Spatial genetic variation of a riparian wolf spider Pardosa agricola (Thorell, 1856) on lowland river banks: The importance of functional connectivity in linear spatial systems

Habitat patches along river systems are often highly isolated and characterised by a high degree of heterogeneity at different spatio-temporal scales. The connectivity between river bank arthropod populations directly adjoining the river channel is often greatly disturbed due to river regulation. While flight-active arthropods easily disperse upstream, less mobile species are expected to show predominant downstream dispersal unless specific upward movements are prevalent. In linear river ecosystems, downstream drift of organisms may therefore prevail with subsequently strong asymmetrical gene flow. We analysed patterns of genetic variation within and among nine spatially structured populations of the highly stenotopic riparian wolf spider (Lycosidae) Pardosa agricola (Thorell, 1856) using Amplified Fragment Length Polymorphism (AFLP) markers. No evidence was found for downstream accumulation of genetic diversity. The high genetic diversity is hypothesised to be the result of historical drift processes. Nearby populations on the same river shore were less genetically differentiated compared with populations further away and/or on the opposite shore. This indicates that short-distance dispersal still occurs on river banks during low water flow levels, but at the same time that the river channel constitutes a physical barrier for species exchange between opposite shores. The rehabilitation of the riparian corridor will increase the amount of suitable habitat, the functional connectivity during periods of low flow-discharges and eventually riparian spider population persistence.