Conclusions
Our results suggest that the effects of biopedturbation by cranes from the conservation and rangeland management viewpoint are complex. Cranes create early-successional open habitat patches that increase the landscape-scale biodiversity and floral resources but decrease the area of intact alkaline grasslands. Crane-disturbed patches can provide forage for livestock early in the season, but later on the forage quality of the vegetation becomes poor. Soil disturbance similar to crane ploughings have been an important component of many open landscapes. Indeed, creating such focal soil perturbations artificially was found to be effective for increasing environmental heterogeneity and provide establishment microsites for subordinate plant species (Limb et al. 2010).
Due to the site fidelity of cranes, biopedturbations seem to be permanent landscape elements, that are maintained in an early successional stage across years. It is an interesting question for future studies why do cranes prefer their already used foraging sites and whether their foraging activity and the associated disturbance increase the abundance of some of their preferred food items, such as certain soil-dwelling arthropods. Given the increasing global population of Eurasian cranes (Wetland International 2015) and the mild winters as a consequence of climate change, it is expected that cranes will be more abundant and stay for longer in stopover areas, and the number of overwintering birds is also expected to increase (König & Mirande 2013). This predicted increase in crane pressure will probably affect not only the grassland patches currently used for foraging but also currently intact grasslands as well. Additionally, the current warming trends of the winters across Europe have already resulted in the northward shift of the wintering ranges of the Eurasian Crane. For example, significant part of the Northwest-European breeding population of the study species has shifted its wintering grounds by over 1000 kilometres within 15 years (Prange 1999). As observational evidence is mounting that the same behavioural change is happening in the Baltic-Hungarian Flyway, the effects of the ploughing of cranes are expected to be stronger in winter than in the autumn months, owing to the increasingly wet soil conditions. This might amplify the transformation of local habitat patches. Therefore, a surveillance system monitoring the landscape-level extent of the disturbed areas, e.g., by satellite imagery would be important to keep track on the vegetation changes.
In case the crane pressure considerably increases in the future, prevention or mitigation actions will be necessary to avoid land degradation. Increased crane pressure in grasslands will cause a conflict within the nature conservation sector: a strictly protected species in strictly protected areas can cause the degradation of the strictly protected habitat. Such conflict can only be managed with a complex prevention and mitigation strategy.
One option would be to use visual clues that can make the focal grassland areas unattractive for cranes. Scarecrows or other devices have been used for such purposes in croplands with mixed success: this option works mostly against small groups of birds and items should be moved or animated because the cranes can quickly habituate to constant objects (Austin et al. 2018). Using scary visual clues can be an option on some particular localities, but cannot provide a full solution for decreasing crane pressure in the whole 80 000 hectares of grassland area in the study region. Other options developed for scaring cranes from croplands (e.g., fencing or using audial clues) cannot work in grasslands inside a nature reserve.
A potentially effective prevention strategy would be to maintain the currently used croplands in the stopover areas to provide primary feeding grounds for cranes and to avoid a considerably larger crane pressure on grasslands (Végvári 2002). In these areas, cultivation of corn would be the best option, as its crop residue is a preferred food item of cranes. Experimental studies are required to analyse the habitat preferences of the Eurasian crane during wintering that helps outlining management plans which distract cranes from sensitive grasslands, which needs to be incorporated into national and international agricultural subsidising systems.
Among management strategies, grassland restoration on disturbed surfaces might be an option. As alkaline grasslands have a good spontaneous recovery potential, passive restoration should be supported in smaller disturbed patches (Valkó et al. 2017). Active restoration by seed sowing should be only used on large disturbed patches where the likeliness of spontaneous recovery is lower, but in this case the trampling and other disturbance associated with restoration measures can be harmful for the surrounding intact grassland areas.
It is important to note that in less harsh environments, intense biopedturbation by cranes might be of greater concern due to the possible encroachment of weed and invasive plant species that were limited by the harsh environmental conditions in our studied system. We can also expect that in lower latitudes the engineering effect of cranes can be even higher (Romero et al. 2014). These call for future studies in other ecoregions and habitat types.