It is critical for our knowledge of biodiversity and ecosystem processes to understand how individual species contribute to ecosystem processes and how these contributions vary in space and time. We used a manipulative field experiment in five locations over 17 degrees of latitude [from southern Portugal to the Isle of Man (British Isles)] to determine the relative response of rocky intertidal algal assemblages released from control by the grazing of limpets. Response ratios showed that when limpets were removed there was a trend of effects from north to south. In the north, grazing had a strong effect on algal assemblages, but removing grazers reduced spatial variability in assemblages. In the south, the effect of limpet grazing was far weaker and removal of grazers had a much reduced impact on spatial variability. Here we show a clear trophic control of an ecosystem in that grazing by limpets not only determines macroalgal abundance overall but also modifies ecosystem stability via variability in cover of algae.

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The distribution of Fucus serratus has a sharp boundary In northern S p a n F serratus dominates the m~d -~n t e r t~d a l l e g~o n west of Novellana, Asturias but is absent further east In a short t r a n s~t~o n zone F serratus 1s abundant within patches The causes underlying this pattern were investlgated by exper~rnental transplants of F serratus to locations In the transit~on zone and o u t s~d e its normal range Plants moved outslde the normal range had slmllar growth and reproductive rates to those transplanted to the transition zone and ivith~n the zone of abundance F serratus recruited at s~t e s outside the normal boundary apparently from transplanted thalli Temperature affected the growth rate of embryos of F serratus in laboratory exper~rnents Though high summer temperatures may be responsible for the lack of F serratus in the inner part of the Bay of Blscay temperature alone cannot explain the observed llm~t of distribution Other tactors such as l~mited dispersal ablllty and Interspecific compet~tion w~t h other macroalgae might also be important In settlng the boundary for F serratus in northern Spain

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.

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