Introduction
Ecosystem engineer organisms create, alter, or maintain environmental conditions in a way that considerably affects several other organisms (Jones et al. 1994). The effect of engineer species varies across spatial scales (Coggan et al. 2018), from local microhabitat creation (e.g., several burrowing vertebrates create distinct microhabitat patches that provide shelter for other animals or establishment microsites for plants) to landscape-scale habitat alteration (e.g., beavers alter landscape-scape habitat structure and water flow that affect a wide range of species). Engineer animals mediate vegetation composition and dynamics through several mechanisms, including soil formation and affecting soil characteristics due to their metabolism (Clyde et al. 2021, Mallen-Cooper et al. 2018, Mosbech et al. 2018), forming establishment microsites through biopedturbation (Cavin & Butler 2015, Davidson et al. 2012), or affecting seed dispersal and seedling establishment by scatter-hoarding (Godó et al. 2022, Pesendorfer et al. 2016).
Most studies on vertebrate engineers focus on mammals in general and rodents in particular (Mallen-Cooper et al. 2018). However, ecosystem engineering is also exemplified by charismatic birds, such as soil and sediment formation by seabirds or ducks on oceanic islands (Clyde et al. 2021, Mosbech et al. 2018), supporting forest recovery by scatter-hoarding corvids (Pesendorfer et al. 2016) or intentionally modifying fire regimes by raptors in Australia (Bonta et al. 2017). Ecosystem alteration by soil disturbance is rarely documented among birds and 96% of the studies on biopedturbation focus on mammals (Mallen-Cooper et al. 2015). Among birds, there are a few examples of burrow-nesting species, such as owls in deserts (Rengifo-Faiffer & Arana 2019) or seabirds on oceanic islands (McKechnie 2006) that are known to affect local habitat conditions by biopedturbation during the construction and use of burrow systems. The burrow systems of birds and mammals are usually permanent landscape features as they are inhabited by successive generations of animals (Whitford & Kay1999). These landscape features are often characterised by a unique vegetation. The structure and species composition of the vegetation developed on burrows are often distinct from the surrounding matrix, due to the continuous and concentrated trampling and nutrient input by the burrow dwellers (Valkó et al. 2021). Burrow networks can introduce a high level of environmental heterogeneity and biodiversity to the landscape (Cavin & Butler 2015, Davidson et al. 2012, Valkó et al. 2021).
Biopedturbation during foraging might also be a relevant process in large-bodied birds inhabiting open landscapes, especially during migration when they forage in large flocks. Biopedturbation at foraging sites are probably less persistent landmarks as these areas are used only temporarily, but as large areas are affected worldwide this process might act as an important factor influencing habitat conditions in certain ecosystems. Despite the potential relevance of this process, we are not aware of any studies on the effect of biopedturbation by foraging birds on the species composition, structure or environmental conditions of natural habitats.
Ecosystem engineer animals, especially those creating biopedturbation are often involved in human-wildlife conflicts and face negative public attitudes. Their effects are often disputed as from the agricultural viewpoint they are often considered as pests but from the conservation viewpoint they are important keystone species. For example, the burrowing activity of the plateau pika (Ochotona curzoniae ) provides a critical ecosystem service by increasing the infiltration rate of water, hence reducing overland flow at the Quinghai-Tibetean plateau (Wilson & Smith 2015). Still, rangeland managers consider pikas as pests, because their burrowing activity can decrease the quality of the pasture, which resulted in mass poisoning campaigns that can have serious negative impacts on regional-scale hydrological functioning. As shown by the above example, maintenance of the integrity of essential ecosystem functions requires the gathering of evidence and informing the decision makers about the full spectrum of impacts, ecosystem services and disservices provided by engineer species. This might contribute to avoiding the damage to these important species and many others that depend on them. This is especially important given the conservation importance and threatened status of many ecosystem engineer species (Davidson et al. 2012). Their global decline goes beyond the loss of certain species and involves several cascading effects on the ecosystem structure and functionality.
Large bird species, such as majority of the 15 crane species of the world are involved in human-wildlife conflicts as they often forage in croplands, even though they mainly feed on crop residue (Austin et al. 2018). König et al. (2021) investigated the ecosystem services and disservices of four iconic animal species, one of them being the Eurasian crane (Grus grus ). They found that the negative effects associated with crane presence were the moderate decrease of yield on croplands and increased labour and prevention costs that act at short term and local scales, affecting mostly private farmers. In contrast, several positive effects were found which were associated with cranes, as these iconic birds can support tourism, quality of life, cultural identity; and these middle- and long-term positive effects go beyond the local scale. Since not the same stakeholders experience the negative and positive effects, the effective conservation of cranes should include the compensation of those stakeholders (i.e., farmers) who are affected by the negative effects. The growing population trends of the world’s two most abundant cranes – the sandhill (Antigone canadensis ) and the Eurasian cranes – are associated with their ability to make use of the expansion of intensive agriculture by opportunistic foraging on croplands (Harris & Mirande 2013, König et al. 2021). The growing abundance of these two crane species will probably further intensify the conflicts between cranes and farmers. There is a chance that these conflicts can spill over to natural habitats, where foraging by large flocks might lead to land degradation. The tendency for using technology that reduces crop residue might affect the habitat selection of cranes and might impose larger pressure on other habitat types including grasslands in the future (Nevard et al. 2018). In grasslands, farmer-wildlife conflicts and conflicts within the conservation sector can both occur if the intense disturbance by cranes decreases forage quality or naturalness of the vegetation.
Crane species foraging in grasslands occur in many parts of the world (Austin et al. 2018); for instance, the sandhill crane in North-America, the Eurasian crane and the demoiselle crane (Anthropoides virgo ) in many parts of Eurasia, the black-necked crane (Grus nigricollis ) in Tibet and the blue crane (Anthropoides paradiseus ) in South-Africa. Grazed grasslands provide suitable feeding grounds for cranes because livestock grazing recycles nutrients, keeps the landscape open, and supports large quantities of invertebrate food resources (Austin et al. 2018). Foraging crane flocks might have a considerable impact on grassland vegetation, although no studies evaluated the effects of biopedturbation by foraging cranes in grasslands.
The aim of our study was to evaluate the effect of biopedturbation by foraging Eurasian cranes on dry grassland vegetation. We used indicators for vegetation naturalness, forage quality and floral resource provision to evaluate the ecosystem state from multiple aspects. We specifically asked the following questions: (i) Do the species composition and structure of the vegetation differ in grasslands used as forage sites by cranes from intact grasslands? (ii) Which plant functional groups are supported and suppressed by biopedturbation of the cranes? (iii) How does biopedturbation by cranes affect the diversity and naturalness of the vegetation? (iv) Does soil disturbance by cranes affect forage quality and floral resources? Our study can contribute to the understanding of the ecosystem services and disservices provided by a large iconic bird species in grasslands. The results can be relevant not only for the Eurasian crane but also for the other crane species.