Dispersal distance and the pool of taxa, but not barriers, determine the colonisation of restored river reaches by benthic invertebrates


Restoration is an increasingly central theme in river ecology. Recent studies have highlighted the importance of the species pool in the surrounding river network for determining colonisation of restored river reaches by both invertebrates and fish. Using a comprehensive data set of 21 river restoration sites and 292 sites in the immediate surroundings, we tested the influence of distance to nearest colonist source on invertebrate colonisation based on a comparison of river network distances and Euclidean distances, expecting river network distances would better align with colonisation rates. We then assessed the importance of dispersal distance in relation to several other parameters, such as the number and intensity of barriers along the river network, surrounding taxon pool occupancy rate, physical characteristics of the restored sites and restoration techniques used in determining colonisation of commonly occurring benthic invertebrates. We hypothesised that (i) distance would be critical, with colonisation of restored sites declining with increasing distance; (ii) barriers between these sites would be a minor, but taxon-specific, influence on the colonisation; and (iii) the higher the regional pool occupancy rate of a certain taxon, the higher its probability of presence at a restored site. Overall, taxon pool occupancy rate was the most important driver of colonisation likelihood, followed by distance to nearest source, with the first kilometre particularly important. The effect of barriers was minor but significant, and taxon identity had no effect on the predictive ability of the model. Factors associated with the restoration projects such as techniques used and physical characteristics had minor influences, being completely outweighed by taxon pool and dispersal-related factors. To gauge the likelihood of successful outcomes of habitat restoration projects, we suggest it is important to assess regional taxon pools and ensure distances between healthy populations are minimised. These results clearly emphasise the importance of spatial planning for restoration projects.

Freshwater Biology
Jonathan D. Tonkin
Senior Lecturer & Rutherford Discovery Fellow