Plant establishment and invasions: an increase in a seed disperser combined with land abandonment causes an invasion of the non-native walnut in Europe.

Successful invasive species often are established for a long time period before increasing exponentially in abundance. This lag phase is one of the least understood phenomena of biological invasions. Plant invasions depend on three factors: a seed source, suitable habitat and a seed disperser. The non-native walnut, Juglans regia, has been planted for centuries in Central Europe but, until recently, has not spread beyond planted areas. However, in the past 20 years, we have observed a rapid increase in walnut abundance, specifically in abandoned agricultural fields. The dominant walnut disperser is the rook, Corvus frugilegus. During the past 50 years, rooks have increased in abundance and now commonly inhabit human settlements, where walnut trees are planted. Central Europe has, in the past few decades, experienced large-scale land abandonment. Walnut seeds dispersed into ploughed fields do not survive, but when cached into ploughed and then abandoned fields, they successfully establish. Rooks preferentially cache seeds in ploughed fields. Thus, land-use change combined with disperser changes can cause rapid increase of a non-native species, allowing it to become invasive. This may have cascading effects on the entire ecosystem. Thus, species that are non-native and not invasive can become invasive as habitats and dispersers change.