Is assemblage variability related to environmental variability? An answer for riverine fish

We examine the variability of riverine fish assemblages in terms of assembl age stability (i.e. variability of numbers of individuals within species ov er lime and variability of assemblage total density), assemblage persistenc e. and assemblage species richness using data from a 9-yr survey of 27 site s within 18 coastal streams of North-western France. To do so, we test a hy pothesized directional model for the expected relationships between environ mental variability, assemblage variability, assemblage persistence, and ass emblage species richness: 1) environmental variability within a given syste m is likely to generate variable local population size within this system, thus increasing local assemblages variability; 2) environmental variability should increase extinction rates (or, under constant colonization rates. d ecrease persistence), because the more population sizes vary within an asse mblage, the more likely they are to become zero in some period of time. 3) assemblage variability should reduce assemblage species richness by increas ing extinction rates within populations composing these assemblages. Result s are compatible with our starting hypotheses and show that assemblage vari ability increased with environmental variability (i.e. discharge variabilit y), that assemblage persistence decreased with environmental variability, a nd that species richness decreased with assemblage variability after enviro nmental factors were controlled for. Thus, disturbance regimes, in our case , can alter the stability properties of assemblages and extrinsic determina nts of assemblage variability may be an important determinant of assemblage species richness. These results have important conservation and management implications, due to the strong impact of river regulation on flow regimes .