Isle Of Rum Research Articles

Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccia

AbstractThe Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.

Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example from the Isle of Rum, NW Scotland

AbstractSr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustal melts and and primary mantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place.

Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

AbstractThe Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.

The Unit 12 allivalite, Eastern Layered Intrusion, Isle of Rum: a textural and geochemical study of an open-system magma chamber

AbstractThe Eastern Layered Intrusion of the Rum Central Complex, NW Scotland, comprises 16 macro-units broadly defined by a basal peridotite body overlain by gabbros and troctolites. The local term for the upper plagioclase-bearing cumulates is ‘allivalite’. The Unit 12 allivalite contains five subsidiary peridotite horizons, each continuous on the kilometre scale. Field, textural and geochemical evidence points to the Unit 12 allivalite forming by progressive accumulation of crystals from up to seven separate magma batches, varying from picrite to basalt. The thickness of the magma lens in the sill-like chamber was as low as 30 m during a significant part of the development of the Unit 12 allivalite, and evidence supports one eruption from the chamber during its accumulation.

Testing for genetic trade-offs between early- and late-life reproduction in a wild red deer population

The antagonistic pleiotropy (AP) theory of ageing predicts genetically based trade-offs between investment in reproduction in early life and survival and performance in later life. Laboratory-based research has shown that such genetic trade-offs exist, but little is currently known about their prevalence in natural populations. We used random regression ‘animal model’ techniques to test the genetic basis of trade-offs between early-life fecundity (ELF) and maternal performance in late life in a wild population of red deer (Cervus elaphus) on the Isle of Rum, Scotland. Significant genetic variation for both ageing rates in a key maternal performance measure (offspring birth weight) and ELF was present in this population. We found some evidence for a negative genetic covariance between the rate of ageing in offspring birth weight and ELF, and also for a negative environmental covariance. Our results suggest rare support for the AP theory of ageing from a wild population.

Habitat Selection and the Evolution of Specialists in Heterogeneous Environments

Recent advances in studies of habitat selection and resource use provide a framework not only for estimating resource specialization, but also for predicting future success of specialist and generalist strategies. The protocol merges resource selection functions with fitness and population dynamics to assess the evolution of competing strategies that change with density-dependent habitat selection. These strategies are revealed by resource selection coefficients derived from marked individuals that can then be used to predict each individual’s fitness at different population sizes. Simulated consumer resource dynamics confirm the theory’s ability to identify strategies of habitat and resource use with simple statistical models that summarize rather complex systems. The theory produces excellent fits with simulated data when strategies depend on density, and when the success of a single strategy interacts with others. Specialist strategies yield highest fitness at low population density, whereas generalists have highest fitness in dense populations. When applied to female red deer living on the Isle of Rum, Scotland, the theory correctly predicts an equilibrium distribution of competing strategies dominated by specialization on Agrostis/Festuca grassland. Specialization declined as population density increased through time. Simultaneously, changes in the genetic structure of the population reflected the increased opportunity for out

The relationship between tooth wear, habitat quality and late‐life reproduction in a wild red deer population

1. Molar tooth wear is considered an important proximate mechanism driving patterns of senescence in ungulates but few studies have investigated the causes of variation in molar wear or their consequences for reproductive success. 2. In this study, we assessed molar tooth wear at death among red deer Cervus elaphus of known age on the Isle of Rum, Scotland. 3. First molar height showed a decelerating decline with age. In females, the rates of molar wear with age varied with location of home range and individuals experiencing low resource competition showed reduced molar wear. We suggest that this spatial variation in molar wear is related to differences in the availability of high-quality grazing habitat and levels of resource competition. 4. There was no evidence that females with more heavily worn molars had reduced reproductive performance late in life or that first molar height was associated with reproductive senescence.

Infiltration Metasomatism of Cumulates by Intrusive Magma Replenishment: the Wavy Horizon, Isle of Rum, Scotland

The Eastern Layered Intrusion of the Rum Layered Suite comprises paired peridotite and allivalite (troctolite and gabbro)layers forming 16 macro-rhythmic units. Whereas the majority of these macro-units are believed to have formed by a process of crystal^liquid differentiation involving successive accumulation of crystals from multiple picritic replenishments of the chamber, the Unit 9 peridotite is interpreted as a layer-parallel picrite intrusion. Closely correlated with this discontinuous peridotite body is a distinctive feature generally known as theWavy Horizon, which divides the overlying allivalite into a lower troctolite and an upper gabbro along a well-defined undulating contact. We propose that the Wavy Horizon is a metasomatic feature formed consequent to the removal of clinopyroxene from an original gabbroic mush. Foundering of the mush into the picritic sill resulted in the replacement of the original interstitial liquid by one saturated only in olivine ( plagioclase). Progressive through-flow of this liquid resulted in the stripping out of clinopyroxene from the lower parts of the allivalite.We interpret theWavy Horizon as a reaction front, representing the point at which the invading liquid became saturated in clinopyroxene.The distinctive pyroxene-enriched zone immediately above theWavy Horizon could have formed when mixing of the interstitial liquids on either side of the reaction front formed a supercooled liquid oversaturated in pyroxene, as a result of the curvature of the olivine plagioclase clinopyroxene cotectic. The presence of many such approximately layer-parallel features, defined by differences in pyroxene content, in the Eastern Layered Intrusion of Rum suggests that such an infiltration^ reaction process was not unique to Unit 9.

Cumulative reproduction and survival costs in female red deer

Successful reproduction in a single breeding event has consistently been shown to reduce condition, fecundity and survival to the following breeding season. Few studies have examined the cumulative costs of frequent reproduction on survival. Here we use a dataset of female red deer (Cervus elaphus) from the Isle of Rum, Scotland, to model survival probability within a mark–recapture framework. By including both recent reproduction and long‐term cumulative reproductive effort in the models we tested whether knowledge of lifetime reproductive effort improves our estimates of survival probability. We found that the fit of the model was significantly improved with the inclusion of longer‐term measures of reproductive history. Heterogeneity in the reproductive performance of individuals influenced the expected survival cost of reproduction, with high cumulative reproductive effort associated with high survival, except with individuals reproducing in their first year where reproduction was associated with a decrease in survival. This work emphasises the need to account for reproductive history when estimating the survival probabilities of animals.

Low-level parasitic worm burdens may reduce body condition in free-ranging red deer (Cervus elaphus)

Regulation of ungulate populations by parasites relies on establishing a density-dependent relationship between infection and vital demographic rates which may act through the effect of parasites on body condition. We examine evidence for parasite impacts in 285 red deer (Cervus elaphus) harvested during 1991 and 1992 on the Isle of Rum. In the abomasa, prevalence of nematodes was 100% and the most abundant genus observed were Ostertagia species, however, mean intensity of infection was low (less than 1000) relative to other studies. Additional species, also present in low numbers, included Nematodirus spp., Capillaria spp., Cooperia spp., Monieza expanza, Oesophagostomum venulosum and Trichuris ovis. Lungworm (Dictyocaulus spp.) and tissue worm (Elaphostronygylus cervi) larvae were also observed in faecal samples. There was no evidence for acquired immunity to abomasal nematodes. Despite low levels of infection, both adult male and female deer showed significant negative correlation between indices of condition (kidney fat index, dressed carcass weight and larder weight) and intensity of Ostertagia spp. infection. However, there was no evidence that pregnancy rate in females was related to intensity of infection. For calves, there was no relationship between body condition and intensity of infection. The apparent subclinical effects of low-level parasite infection on red deer performance could alternatively be due to animals in poorer nutritional state being more susceptible to infection. Either way the results suggest that further studies of wild populations are justified, in particular where high local host densities exist or alternative ungulate hosts are present, and, where experimental treatments are tractable.

Sexual segregation among feral goats: testing between alternative hypotheses

Sexual segregation occurs widely in ungulates, and four principal hypotheses (predation, forage quality, social preferences and activity patterning) have been suggested as possible explanations. However, no single explanation has so far received widespread support. We used data on the grouping behaviour of feral goats, Capra hircus, on the Isle of Rum, Scotland, U.K. to test between competing hypotheses. We first used Conradt's (1999, Animal Behaviour, 57, 1151–1157) test to show that sexual segregation is not a consequence of sex differences in habitat preference. Since the predation risk hypothesis can be ruled out by both the absence of important predators on Rum and the tendency for those individuals most sensitive to predation (mother–offspring pairs) to be the furthest away from their neighbours, any explanation for sexual segregation must either lie in differences in activity patterning or be the consequence of a social preference for associating with same-sex individuals. We analysed data on the patterns of fission, behavioural synchrony and neighbourhood in relation to party size and composition to show that the activity budget hypothesis is the more likely. However, the data also suggest that the social preferences hypothesis none the less has a residual influence on segregation tendencies of the Rum goats. We conclude that one likely reason for the conflicting results in the literature may be that all four hypotheses in fact apply simultaneously, but that their relative weightings may depend on habitat- and species-specific characteristics.

Genetic consequences of human management in an introduced island population of red deer (Cervus elaphus)

We investigated phylogeography and spatial genetic structure in an introduced island population of red deer (Cervus elaphus) on the Isle of Rum, Scotland, experiencing spatial variation in management regime. Five different mitochondrial DNA (mtDNA) haplotypes were present among female red deer on Rum. These comprised two phylogenetically divergent groups, one of which clustered with red deer from Sardinia and North Africa, while the other four grouped with other Western European red deer. Recent and historical red deer management practices explain this result. The Rum population is descended from recent introductions from at least four different UK mainland populations, and translocation of red deer within the UK and across Europe is well documented. We found significant spatial genetic structure across Rum in both mtDNA haplotypes and microsatellite markers. Mitochondrial spatial structure was over an order of magnitude greater than structure in nuclear markers. This extreme difference is explained by the fact that the Rum population was introduced from different source populations, the highly male-biased dispersal patterns of red deer and the much smaller effective population size of mitochondrial compared to nuclear markers. Spatial structure in mtDNA conformed to a pattern of isolation by distance, while nuclear DNA did not. Apparent structure in the nuclear markers was driven by differences between the North Block and the rest of the island. We suggest that recent differences in the management regimes in different parts of the island have led to differences in effective male migration that would account for this observation.

Feeding competition within a feral goat population on the Isle of Rum, NW Scotland

This study investigated feeding competition within and between different age–sex classes of feral goats (Capra hircus) on the Isle of Rum (northwest Scotland) from August to November 2000 (inclusive). Although contests in a feeding context were common, most were relatively passive: little overt agonistic behaviour was observed between opponents and the distance between feeding animals involved did not change significantly after an interaction. Month (but not sex or habitat type) had a significant effect on feeding interaction rates, and the proportion of interactions involving more intense forms of conflict was highest in November when forage availability was beginning to decline. The results show that the initiator won most feeding encounters, with adult males being dominant over females. The ability to win conflicts increased with age for both males and females. However, it decreased sharply for adult males older than 5 years, which may, in part, explain the reduced overwinter survival of these individuals.

Constraints on plastic responses to climate variation in red deer

Influences of climate on life history traits in natural populations are well documented. However, the implications of between-individual variation in phenotypic plasticity underlying observed trait-environment relationships are rarely considered due to the large, long-term datasets required for such analysis. Studies typically present correlations of annual trait means with climate or assume that individual phenotypic responses are constant. Here, we examine this additional level of variation and show that, in a red deer population on the Isle of Rum, Scotland, changes in climate generate changes in phenotype only amongst individuals who have experienced favourable ecological conditions. Examination of relationships between offspring birth weight and spring temperature within the lifetimes of individual females revealed that the tendency to respond to climate declined as the population density experienced early in life increased. The presence of such systematic variation in individual plasticity is rarely documented in the wild, and has important implications for our understanding of the environmental dependencies of traits under varying ecological conditions.

Dynamics of grouping patterns and social segregation in feral goats (Capra hircus) on the Isle of Rum, NW Scotland

We investigated group size, grouping patterns and sexual segregation in feral goats on the Isle of Rum (NW Scotland) from January 1981 to September 1982 and May to November 2000. A total of 7396 groups of feral goats were recorded, comprising 30356 adult individuals and 13119 yearlings and kids. The most commonly observed group size was 1-3 individuals. Mean group size was larger in 2000 than in 1981-82, possibly reflecting an increase in population size. In both study periods, group size varied throughout the year: mean group size was smallest in late winter and early spring, and increased through summer to reach a maximum during the August/September rutting period. Mixed-sex groups were larger than single sex groups. Feral goats on Rum showed obvious social segregation during late spring and summer months (April to July) as demonstrated by a lower percentage of mixed-sex groups, lower proportions of adult males and females observed in mixed-sex groups and a higher social segregation coefficient in these months. The extent to which the population became sexually segregated correlated most strongly with daylength. This probably reflects the goats' pattern of habitat use during foraging: large mixed sex parties, which use caves for overnight shelter, forage slowly up the hill sides during the day, gradually dispersing more widely as they go. In winter months, when days are short, the animals are able to move less far, and so become less dispersed.

Phenotypic plasticity in a maternal trait in red deer

Summary Phenotypic plasticity and microevolution represent the two processes by which phenotypic traits in a population can track environmental change. While there is a growing literature documenting microevolution in reproductive traits in naturally occurring animal populations, few studies to date have examined either between‐individual variation in levels of plasticity or how selection acts on plasticity. We present here mixed‐effect linear models analysing changes in calving date in relation to autumn rainfall observed over a 30‐year study of 2147 red deer on the Isle of Rum, Scotland. The study period is characterized by a phase of low and rising population density (up to and including 1980), followed by a phase of high and fluctuating population density (1981 to present). Variation within individual females explained a population‐level trend of delayed calving dates following years of high autumn rainfall. There was significant variation between females both in their average calving dates and in their individual plastic responses of calving date to autumn rainfall. Females born in the low population density phase were, on average, phenotypically plastic for the calving date–autumn rainfall relationship, and showed significant variation in plasticity. Selection favoured individuals with early average calving dates among these females. Among females born at high population density, there was on average no significant plasticity for calving date, but variation in plastic responses was still present. Selection favoured females with increasingly positive plastic responses of calving date to autumn rainfall. We argue that early experience of high population density affects the physiological condition of females, making an environmental response (calving early following dry autumns) in later life physiologically untenable for all but a few high quality individuals. These same few individuals also tend to be fitter and have higher reproductive success.

Walking the Talk: Philosophy of Conservation On The Isle of Rum

AbstractThis paper describes our experience of teaching environmental ethics as part of a Philosophy of Conservation field trip to Rum, off the West coast of Scotland. The field trip was formalised into an M. A. Module in 1999. After outlining the educational aims of the module, and how these are implemented in this setting, we indicate some of the key issues in ethics and aesthetics that emerge as we explore two specific conservation sites. We close with a reflection on the value of experiential education in this area, and the importance of combining experiential, emotional and intellectual engagement in any exploration of normative issues.

Daytime activity budgets of feral goats (Capra hircus) on the Isle of Rum: influence of season, age, and sex

The activity budgets of feral goats (Capra hircus) on the Isle of Rum, northwest Scotland, were studied from January to December in 1981 and from May to November in 2000. We investigated differences in activity patterns in relation to season, time of day, and age–sex classes of goats, and their possible causes. Since the goats spent the night sleeping (or resting) in caves or other sheltered sites, only diurnal activity budgets were considered. Activity patterns were very similar between the two study periods (from May to November). All age–sex classes of feral goats spent most of the daytime feeding in both years, especially during autumn and winter. Feral goats increased the percentage of daytime that they spent feeding and decreased their resting time from summer to autumn–winter in response to the decrease in available day length and, possibly, the decrease in forage quality and biomass. Feral goats on Rum had two or three daily peaks of feeding in summer, but showed no obvious daily feeding peaks in winter, although their percentage of feeding time remained relatively high throughout the daytime in winter. No significant differences in general activity budgets between adult males and females were found, but yearlings were found to spend more time feeding than adults and kids. Adult males spent more time in social activities than did other age–sex classes, and adult males sharply decreased the percentage of daytime that they spent feeding during rutting periods, which may have contributed to their higher mortality over the subsequent autumn and winter.

Changes in vigilance with group size under scramble competition.

Animals often interrupt foraging to scan the surrounding environment. In several taxa, such vigilance decreases with group size (Elgar 1989). The decline in vigilance is usually interpreted as a decrease in the predation risk to an individual in large groups. Mechanisms such as increased detection ability and numerical dilution of risk may provide more protection to foragers in large groups, thus leading to lower vigilance (Pulliam 1973; Roberts 1996; Bednekoff and Lima 1998). Nevertheless, recent work indicates that the decrease in vigilance with group size may be mitigated against if individuals in large groups scan more to avoid displacements by aggressive companions (Slotow and Coumi 2000; Treves 2000) or to locate the food discoveries of other group members (Beauchamp 2001). We explore the role of resource availability on vigilance patterns. In earlier vigilance models, a decrease in vigilance would allow foragers either to decrease the total amount of time needed to accumulate a fixed amount of resources (Pulliam et al. 1982) or to increase feeding rate over a given time horizon (McNamara and Houston 1992). In either case, no competition for resources is assumed to take place. However, when resources are limited, a reduction in vigilance may allow individuals to allocate more time to foraging and so obtain a greater share of the re-

Spherulitic textures formed during crystallization of partially melted arkose, Rum, Scotland

Melting of Torridonian arkoses, due to intrusion of shallow bodies of mafic melts on the Isle of Rum, was locally extensive, reaching up to 95 vol.%. Crystallization (to form granophyre) of these highly melted rocks was generally static in the vicinity of minor intrusions, although disruption of bedding is apparent in the aureole of the main mafic pluton. A distinctive millimetre- to centimetre-scale spherulitic texture developed during solidification of the partially melted arkose, in both the aureole of the main intrusion, and also in those of associated minor gabbro plugs. The spherulites are formed of radiating, fan-shaped intergrowths of plagioclase and quartz set in a matrix of quartz, plagioclase and K-feldspar. The importance of nucleation rates in determining texture development is demonstrated by a dominance of Na-rich compositions in the first feldspar to form, even in non-spherulitic rocks. It is suggested that the spherulites formed by the early growth of metastable plagioclase due to difficulties in nucleating feldspar.