Estimation of intergenerational drift dispersal distances and mortality risk for aquatic macroinvertebrates

Estimates of the total lifetime dispersal capacity of individuals comprising invertebrate drift in streams and rivers have proved very difficult to determine empirically. Here we use recent data on dispersal in the amphipod Gammarus pulex L. to illustrate a method for estimating the total distance an individual is likely to travel downstream during the period between hatching and its first reproductive episode. For the system we consider, this estimate is on the order of 1.5 km. This method may be useful in future explorations of the ecological relevance of within-stream displacement and population persistence. Furthermore, we are able for the first time to estimate an upper bound for the mortality risk associated with entering the drift, which, for the G. pulex population under consideration, is less than 1% for individual drift events. We suggest that this risk may not be nearly as high as had previously been thought. These mortality and distance estimates may illustrate a fundamental difference between dispersal in lotic systems and those in other habitats, in that the mean dispersal distance is of much higher ecological relevance than that of rare long-distance events. Dispersal from one habitat patch to another is one of the cornerstones of modern ecological theory, and underpins the large body of work devoted to the study of population dynamics (Palmer et al. 1996; Hanski and Gilpin 1997; Bilton et al. 2001). Studies of dispersal of the aquatic stages of macroinvertebrates within lotic (flowing water) systems are very common in the literature. Rapid colonization and movement of animals between habitat patches on several spatial scales appears to be a key component of the dynamics of many benthic invertebrate populations (e.g., Mackay 1992; Allan 1995; Anholt 1995; Speirs and Gurney 2001). The physical properties of water mean that animals are more easily entrained within it than they are in air (Denny 1993; Vogel 1994), and the unidirectional nature of flow in streams and rivers means that studies of dispersal in streams generally focus on invertebrate drift: the downstream transport of benthic invertebrates in the water column. Movement on or within the streambed has also been considered, especially in studies of colonization, but the consensus is that the most important dispersal mechanism for benthic invertebrates is likely to be drift in the water column (Mackay 1992; Allan 1995; Malmqvist 2002). Consequently, the literature is filled with studies describing the number of 1 Corresponding author ([email protected]).