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.