Aim: In the current Anthropocene, many ecosystems are being simultaneously invaded by multiple alien species. Some of these invasive species become more dominant and have greater environmental impacts than others. If two potentially dominant species invade the same area, the combined impact has been reported to be either (a) domination by one species, that is, the competitive dominance of one invader, or (b) invasion meltdown, where the combined impact is much greater, that is, a synergistic effect. We studied the effects of the invasion of two alien plant species that are known to strongly decrease native plant species diversity: the Persian walnut Juglans regia and goldenrod Solidago canadensis. Location: We examined native vegetation diversity in abandoned fields (in Poland)where neither species had invaded, only one species had invaded, and both species had invaded.Methods: Field survey data were analysed using generalized linear mixed models and ordination techniques.Results: When goldenrod invaded alone, it caused a larger decrease in species richness and cover (74%) than when walnut invaded alone (58%). When walnut and goldenrod co-occurred in abandoned fields, walnut was dominant and strongly decreased goldenrod density by 87%. However, the combined impact on native species diversity was much lower (15% decrease in native plant diversity) than when either goldenrod or walnut invaded alone. Main conclusions:In contrast to many other studies, our study does not support the occurrence of an invasion meltdown. Instead, our results show that even when one invader dominates, its negative effect on plant diversity can be strongly modified by the presence of another invasive species. K E Y W O R D Sbiodiversity, competition, competitive hierarchy, dominant invader, facilitation, invasion ecology, invasion meltdown, nitrogen, theory of coexistence

1. Plant invasions are affected by many factors that must be favourable in order for invasions to occur. Factors can be grouped into three major categories: propagule pressure, biotic factors and abiotic characteristics; all may be moderated by human activity. However, studies examining all factors simultaneously are rare, and most are limited to a single factor. This hampers our understanding of the mechanisms driving invasions. In recent decades, an alien walnut (Juglans regia) has become invasive in CentralEurope due to an increase in the populations of native dispersers, rooks (Corvus frugilegus) and political changes that have resulted in the abandonment of farmland.Here, we test whether increased propagule pressure resulting from the presence of seed-bearing walnuts in abandoned fields interacts with native forest seed-dispersing animals (biotic factors), landscape structure and management, and passive secondary dispersal by gravity (abiotic factors) to facilitate the further invasion of walnuts into forests.3. Seed-dispersing animals were observed visiting walnuts growing in abandoned fields and in human settlements. Walnuts and seed dispersers were surveyed in 102 forest patches. Forest characteristics were examined, and field experiments examining passive dispersal were conducted. Walnut seeds were carried into forests by native birds, jays (Garrulus glandarius).Jays harvested seeds from both wild walnuts growing in abandoned fields and walnuts planted in human settlements. The density of walnuts in forest patches was correlated with jay density. Forest occupancy by walnuts and walnut densities were correlated with the cover of abandoned fields and human settlements containing seed-bearing walnuts. Secondary seed dispersal also affected forest colonization. Walnut seeds hidden by native rooks in sloping, arable fields may be transported passively to forest edges. 5. Synthesis. Our results show that this invasion is a multifaceted process in which human-related alterations to propagule pressure and biotic and abiotic factors have led to the spread of alien walnuts from human settlements and abandoned fields into forest ecosystems. Thus, politically related land use changes can create an invasion debt that causes unexpected linkages among the invasive plant, native Paper previously published as Standard Paper K E Y W O R D S invasion debt, invasive species, lag phase, land use change, landscape structure, politics, seed dispersal | 673 Journal of Ecology LENDA Et AL.