Main text
Agricultural intensification and further conversion of natural
ecosystems into agricultural lands are global threats to biodiversity,
causing extinctions of native fauna and flora. By 2050, the human
population may reach 9.6 billion (Gerland et al. 2014)—feeding more
and more people will increasingly demand more agricultural production
and land—leading to continuous agricultural intensification. This is
expected to add to further land clearing, pesticide and herbicide use,
soil pollution, water shortages, and greenhouse gas emissions
contributing to climate change.
Two concepts from classic landscape ecology have been proposed to
address this looming biodiversity crisis: 1) land
sparing and 2) land sharing (Green et al. 2005). Land sparing
refers to ‘sparing’ of nature; coupling the intensive use of
agricultural land on one hand and saving intact areas where wildlife and
biodiversity thrive. Land sparing assumes that primary habitats are the
most biodiverse; that agricultural lands have very little biodiversity
value and that preserving natural lands, while intensifying management
of agricultural areas can maintain biodiversity while achieving
sufficient yields to feed humans (Gibson et al. 2011, Laurance et al.
2014). In contrast, land sharing is an integrative approach, defined as
accepting lower yields in agricultural lands combined with biodiversity
conservation and extensive land management (Lindenmayer and Cunningham
2013). Land sharing/sparing concepts have been developed and tested
mostly in pristine areas, such as intact tropical forests, where
agricultural land clearing creates a drastic land change into row crop
agriculture or rangelands. Studies conducted in tropical forests suggest
that land sparing is a better strategy for maintaining regional species
diversity while achieving agricultural yield goals (Phalan et al. 2011,
Kamp et al. 2015). However, this situation is not relevant to regions
invaded by alien species, which may complicate land sharing and land
sparing models.
Most of the world is not pristine and has been in human use for
hundreds, if not thousands of years, which includes virtually all of
Europe. In the European Plain, the only temperate, terrestrial pristine
nature remaining is the Białowieża Forest in Central Europe. Outside of
Antarctica, thousands of years of agricultural management have shaped
cultural landscapes often with unique flora and fauna, which has adapted
and coevolved with land management. Over 8000 years ago, prior to
Neolithic agriculture, Europe was mostly a wooded continent. Starting
6000 years ago, forests have been progressively cleared for agriculture
and transformed into managed grasslands and tilled crop fields (Roberts
et al. 2018). The management of these agricultural landscapes created by
humans has shaped complex ecosystems and regions with high biodiversity
values, that depend on continuous agriculture (Rosin et al. 2016).
Nature conservation programs such as “Natura 2000”, which is the
oldest and largest in the European Union, promote extensive land
management to maintain local biodiversity. For example, in Central
Europe, many protected insects, birds, and plant species depend on
extensive land management, such as mowing or cattle grazing.
Low-intensity management of grasslands, heathlands, and peatlands
supports threatened and declining species, such as large blue
butterflies, Phengaris teleius and P . nausithous ,
both flagship species in European biodiversity conservation. Many
protected bird species occur only in managed landscapes, such as ortolan
bunting (Emberiza hortulana ) or corncrake (Crex crex ), a
grassland-specialized bird vulnerable to extinction (IUCN). Mown or
grazed meadows are also important for plants such as threatened orchids,
Siberian iris (Iris sibirica ), globeflower (Trollius
europaeus), chess flower (Fritillaria meleagris ), and crocus
(Crocus scepusiensis ). However, currently, these
agriculture-associated habitats have the worst conservation status among
all ecosystems (Pe’er et al. 2014). The increasing demand for food
continues to drive agricultural intensification in Europe. Aided by
subsidies, the scale and intensity of agricultural operations is
increasing throughout the EU with an increase in agrochemical inputs,
such as fertilizers. These processes have led to a continuing decline in
farmland biodiversity (Tryjanowski et al. 2011).
Although managed agricultural landscapes can harbor high levels of
biodiversity, in theoretical nature conservation, one may suggest
abandoning this land to create new spared areas designated for nature
conservation. Such theoretical considerations and the establishment of
new spared areas from already managed land due to agricultural
management cessation have already been applied in Europe. An
international nature conservation idea, “Rewilding”, described
recently in the journal Science proposed to abandon agricultural land or
repurpose previously abandoned post-agricultural land into new nature
conservation areas (Navarro and Pereira 2012, Sylvén and Windstrand
2015, Perino et al. 2019). Similarly, since 2013, the European Union
(EU) “Greening policy” has advocated abandoning at least 5% of arable
land to create permanent set-aside land (i.e., ecological focus areas)
such as fallow lands, afforested areas, field margins, hedges, buffer
strips, etc. (Van Zeijts et al. 2011, Hauck et al. 2014). Both the
“Rewilding” and “Greening policy” resemble the land sparing concept,
as both rely on reestablishing natural lands within landscapes that are
currently entirely, actively managed. In these policies, agricultural
land abandonment and natural secondary succession are usually combined
with further agricultural intensification (Van Zeijts et al. 2011, Hauck
et al. 2014, Sylvén and Windstrand 2015, Perino et al. 2019). The
Rewilding idea emphasized the positive effect of land abandonment on
nature conservation, as has happened in Chernobyl after the nuclear
catastrophe (Perino et al. 2019). However, none of these sparing
concepts propose a specific strategy for landscapes threatened by alien
species invasions. When invaded, such areas also become propagule
sources that further threaten other areas designated for nature
conservation. In a parallel to the Chernobyl vegetation succession,
abandoned paddy fields at Fukushima have become dominated by alien
invasive goldenrod Solidago altissima within one year after the
nuclear catastrophe, which resulted in agricultural land abandonment in
the area (Fig. S1, Yamashita et al. 2014).
The theoretical concept of land sparing and land sharing together with
the applied nature conservation policies as “Rewilding” and
“Greening” omit scientific, published data which show that abandoned
agricultural land is frequently threatened by invasive, alien plant
species (Figs. 1 & 2, Cramer et al. 2008, Lenda et al. 2021). This
plant invasion risk has not been addressed in the land sharing/land
sparing conceptual framework and in practical solutions. Invasive
species need to be considered because they can disturb natural
succession (Gusev 2015), affect fire regimes (Otero et al. 2015),
decrease native biodiversity of plants, pollinators, ants, and birds
(Fig. 2, Moroń et al. 2009, Skórka et al. 2010, Lenda et al. 2013), and
homogenize ecosystems (Lenda et al. 2019). For example, in Central
Europe, up to 90% of abandoned agricultural land is dominated by alien
goldenrods Solidago sp. (S. canadensis and S.
gigantea ) (Szymura et al. 2016, Lenda et al. 2019, 2021). Goldenrods
create homogenous habitat patches with up to 100% dominance within just
a few years, (Fig. 1, Moroń et al. 2009, Lenda et al. 2019) that
negatively affect ecosystem service providers (Fig. 2) and decrease
their functional diversity (Patchey and Gaston 2006). Invasive species
can cause up to 70% decline in wild pollinator abundance (Fig. 2, Moroń
et al. 2009, 2019), and a 50% decline in farmland bird abundance and
ant diversity (Fig. 2, Skórka et al. 2010, Lenda et al. 2013). Thus, if
invasive species establish dominance in spared “natural” land, the
biodiversity of such areas would be much lower than that of extensively
managed agricultural habitats (Fig. 1; Moroń et al. 2009, Skórka et al.
2010). In addition, creating such “natural areas” using agricultural
land abandonment and land sparing in the presence of invasive plant
species will lower native species richness and abundance by increasing
invasion on new, post-agricultural land. We also demonstrated this
effect on birds recorded from managed fields, non-invaded abandoned
fields, and invaded abandoned fields (Fig. 2). Using simulations, we
created virtual landscapes varying in cover of abandoned fields in two
scenarios: with and without invasions (see description of Methods in
Supplementary material). It was clear that non-invaded land sparing
(land abandonment) increases bird species richness (up to about 40%
cover in a landscape) and abundance in an agricultural landscape,
however, there was no gain in bird diversity if the abandoned land was
invaded (Fig. 2, details in Supplementary material).
The problem of alien plant species invading abandoned lands is a global
problem. For example, dense stands of the invasive grass Saccharum
spontaneum prevent forest regeneration in abandoned pastures in Panama
(Joo Kim et al. 2006). The grass invasion of Ampelodesmos
mauritanica on abandoned farmland in Catalan, Spain increases fire
frequency and intensity (Grigulis et al. 2005), which increases soil
erosion (Otero et al. 2015).
In this study, based on our new and earlier results, we suggest that
land sharing may be the best solution for sustaining biodiversity when
the risk of invasion is high. This is because land management practices
such as plowing, cutting, and grazing, even if they are undertaken
extensively, usually prevent the successful establishment and spread of
invasive alien species (Fig. 1). A clear example of this are the recent
Persian Walnut and goldenrod invasions in Central Europe. Seed catching
birds each year carry Persian walnut seeds over distances up to 1 km
from a seed source, and bury them in arable fields, creating a seedbank.
These walnut seeds germinate and grow; however, each year’s land
management, such as plowing and cutting, prevents establishment and
thereby invasion (Fig. 1, Lenda et al. 2012, 2018). The mechanism of
goldenrod invasion is similar, with the difference being that goldenrod
seeds are wind-dispersed (Fig. 1). In this case, with land abandonment
and management cessation, goldenrod seedlings can establish and become
dominant in agricultural lands.
Vast land abandonment and/or management cessation allows the
establishment of invasive plants in agricultural lands. Seeds of many
invasive species are already present in the soil; hence, after
germination, fast-growing, highly competitive, and often allelopathic
seedlings can achieve dominance. Furthermore, these invaded abandoned
agricultural lands can become hotspots of invasive species dominance
that can spread further into other natural landscape elements. Thus, a
land sparing strategy in such an environment promotes further
progression of plant invasions.
If the land sparing concept is intended to improve food production and
biodiversity conservation, intensively managed cropland alongside
invaded spared land could be a catastrophe for both agriculture and
biodiversity. This is because biodiversity in agricultural ecosystems
provides biodiversity and ecosystem services. We believe that land
sharing mitigates many of the dangerous consequences of large-scale
invasions.