Non-random dispersal in sympatric stream fishes: Influences of natural disturbance and body size

Although the prevailing paradigms in spatial ecology have treated dispersal as a stochastic process, there is an increasing awareness that spatial processes are non-random such that individual characteristics and ecological contexts influence dispersal. Natural disturbance, such as river flooding, is known to stimulate dispersal behaviour, but its interactive effects with individual-level characteristics (e.g. body size) of potential dispersers remain elusive. It is critical to fill this knowledge gap because anthropogenic impacts (including climate change) alter both disturbance regimes and population structures. Here, we examined how extreme high flows and individual body size combined to influence dispersal of three fishes (creek chub Semotilus atromaculatus, bluehead chub Nocomis leptocephalus, and striped jumprock Moxostoma rupicartes) in two streams (740 and 520 m long) in South Carolina, U.S.A. We focused on extreme high flows as a natural disturbance, whereas body size was used as an individual-level characteristic of dispersers. A total of 5,604 individuals were uniquely marked in the two streams over a >2-year study period, during which sampling occurred every 2 months. The intensive capture–recapture study revealed differential effects of disturbance and body size among sympatric stream fishes. Extreme high flows increased dispersal of striped jumprock and creek chub, whereas body size influenced dispersal of striped jumprock with larger individuals traveling longer distances. Bluehead chub showed a complex dispersal response. That is, size-dependence in dispersal emerged only during high-flow periods. The results of this study build upon previous efforts by providing field-based evidence of how disturbance and individual characteristics (body size) combine to drive non-random dispersal and how it varies among sympatric species. This finding is important because metapopulations that are maintained by different non-random dispersal (i.e. externally or internally driven) may show varied sensitivities to human-induced environmental changes.