Understanding the factors determining geographic ranges and range shifts of species is a central issue in ecology and evolutionary biology. Research addressing distributional borders from a demographic perspective frequently focused on reproductive traits, finding reproduction reductions or failure at the range margin. However, some of the observed changes in marginal locations could be the result of adaptive adjustments to local, unfavourable conditions, though they have been rarely interpreted from this point of view. In this study we investigated the reproductive patterns of the seaweed Fucus serratus in central and southern marginal locations (SW UK, N Spain) over a 3-yr period. Our main goals were: 1) to determine the spatial (centre-margin) and temporal variation in reproductive traits and 2) to test if this variation fits with life-history predictions for stressful environments. Threshold size for reproduction declined at the range margin, in accordance with life-history predictions. Nevertheless, we also observed parallel drastic reductions in the percentage of reproductives, reproductive allocation and plant size. The reproductive capacity of marginal locations was thus dramatically reduced in relation to central ones. Furthermore, the decline became more pronounced over the study period. Our results suggests that the viability of marginal populations is at risk. This situation clearly differs from the pattern observed during the last decade. At that time, the species was able to growth and reproduce beyond its distributional boundary at similar rates than inside its range in N Spain. The seaweed was then expanding its distribution and the position of the boundary was set by dispersal limitations. At present, the southern boundary of this species seems to be directly influenced by very unfavourable abiotic conditions, which may be linked to the present scenario of climatic change or to environmental fluctuations acting at shorter-time scales.