Climatic fluctuations over the last 120 years have been well described in northern Europe, showing alternation of colder (1910s to 1920s, 1960s to 1980s) and warmer (1930s to 1950s) periods upon which recent anthropogenic warming has been superimposed. Biological manifestation of such changes in climate may be determined by examination of shifts in species distribution and performance traits. In Britain, shifts in biogeographic ranges of species have been well documented because of the biogeographical boundary, which crosses the British Isles featuring the overlapping geographical distributions of cold- and warmwater species. Patellid limpets have been proposed as climatic indicators and they are considered a keystone species on exposed- and semi-exposed rocky shores in the British Isles. Changes in demographic attributes across the distributional range will therefore have consequences on intertidal communities. This PhD thesis has studied two patellid species: the warm-water species Patella depressa, which reaches its northern limit in Britain and its congener, and putative competitor, the cold-water species Patella vulgata. The overall
question was addressed towards what processes at individual and population levels are setting both poleward range limits of P. depressa distribution. Thus, in order to determine how geographical distribution in relation to range edge affects individual and population traits, the southern limpet P. depressa was studied in detail at two poleward range limits in Britain (North Wales [N-Wales] and South-east [S-East] England) and at more central populations in South-west (S-West) England. Comparisons based on both historical and current abundances, size, growth, mortality and competition were made with its sibling species P. vulgata, which is at its centre of distribution in Britain.
Results showed that shifts in the two-poleward limits of P. depressa have occurred in different ways over the past decades, evidenced by historical records in Britain (1950s). The current range edge of P. depressa in N-Wales has not fully recovered to that occupied in the warm 1950s after the cooler 1960s to early 1980s. Nowadays, breeding populations reach the northern edge of Cardigan Bay and have not reextended around the Lleyn Peninsula. Scattered individuals are no longer found on Anglesey as in the early 1980s. In contrast, in the English Channel, its abundance was higher in the warm 1950s than the cooler early 1980s, but its range has now extended to the east of the Isle of Wight, forming a breeding population at Southsea. Further, long-term comparisons (1980-2016) have indicated that P. depressa has different abundance patterns towards the two leading edges in Britain. P. depressa has been much less abundant in
N-Wales when compared with populations in S-East England in the last 35 years. Furthermore, a significant fraction of individuals of juvenile P. depressa (less than 15 mm in length) were found in S-East England, indicating much more consistent recruitment when compared to N-Wales. Based on Brown (1984), I have explored the Abundant-Centre Hypothesis (ACH) to understand the mechanisms that determine patterns of growth and mortality rates across a species range. I found that growth and mortality - both variables measured by marking limpets in the field - do not differ substantially between P. depressa and P. vulgata across regions in Britain. However, growth and mortality rates of both limpet species were higher at the range edge of P. depressa in S-East England, when compared with populations in the range edge in N-Wales and in central populations in S-West England. Surprisingly, P. vulgata, which may be considered at the centre of its range of distribution in the British Isles, showed
patterns very similar to P. depressa. Hence, these patterns of growth and mortality of both Patella species do not support the ACH. Further investigation of growth patterns through analysis of inner growth lines visible in shell sections have showed that populations of P. depressa nearer to their poleward edge in N-Wales are characterized by older individuals, when compared to populations in S-East England. Growth performance calculated by using annual lines on shells supported the results of limpet growth obtained by marking limpets; similar growth performance values were obtained by using both approaches. Furthermore, I also found that P. depressa growth patterns were influenced by density-dependent effects over different spatial scales. Thus, across locations, high limpet density does not necessarily lead to a reduction in growth rates
in Patella species. In contrast, at a local quadrat-scale, I found a localised effect of limpet density on growth performance of P. depressa. These patterns suggest that growth rates in limpet species are highly variable, subject to drivers and limitations at various levels of geographical scale, with local processes being
important. Competitive interactions between both limpet species were explored by determining the biological and physical factors, which are setting the leading edges of P. depressa at poleward populations. Individuals of P. depressa were strongly affected by inter-specific competition at its range edge in N-Wales,
particularly in mixed plots with P. vulgata under shading treatments. By contrast, P. vulgata was affected by intra-specific competition in no-shading plots, particularly in central populations in S-West England. These results suggest that both biological and physical factors are modulating the poleward range of P.
depressa in populations in N-Wales. Therefore, a better understanding of the differences in competitive ability between Patella species over a large spatial scale will improve our understanding of the role of competition in determining the performance of range edge individuals and hence range limits of populations. Finally, as the British Isles are affected by different weather conditions, limpets performance traits may be vary leading to regional variations irrespective of the species. Limpet competitive interactions and local physical conditions must be considered as important local drivers within a large geographical scale. My results do not support the ACH in populations at the northern fraction of P. depressa distribution. As Patella species have been proposed as climate indicator species, the effects of climate must be considered
in terms of both limpet life-history traits and the interaction between the ultimate and a multitude of local proximate factors.