Self-organisation in streams: the relationship between movement behaviour and body condition in a headwater salamander

1. When movement behaviour is correlated with traits affecting fitness, it may affect population performance directly, independent of extrinsic habitat conditions. 2. In a previous 3-year, capture-recapture study, upstream movement by the salamander Gyrinophilus porphyriticus compensated for low reproduction in the upper 500 m of Merrill Brook, a first-order stream in northern New Hampshire (U.S.A.). This immigrant subsidy resulted from excess reproduction in the downstream section (i.e. the lower 500 m of stream length) and from consistently upstream-biased movement by salamanders. Reproduction in the two stream sections was positively correlated with mean body condition. Using 6 years of capture-recapture data, this study examines whether the movement behaviour of G. porphyriticus is related to body condition and thereby directly influences mean body condition and reproduction in the two sections of Merrill Brook. 3. Upstream-biased movement and greater mean body condition in the downstream section were consistent across 6 years of data collection. In Merrill Brook and four other streams, however, individuals with high body condition were more likely to move upstream and low-condition individuals were more likely to move downstream. Movement direction was unrelated to the size, sex and initial location of individuals. Body condition was positively related to growth rate, further supporting its link to reproductive potential, and positively autocorrelated through time in individuals that moved. 4. Results of this 6-year study suggest that the movement behaviour of G. porphyriticus partially compensated for environmental factors differentiating mean body condition and reproduction along Merrill Brook and illustrate the potential for this form of self-organisation to occur in linear habitats such as streams and rivers.