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.