Mind the gap – context dependency in invasive species impacts: a case study of the ascidian Ciona robusta

Ecosystems worldwide are losing some species and gaining others, resulting in an interchange of species that is having profound impacts on how these ecosystems function. However, research on the effects of species gains and losses has developed largely independently of one another. Recent conceptual advances regarding effects of species gain have arisen from studies that have unraveled the mechanistic basis of how invading species with novel traits alter biotic interactions and ecosystem processes. In contrast, studies on traits associated with species loss are fewer, and much remains unknown about how traits that predispose species to extinction affect ecological processes. Species gains and losses are both consequences and drivers of global change; thus, explicit integration of research on how both processes simultaneously affect ecosystem functioning is key to determining the response of the Earth system to current and future human activities.

Determining the degree of population connectivity and investigating factors driving genetic exchange at various geographical scales are essential to understanding population dynamics and spread potential of invasive species. Here, we explore these issues in the highly invasive vase tunicate, Ciona intestinalis, a species whose invasion history has been obscured by its poorly understood taxonomy and population genetics. Recent phylogenetic and comparative genomic studies suggest that C. intestinalis is a cryptic species complex consisting of at least three species. We reconstructed phylogenies based on both mitochondrial (cytochrome c oxidase subunit 3--NADH dehydrogenase subunit 1 region and NADH dehydrogenase subunit 4 gene) and nuclear (internal transcribed spacer 1) sequences, results of which support four major phylogroups corresponding to the previously reported spA, spB and Ciona spp. (spC) as well as an undescribed cryptic species (spD). While spC and spD remain restricted to their native ranges in the Mediterranean Sea and Black Sea, respectively, the highly invasive species (spA and spB) have disjunct global distributions. Despite extensive interspecific divergences, we identified low phylogeographical structure within these two invasive species. Haplotype network analyses revealed comparatively limited mutation steps among haplotypes within each species. Population genetic analyses based on two mtDNA fragments and eight unlinked microsatellites illustrated relatively low population differentiation and high population connectivity at both regional and continental scales in the two invasive species. Human-mediated dispersal coupled with a high potential for natural dispersal is probably responsible for the observed genetic homogeneity. © 2010 Blackwell Publishing Ltd.