Historical contingency and spatial processes rather than ecological niche differentiation explain the distribution of invasive goldenrods (Solidago and Euthamia)

Abstract

The effectiveness of invasion depends on the interacting environmental factors and biology of the invader. The effects of these interactions are contingent on their contexts and difficult to predict. Among the most successful plant invaders in Europe are the goldenrods (Solidago and Euthamia genera). We assessed the roles of ecological niche differentiation and historical contingency on their distribution in Silesia (Central Europe, studied area approximately 32,000 km2). The distributions of the two most common species (S. gigantea and S. altissima) were clumped, and the species dominated different areas. The distribution was unrelated to ecological niche differences but originates rather from the history of invasion and subsequent density-dependent spread; the species that was first introduced occupied available habitats and prevented the establishment of other species. Solidago canadensis was distributed randomly throughout the entire region and did not differ from S. altissima in its ecology. Euthamia graminifolia differed from the other goldenrods in habitat preferences and environmental requirements. Its distribution was strongly spatially structured, clumping around initial infestation sites. The extents of the differences resulting from ecological niche differentiation and the limitation of long-range dispersal are unclear. We argue that the under-representation of E. graminifolia in abandoned fields originates from the inhibition of establishment in habitats already occupied by other goldenrods. The results underline that the effect of historical contingency can be more prominent than ecological niche differentiation on invasive species distribution and invasion effectiveness.