because Achtziger et al. (1999) showed that with decreasing land use intensity the number of species, indi-viduals and specialists increased. However, not only the number of mowing events but also the timing plays an important role. It seems that mowing in autumn or early in spring leads to species rich and specialised Auchenorrhyncha communities (Achtziger et al. 1999, Morris and Lakhani 1979). Mowing early in spring is not debatable when talking about conservation management for the benefit of the corncrake as the birds need sufficient cover when establishing their mating territories. If both corncrake and plant- and leafhoppers should profit from the conservation management, it seems that

question satisfactorily. An experimental approach would be necessary to investigate the impact of mowing at different times in the year on the diversity of plant- and leafhoppers as well as on other insect groups. In addition, a socio-economic study would be necessary to assess the costs for compensating the farmers for the loss in fodder quality due to later mowing activity i.e. for conservation management. It seems that by changing the mowing practice, plant- and leafhoppers will not necessarily be the

“losers”. It is more a question of whether the farmers or the plant- and leafhoppers will be the “winners”.

Against these results, one can call the selection of the corncrake as target-species into question. Corncrakes, having a large home range, are considered to be an umbrella spe-cies and thus it is supposed that a large number of smaller spespe-cies will profit from the conservation efforts made for the corncrake. However, the results presented above do not support this assumption. At least Auchenorrhyncha and Anura, being typical species of wet grassland habitats, do not profit from the conservation management. On the con-trary, conservation measures even negatively affect the selected taxa.

Besides conservation management for target species conservation of processes will be realised in the national park. It is planned that until 2010 50% of the national park’s area will be declared as core zone. These areas will be set aside and left for succession.

Furthermore, process conservation includes that flooding will be altered towards a more natural flooding regime i.e. longer and more frequent flooding events. The artificial lowering of the ground water level is planned to be stopped as soon as the area of the respective polder is completely set aside.

For the corncrake, the results show that regular mowing events are essential to maintain suitable breeding habitats. Thus setting aside grassland will surely make grassland un-suitable to be used by corncrakes’ population after a few years. As the case study of the amphibian community presented above focused on the impact of the long winter flood-ing on the Anura, it is not possible to assess the impact of reduced mowflood-ing activity on the amphibians. However, the reduction of land use might even have a positive effect on the amphibian community (Blab 1986, Beebee 1996). For Auchenorrhyncha, it was previously stated that stopping mowing activity will lead to a community consisting of only a few, but specialised species.

At present it is not clear how long the floodplain would be inundated without regula-tions as the flooding regime is still managed and ideas of how to realise the change in flooding regime are still under discussion. Independent of how the changes will be re-alised they will lead to longer flooding time in spring.

A study on Anurans revealed that the current flooding regime already negatively af-fects amphibian diversity. In the “wet polder” two species of the water frog complex were found whereas in the “dry polder” seven species were caught. But even longer flooding periods might not negatively affect the water frog complex. Although their mating time starts in May, it can continue until July. Furthermore, in contrast to the frogs occurring in the “dry polder”, species of the water frog complex are able to

find suitable waterbodies for breeding in the “wet polder” during their mating period (Blab 1986, Günther 1996a, 1996b). A reduction of the artificial lowering of the ground water level throughout the “dry polder” might have a positive effect on the amphibians.

Besides other species, the fire-bellied toad and the moor frog which are bound to high levels of ground water, will find more waterbodies suitable for breeding and thus have the possibility to spread (Günther and Nabrowsky 1996, Schneeweiß 1996).

Unfortunately, the “dry polder” is the area where conservation management for corncrakes is planned, thus mowing must still be possible and this includes maintaining the regulation of the ground water level.

For Auchenorrhyncha, the study revealed that the species assemblages on fallows differ in regard to flooding period and frequency. Significant differences in specialisation of the species to moisture conditions and diet width were identified. Communities in sites that are subject to long winter floods are dominated by specialised plant- and leafhop-pers, while sites that are subject to short winter floods or to summer and winter floods host more generalists. Ideally, the change in flooding regime will lead to a patchy envi-ronment that is subject to dynamic changes due to flooding influence. High summer floods are a more rare event in the study area. High water levels similar to those of the winter floods occur on average only once in ten years (Vössing 1998). Changes towards longer flooding in combination with higher ground water levels throughout the whole year will therefore increase the number of patches in the wet polder, similar to the areas subject to long winter flooding. Additionally, areas subject to winter and summer floods that are comparable to the study sites on the river bank will increase as the more regularly occurring smaller summer floods will no longer be restricted to the river bank.

The more elevated areas are comparable to those study sites subject to low flooding impact. The changes will favour on the one hand specialised plant- and leafhoppers, but will host on the other hand a number of generalists that can cope with the more unpredictable flooding conditions. Thus, besides the preservation of specialists and en-dangered species, process conservation might lead to a high β-diversity.

To summarise, conservation management for the target species corncrake will certainly have a positive effect on the corncrake population. Its effect on the anuran community is not assessable by this study because the impact of mowing on amphibians was not in-vestigated. For the plant- and leafhoppers, as an example of typical grassland insects, benefit and loss seems to be dependent on the date of mowing which is in turn, depend-ent on the money that can be spdepend-ent for conservation managemdepend-ent. However, currdepend-ent plans concerning the conservation management for the target species corncrake seem to have negative effects on Auchenorrhyncha. The studies reveal that the corncrake seems an unsuitable umbrella species for wet grassland habitats. Process conservation, on the other hand will clearly have negative effects on the corncrake population. The anuran community in the “wet polder” seems to be neither positively nor negatively affected by a change towards more natural flooding conditions. In the “dry polder” a change to-wards a higher ground water level might be beneficial for many anurans. The changes in the Auchenorrhyncha community due to an altered flooding regime seems to be positive

With these results, it seems that the idea of realising the two conservation strategies side by side can be a successful approach to combine conservation of rare endangered spe-cies as well as of biological diversity. But for the Lower Oder Valley National Park as well as for other protected areas following a similar approach, some open questions still remain. Is the protected area big enough to host (in just a part of its area at least), a MVP of the selected target species? Is the remaining area sufficient to host all seral stages created by natural dynamics side by side? Are the revitalised dynamics sufficient to create a patchy environment comparable to natural conditions? How can e.g. farmers currently using large parts of the floodplain be integrated into the conservation plans?

The answers to these questions are out of the scope of the studies presented above and thus further investigations are necessary.

6 Discussion

6.1 Impact of mowing and flooding on the diversity of arthropods