Fig. 1. Conceptual framework of invasive plant species spread into
abandoned agricultural lands, showing the ecological mechanism driving
the invasion. This model is applicable when invasive plant species are
present in the landscape and agricultural land is abandoned for nature
conservation. The model is based on walnut Juglans regia and
goldenrod Solidago sp. invasions but can also be applied to other
taxa. A) Landscape elements, such as backyards and already abandoned
fields contain walnut and goldenrod plants that are seed sources. B)
Birds collect and disperse seeds (walnut) or seeds are dispersed by wind
(goldenrod). C) Birds cache seeds into managed arable fields, pastures,
and meadows (Lenda et al. 2012); here rooks (Corvus frugilegus )
cache walnut seeds. Wind dispersed seeds are carried and deposited
randomly, including in sites such as managed agricultural land
(goldenrod). In both cases, dispersed seeds form a soil seedbank. D)
Seeds germinate and grow. E) Invasive plant seedlings are eliminated by
agricultural management in a land sharing strategy. F) Although the
plant invasion risk is high because seed sources and dispersers are
present, the invasion does not progress. The invasion is stopped by
agricultural land use (here plowing), which damages seeds and seedlings,
thus the native biodiversity remains. G) In a land sparing strategy,
seedlings are no longer damaged by agricultural management after land
abandonment. H) Land use change and land abandonment including
management cessation allow seeds of invasive species to germinate, grow
into mature plants, and create monocultures. Here, goldenrod invasion
can be seen after 2 years of agricultural field abandonment. The
monospecific goldenrod patches drastically reduce biodiversity of native
plants, pollinators, ants and birds, while increasing soil N (see Fig.
2). Scenario for the walnut invasion is similar. This mechanism clearly
shows that land sparing is an unfavorable strategy in the presence of
invasive species. In such sites, land sharing is the best strategy,
because it prevents local biodiversity from extinctions caused by
invasive species.