Sable Island Horse Project

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For a summary of our program, please visit the Sable Island Horse Project on Facebook, where you will also find updates on students’ projects. 

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Sable Island is a 49 km-long, crescent-shaped sand bar located 160 km off the east coast of Nova Scotia, Canada. Renowned for its windswept and grass-covered dunes, wild horses, and shipwrecks, Sable Island is as unique in character as it is in biological diversity. More than 320 species of birds have been observed on Sable Island, including breeding populations of endangered Roseate Terns and the ‘Ipswich’ subspecies of the Savannah Sparrow, which nests exclusively on Sable. Sable is also home to a number of species of seals, including the world’s largest population of grey seals. The island’s most iconic species—the wild (feral) horse—was introduced in the mid-1700s. The Sable Island horse is of exceptional cultural value to the people of Nova Scotia and is noted as a breed of significant conservation interest due to its distinct genetic heritage. Human presence on the island is restricted to a handful of researchers and visiting scientists, tourists, and managers of the island’s meteorological station. On December 1, 2013, Sable Island came under management of Parks Canada Agency as Sable Island National Park Reserve, and we very much look forward to working with Parks to further our joint interests in ecology and applied conservation biology. Sable Island is truly one of the most interesting outdoor laboratories a population ecologist could ask for! We have a number of projects on the go on Sable Island.

Feral HorsesIMG_0019

I have been interested in the ecology and evolution of the Sable Island horse since 2007, when I first set foot on the island (with annual field trips every year since then). Having freshly returned from working on a long-term, individual-based project on red deer of the Isle of Rum in Scotland, I thought it would be neat to start something similar here in Canada. In particular, I set out to establish a program aimed at filling a specific niche in ecology in Canada: the individual-based study of a wild vertebrate that lives free from predation, interspecific competition, and human interference (to allow me to focus on the role of intraspecific density-dependent phenomena in ecology and evolution). After scouting several locations across Canada, I quickly realized that the horses of Sable Island presented an almost ideal system for the study of population ecology and evolution. The horses are the island’s only terrestrial mammal, and have been feral now for more than 30 generations. Sample sizes are large (n = 559 horses in 2013) and each horse is recognizable from photographs; the lack of trees on the island makes it easy to relocate each animal several times a season. The study also provides some new perspectives on the evolution of wild ungulates: e.g., feral horses exhibit a social structure that is very different from other polygynous ungulates, being more similar to that of primates. Also advantageous for the project is that we already know so much about horses—their natural history, behaviour, and even a genome sequence—so we begin our study with a  head start compared to programs on little studied, non-model organisms. For example, linking evolutionary change with molecular genetic variation in natural populations is one of the fundamental but largely inaccessible goals of evolutionary biology. For the Sable Island horses, however, this is not an unrealistic feat.

Since 2007, each summer my collaborators, students, and I have been naming and keeping track of the life histories and movements of every horse that lives on Sable Island, as well as a number of other parameters. We have high hopes for this long-term project to make some significant in-roads into our understanding of how individual dynamics can help us to understand population-level phenomena. We now have a fantastic and continually growing data set, as our study is entering its eighth year of whole-island data collection in summer 2015. Data available for new student projects (or collaborations) includes specifics of individual contributions to population growth (survival and recruitment), movement and dispersal patterns, individual exposure to local population density and sex ratios, condition, body size for all individuals and ages (from parallel-laser calipers), colouration patterns, parasitology, and for many individuals biological samples for DNA, cortisol, and diet.

We are strict in our application of non-invasive methods to study the horses and to count them in our census (always carried out on foot; we visit each horse on the island between 2 and 10 times a summer for only a few minutes each time), and carefully approach horses for sampling to minimally interfere with their normal activities (following Animal Care Protocol 2009032 of the University of Saskatchewan under guidance of the Canada Council on Animal Care).

A Collaborative Effort

The Sable Island project is a truly collaborative research program with many contributors, especially my co-PI on the Sable Island project, ecological evolutionary geneticist Dr. Jocelyn Poissant. Jocelyn is currently cross-appointed with my group and the University of Calgary, Department of Veterinary Medicine). We also have several metoddcurrent collaborators at the University of Saskatchewan campus (e.g., Dr. David Janz, Dr. Keith Hobson [Environment Canada], Dr. Jill Johnstone, Dr. Emily Jenkins, Dr. Joe Rubin, and Dr. Todd Shury [who also holds a post with Parks Canada Agency, Office of the Chief Ecosystem Scientist]). Additional collaborators are with the Faculty of Veterinary Medicine, University of Calgary (Dr. John Gilleard). We also have had links with the University of Oxford (Dr. Tim Coulson), University of Sheffield (Dr. Jon Slate), and University of Exeter (Dr. Alastair Wilson and Dr. Britt Koskella). Dr. Floris van Beest and Dr. Eric Vander Wal (Memorial University of Newfoundland) have also been working closely with my lab to develop questions of habitat selection using the horse population as a model. With Fisheries and Oceans Canada we maintain a long-term collaboration with Dr. Don Bowen and Jim McMillan. From behavioural to population ecology, conservation biology, population genetics and genomics, and natural and sexual selection, this project is going to be a fantastic one to watch in the future. Our first papers are just coming out, so stay tuned!

A Conservation Mandate, On Sable Island…And Off!

Conservation of Sable Island and management of its species and resources will require more than designation as a National Park Reserve. Management requires knowledge, and conservation strategies can only be developed from a base understanding of how and why a system works the way it does. Our goal is to advance conservation initiatives on Sable Island by developing a complete and thorough understanding of the role of the horses in the ecosystem, what they might mean (for better or worse) to other species that call the island home, and what we can expect for long-term population viability of the horse population. Our research questions are directed at both theoretical and applied ecology: both are necessary for conservation biology, as insight needed to apply research follows firstly from applying the scientific method to understand the fundamentals of a system.

Underlying our research objectives is the notion that if the Sable Island horse was formally classed as a distinct taxon—an idea that is functionally accepted to be true by the public (there is only one ‘Sable Island horse’, and no substitute)—the horses would be designated Endangered based on the criteria of their being fewer than 250 mature individuals in the wild (e.g., there are fewer mature Sable Island horses in Nova Scotia than there are Blanding’s Turtles, which are also Endangered). Few numbers of mature individuals (<250) is one criteria used by the Committee on the Status of Endangered Wildlife in Canada and IUCN-World Conservation Union to identify taxa (species or designated population units) as Endangered and thus at ‘very high risk’ of extinction (see: http://www.iucnredlist.org/static/categories_criteria_3_1#critical). Note that there are also additional criteria that the Sable Island horse meets to identify it as being of special conservation concern: the horses occupy an area of less than 500 km2 and occur only in one place, the population experiences large fluctuations in size through time, and the horses are known to be inbred relative to other horse populations (demonstrated recently by Prystupa et al. [2012a] to have the highest inbreeding coefficient of 24 populations of Canadian, Mountain and Moorland and Nordic populations of horses and ponies). We  believe that the Sable Island horse should be treated as and studied as diligently as any other Endangered population of wildlife. Only by understanding as much as we can about the population can we plan for its conservation (in addition to learning about how the horses might affect the conservation prospects for native plants and animals on Sable Island). The opportunity to learn about the functioning of isolated, vertebrate populations using the Sable Island horse as a model is also of national and international conservation interest: observations in this system can and do apply to problems faced by other at-risk species (e.g., see our recent paper on the functioning of source-sink population dynamics using the Sable Island horses as a model [Contasti et al. 2012]).

Gorilla for website2

Over the next five years, our research program will be targeting projects concerning four inter-related themes of research. These themes address fundamental and applied projects on (I) Population ecology; (II) Inter-species interactions and community ecology; (III) Population genetics; and (IV) Parasitism, disease, and health of Sable Island horses.

Theme I. Population ecology of the Sable Island horses. 

Long-term, individual-based study of free-living populations provides a rich resource for understanding ecology, evolution, and conservation because individuals’ life histories can be measured by tracking them from birth to death. There are several famous examples of the individual-based research model (review in Clutton-Brock and Sheldon 2010), like the famous chimpanzee project at Gombe National Park in Tanzania, and the equally famous projects on the Soay sheep of St. Kilda (Scotland), red deer of the Isle of Rum (Scotland), and great and blue tits at Wytham Woods (Oxford, England). In Canada, examples of long-term, individual-based population studies exist for red squirrels at Kluane (Yukon), Columbian ground squirrels at Kananaskis (Alberta), and bighorn sheep and mountain goats in the Alberta Rockies. To this list we aim to add our research program on the Sable Island horses.

The horses of Sable Island are particularly amenable to individual-based research: each horse is recognizable from photographs, and the horses are wild but approachable, as they do not fear humans since we stopped attempting to manage the population in the 1960s (see Christie 1995; the horses are not ‘habituated’ to people but rather tend to ignore humans, which they do not view as a threat nor source of benefit).

Sarah

Our research program is based on a non-invasive monitoring program in summer that collects information for every living horse (which we number and name), including data on survival, reproduction, habitat selection, movement and dispersal patterns, body size, condition, coat colour, associations and behaviour (time budgets for focal individuals), parasite loads, and samples for DNA and hormone (cortisol) and stable isotope analyses. We currently have life history data for all 801 individuals that have lived on the island since 2008 to 2013, which is already comparable to many other long-term studies of wild mammal populations. Our most recent papers on the population ecology of the Sable Island horses document spatial heterogeneity in population growth and genetic diversity and effects of density on movement patterns and dispersal (e.g., Contasti 2011; Contasti et al. 2012; 2013; Marjamäki et al. 2013; van Beest et al. 2014).

Our approach to answering questions about the population ecology of the horses has been firstly directed at decomposing the dynamics of the horse population into individual contributions (something only possible by monitoring all individuals on the island), and then asking what is it about some individuals that allows them to survive and reproduce at rates that differ from others. To answer our questions we collect information on where each horse lives on the island and what it has for access to resources (forage and water), where it moves and with whom it associates (band dynamics), and details about the physical state of individuals (sex, age, health, parasite load, body size, body condition, stress levels, genetics) that can account for individual contributions to population growth. There is much to be learned from linking individual demography to population growth, including opportunities to understand eco-evolutionary dynamics (see Pelletier et al. [2007] for an example). Further, one of our goals is to link individual performance (survival and reproduction) to each individual’s unique experience of the environment. In doing so, we can identify what is most critical to the population and directly define critical habitat (e.g., what resources increase reproduction the most when population size might be at its lowest and most critical density [for examples of this approach, see McLoughlin et al. (2006, 2007, 2008]).
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Fig 2 Website

Over the next five years, we hope to address the following questions on horse population ecology:

a) Determine how density affects horse population dynamics along the length of Sable Island, including dispersal patterns, patterns in band size and structure (sex ratios), and individual contributions to population growth;

b) Identify how spatial heterogeneity in population growth and density (local carrying capacity) is influenced by habitat, including availability of water and vegetation associations;

c) Determine how habitat and abiotic effects (weather) interact to affect the social dynamics of the horse population, movements, and population growth;

d) Understand the nature of population crashes (both historic and if occurring in the next five years); and,

e) Determine the extent to which the horse population dynamics is being influenced the species domestication past, and if these effects are being reversed by natural selection.

II. Inter-Species Interactions

There are several applied questions about horse conservation with respect to impacts on the environment that we are interested in: e.g., how does disturbance affect vegetation communities on Sable Island; what effect does high density have on distribution patterns, especially with respect to other species on the island (terns); and why are abundances so high in recent years (is this related to grey seals which now number more than 200,000 from <10,000 in the 1960s [Bowen et al. 2003, 2007]). We are very interested in the latter. Mediated transport of nutrients from marine to terrestrial environments by species that feed in oceans but also occupy terrestrial systems (e.g., seabirds, spawning salmon, seals, sea turtles) can cause considerable changes in the structure of land plant communities (review in Lysak [2013]). Less understood are the implications of such changes to higher trophic levels, like in herbivores. Sable Island presents us with an exciting opportunity to study mediated sea-to-land nutrient transfers and their implications to the space-use and dynamics of a naturally regulated large herbivore, with applications to the long-term sustainability of biodiversity in the system.

By fertilizing the nutrient-poor sands of Sable Island, increasing populations of terns and gulls (which nest inland during summer) and whelping grey seals may be potential vectors for the input of nitrogen to the dune ecosystem. We hypothesize that if the dynamics of seals and seabirds correlates with the transfer of nutrients from ocean to land, regulating patterns in island vegetation, there may be significant impacts to the resident population of feral horses, through, e.g., changes in space use and population size and carrying capacity on the island.

Our most recent research (Lysak 2013) shows that this does appear to be the case, with nitrogen cycling within the island’s ecosystem being dependent on the input of nutrients from seals and seabirds, affecting primary production and higher trophic levels (i.e., horses). We examined this by developing a spatially-explicit ‘isoscape’ for Sable Island, examining nitrogen isotope signals (δ15N) in samples of marram grass (Ammophila breviligulata), which occurs throughout Sable Island. The most important predictor variables describing the spatial distribution of marram δ15N were distance to seal and seabird pupping/nesting colony and distance to shoreline. The greatest 15N enrichment occurred within the tips and along the perimeter of the island, coinciding with greater densities of grey seals, while the lowest values occurred within the centre of the island. Marram grass exhibited 15N enrichment within seal (7.5‰) and tern (5‰) colonies, while horses contributed to the homogeneity within the centre of the island (3.6‰). Due to the high densities, wide distribution, and greater 15N enrichment, grey seals appear to be the most important vector species while seabirds appear to have a more localized effect. The enrichment within vector colonies extended into the local communities dynamics, contributing to greater vegetation cover within the tips of the island where seal permeability was highest. This relation permeated into the horse population, which showed correspondingly higher horse δ15N values within the tips of the island (δ15N = 1.6‰ higher) due to consumption of enriched forage. We believe that vector species promote vegetation growth and nutrient enrichment by establishing nutrient gateways which indirectly cause cascading effects throughout the food web.

What are the long-term implications of the above? High horse densities may have unforeseen but potentially important consequences for the dune ecosystem, affecting plant diversity, bird life, and other animals. High numbers of horses on the island may be of concern because in systems similar to that of Sable (i.e., marram grass-dominated dune systems), grazing and trampling has been shown to reduce plant cover, vegetative spread of plants, biomass, flowering, and seed production, making dunes vulnerable to erosion. But on the other hand, grazing and recycling of nutrients by feral horses—in cases of intermediate intensity disturbance—may increase plant species diversity. We believe that the horse population and their importance to island ecological integrity may be indirectly regulated by the mediated transport of nutrients from ocean onto land by seals and seabirds. How the dynamics of seals, seabirds, island vegetation, and horses are inter-related is something that managers of Sable Island must be made aware of.

Over the next five years, we hope to address the following specific questions on horse community ecology:

a) Determine how seals and seabird influences on vegetation patterns affect horse population dynamics, social structure, and life history;

b) Determine what the consequences of a decline in the grey seal population might mean to local carrying capacities on Sable Island and horse population densities and dynamics.

Data for this project is already collected (field vegetation work, see Tissier [2011] and Tissier et al. [2013] and Lysak [2013]), or will be collected  (animal locations) as part of sampling identified in Theme I.

Theme III. Population genetics and adaptive evolution of Sable Island horses

A few researchers have published on the genetics of the Sable Island horses (e.g., Plante et al. 2007; Lucas et al. 2009; Prystupa et al. 2012a,b), and all have shown that, as expected for an isolated population founded by a small number of individuals, the Sable Island horses have a relatively high inbreeding coefficient compared to other horse populations and breeds (the horses are most closely related to the Nordic breeds of horses and ponies [Prystupa et al. 2012b]). One of the unintended consequences of isolation and protection of the Sable Island horse in the 1900s has been cessation of adding any new individuals from the mainland and their genetic contributions, with the last recorded arrival being a ‘Hunter-type’ male in 1935, which followed a small series of arrivals of horses of mixed breeds from 1900 to 1904 (see Welsh 1975). This means that for almost 80 years the Sable Island horse has evolved without any genetic introgression. Coupled with meeting the functional definition of being an Endangered species based on population characteristics, and knowing that the horses show high levels of inbreeding, we are concerned about the long-term prospects of population viability in the Sable Island horses from demographic stochasticity and inbreeding depression.

Our aim is to complement our well-established field sampling program on population ecology with molecular and quantitative genetics components (from past collections of hairs, and currently being collected saliva and feces to extract DNA). Led by co-PI on the Sable Island horse project, Dr. Jocelyn Poissant, this includes reconstructing a multigenerational pedigree using molecular markers, and applying this pedigree to estimate selection as well as important quantitative genetic parameters such as the proportion of phenotypic variation explained by direct or indirect genetic effects and genetic correlations among traits. Ultimately, this will allow understanding of how selection and genes interact to shape adaptive evolution in the horses, and the extent to which inbreeding depression may pose a conservation risk.

Fitness in wild populations is most readily studied by performing long-term field studies where most or all individuals of a population are monitored throughout their lives (Clutton-Brock and Sheldon 2010). Advanced quantitative genetic analyses in such systems have yielded insightful results, indicating that genetic variance and heritability for fitness could vary substantially (Walsh and Blows 2009; Shaw and Shaw 2014). However, very few such estimates have been published, and even less is known about the factors influencing their variability, such as the contribution of individual traits and ontogenetic stages, or the impacts of genetic covariances. Quantifying genetic variance for fitness in additional study systems, and determining the factors contributing to its variability within and among populations is greatly needed. We aim to help fill this gap using the Sable Island horses.

jocelyn

Classical quantitative genetic approaches are essential to quantify adaptive evolution in variable environments. However, to fully understand evolutionary processes involved we must extend our analyses to the actual genes underlying fitness variation (Ellegren and Sheldon 2008; Slate et al. 2010). While large genome-wide sets of markers are ideal to make inferences about the genetic architecture of fitness and identify the loci involved, a more direct, economical, and efficient approach consists in targeting genes with well-understood major phenotypic effects for which there is compelling evidence of adaptive significance. Melanocortin genes, such as MC1R and agouti, fit these criteria. Indeed, they have confirmed major effects on conspicuous colour polymorphisms, and these polymorphisms are usually adaptive. For example, melanin-based colour polymorphisms are often involved in camouflage against predators or mate choice. In addition, melanocortins are believed to have major pleiotropic effects on a variety of fitness-related behavioural and physiological traits including aggressiveness, stress response, immune function and energy homeostasis. This rare combination of high tractability, confirmed major phenotypic effects, and compelling adaptive significance makes melanocortin genes outstanding models to study adaptive processes at the molecular level in wild populations.

Colour for website

 

Over the next five years, the lab will be combining quantitative and molecular genetics approaches to study the extent, causes, and consequences of genetic variation in fitness in the Sable Island horses, with a particular focus on (potentially adaptive) melanin-based polymorphisms. We hope to address several objectives:

a) Reconstruct a population-wide pedigree using social (suckling behaviour and identity of band stallion) and genetic markers;

b)  Document the population’s fine-scale spatial genetic structure and its biotic and abiotic determinants;

c) Quantify current heterozygosity among the living horse population, and determine if the population is impacted by inbreeding depression;

d) Quantify the contribution of additive genetic effects to phenotypic variation; and the contributions of individual phenotypic traits and genetic covariance to genetic variance for fitness;

e)   Determine if processes are maintaining adaptive genetic variation in Sable Island horses; and,

f)  Test for the presence of a ‘melanism syndrome’ by determining if melanocortin genes have pleiotropic effects on traits other than color such as dominance, growth, stress response and parasite load.

labweb

Samples already available for our genotyping include some rooted hairs and hundreds of fecal swabs; however, in an effort to collect higher amounts of DNA we are also collecting whole saliva. Whole saliva is readily available from foraging horses, e.g., when they selectively drop unpalatable components of vegetation while feeding.

IV. Parasitism, disease, and health of Sable Island horses.

The assimilation of food in horses, as in all vertebrates, depends on symbiotic microorganisms inhabiting the gastrointestinal tract and the parasites that act to interfere with this process. Since energy and nutrients obtained from food have major impacts on health and body condition, the gut biome (GB, the community of organisms inhabiting the gastrointestinal tract) is predicted to have a profound influence on the well-being and life history (longevity and reproduction) of host organisms. Yet there is a clear dearth of research on the causes and consequences of GB variation.

To directly study resource assimilation in free-living populations we must identify aspects of individual phenotype that can be assayed in the field and that directly influence assimilation. The GB fits both criteria: it can be readily obtained from fecal samples collected non-invasively, and is known to critically influence the uptake of energy from food consumed in herbivores. This is particularly true for horses which obtain over 60% of their energy from the activity of fibrolytic gut bacteria (Costa and Weese 2012). We will study the GB of Sable Island horses (both bacteria and parasites) to understand how environmental and genetic processes link resource acquisition, assimilation, health, and life history. Sampling for this project is ongoing, with >600 fresh feces samples collected from known individuals in both 2013 and 2014). This is a multi-faceted project with co-PI Dr. Jocelyn Poissant, and collaborators at Parks Canada Agency (Dr. Todd Shury), the Western College of Veterinary Medicine (Dr. Emily Jenkins and Dr. Joe Rubin), and the University of Calgary (Dr. John Gilleard), and the Universities of Sheffield (Dr. Jon Slate) and Exeter (Dr. Alastair Wilson, Dr. Britt Koskella).

Gastrointestinal parasites including nematodes and tapeworms are known to occur in Sable Island horses (Welsh 1975). Of particular prevalence are strongyles of the Suborder Strongylida (determined in 2013 via our pilot surveys for samples from 63 horses), which are important parasites in both domestic and feral horses (e.g., Rubenstein and Hohmann 1989; Young et al. 1999). We are using a combination of quantitative and molecular techniques to obtain a detailed description of nematode prevalence and community composition in most horses currently alive. Strongyle-egg count in horses is highly repeatable and thus provides a reliable index of parasitic load (Carstensen et al. 2013). There are many different species of strongyles and eggs are not identifiable to genus and/or species when detected visually in a fecal sample. To resolve this, we are adopting a ‘microbiome-type’ approach that has recently been developed to quantify cattle parasite species in cattle fecal samples by collaborator Dr. John Gilleard (University of Calgary) and colleagues. A combination of egg counts and genetic analysis will serve to measure overall parasitic (nematode) load as well as community structure. In addition to providing a unique opportunity to conduct research on the ecology and evolution of host-parasite interactions in a wild long-lived vertebrate, this project will provide baseline parasitology information for an equine population that has never been exposed to anthelmintic drugs and contribute to the development of new molecular diagnostic tools; both of major importance to the equine veterinary industry.

The Sable Island horses also present an opportunity to study what we expect to be unique antimicrobial resistance patterns in a previously unstudied horse population, and given the population’s long history of isolation (Welsh 1975; Christie 1995), lack of modern veterinary care, and no history of drug administration. Using fecal samples collected in 2014 and 2015, we will be conducting a survey of antimicrobial resistance with emphasis on extended-spectrum β-lactamases (ESBLs) in E. coli of the GB of Sable Island horses. This aspect of the project proposes to determine the frequency of colonization of Sable Island horses with ESBL-producing E. coli in the GB. Observations of resistant strains of E. coli may shed light on the importance of non-direct environmental sources, including migratory birds and human traffic, to the problem of antimicrobial resistance in animal (and human) health (Allen et al. 2010). This project will use bacteriological and molecular techniques at the University of Saskatchewan’s Western College of Veterinary Medicine (lab of Dr. Joe Rubin).

Over the next few years we aim to:

a) Quantify each horse’s individual GB from fecal egg counts of parasites and ultra-high throughput sequencing of nematode parasites and bacterial communities;

b) Investigate the causes and consequences of GB diversity through the individual-based study of the Sable Island horses; and,

c) Study what we expect to be unique antimicrobial resistance patterns in a previously unstudied horse population, and given the population’s long history of isolation, lack of modern veterinary care, and no history of drug administration.

Stay tuned to this website and our Facebook page and Twitter feed (links above) as these projects develop and results of our research are shared!

Some References Cited Above

Allen, H.K., Donato, J., Wang, H.H., Cloud-Hansen, K.A., Davies, J., and Handelsman, J. 2010. Call of the wild: antibiotic resistance genes in natural environments. Nature Reviews Microbiology 8:251–259.

Beson, K. 1998. A conservation strategy for Sable Island. Prepared for the Canadian Wildlife Service, Environment Canada Atlantic Region, Sackville, NB.

Bowen, W.D., McMillan, J., and Mohn, R. 2003. Sustained exponential population growth of grey seals at Sable Island, Nova Scotia. Journal of Marine Science. 60:1265–1274.

Bowen, W.D., McMillan, J.I., and Blanchard, W. 2007. Reduced population growth of gray seals at Sable Island: evidence from pup production and age of primiparity. Marine Mammal Science 23:48–64.

Carstensen, H., L. Larsen, C. Ritz, and M.K. Nielsen. 2013. Daily variability of strongyle fecal egg counts in horses. Journal of Equine Veterinary Science  33: 161-164.

Christie, B. J. 1995. The Horses of Sable Island. Pottersfield Press. Lawrencetown, Nova Scotia, Canada.

Clutton-Brock, T. H., and B. C. Sheldon. 2010. Individuals and populations: the role of long-term, individual-based studies in ecology and evolutionary biology. Trends in Ecology and Evolution 25:562–573.

Contasti, A.L. 2011. Structure in vital rates, internal source-sink dynamics, and their influence on current population expansion for the horses of Sable Island, NS. M.Sc. University of Saskatchewan.

Contasti, A.L., Tissier, E.J., Johnstone, J.F., and McLoughlin, P.D. 2012. Explaining spatial heterogeneity in population dynamics and genetics from spatial variation in resources for a large herbivore. PLoS ONE 7:e47858.

Contasti, A.L., van Beest, F.M., Vander Wal, E., and McLoughlin, P.D. 2013. Identifying hidden sinks in growing populations from individual fates and movements: the feral horses of Sable Island. Journal of Wildlife Management 77:1545–1552.

Costa, C.C., and J. Weese. 2012. The equine intestinal microbiome. Animal Health Research Review 13:121–128.

Ellegren, H., and B.C. Sheldon. 2008. Genetic basis of fitness differences in natural populations. Nature 452:169–175.

Lucas, Z.N., McLoughlin, P.D., Coltman, D.W., and Barber, C. 2009. Multiscale analysis reveals restricted gene flow and a linear gradient in heterozygosity for a island population of feral horses. Canadian Journal of Zoology 87:310–316.

Lysak, K. 2013. Sea-to-land nutrient transfer by seals and seabirds on Sable Island: isoscapes revealed by stable isotope analysis of vegetation with an echo in the island’s feral horses. M.Sc.thesis, University of Saskatchewan, Saskatoon.

Marjamäki, P.H., Contasti, A.L., Coulson, T.N., and McLoughlin, P.D. 2013. Local density and group size interacts with age and sex to determine direction and rate of social dispersal in a polygynous mammal. Ecology and Evolution 3:3073–3082.

McLoughlin, P.D., Boyce, M.S., Coulson, T., and Clutton-Brock, T. 2006. Lifetime reproductive success and density-dependent, multi-variable resource selection. Proceedings of the Royal Society: Biological Sciences 273:1449–1454.

McLoughlin, P.D., Coulson, T., and Clutton-Brock, T. 2008. Cross-generational effects of habitat and density on life history in red deer. Ecology 89:3317–3326.

McLoughlin, P.D., Gaillard, J.-M., Boyce, M., Bonenfant, C., Messier, F., Duncan, P., Delorme, D., Van Moorter, B., Saïd, S., and Klein, F. 2007. Lifetime reproductive success and composition of the home range in a large herbivore. Ecology 88:3192–3201.

Pelletier, F., Clutton-Brock, T., Pemberton, J., Tuljapurkar, S., and Coulson, T. 2007. The evolutionary demography of ecological change: linking trait variation and population growth. Science 315:1571–1574.

Plante, Y., J.L. Vega-Pla, Z. Lucas, D. Colling, B. de March, and F. Buchanan. 2007. Genetic diversity in a feral horse population from Sable Island, Canada. Journal of Heredity 98:594–602.

Prystupa, J. M., P. Hind, E. G. Cothran, and Y. Plante. 2012b. Maternal lineages in native Canadian equine populations and their relationship to the Nordic and Mountain and Moorland pony breeds. Journal of Heredity 103:380–390.

Prystupa, J.M., Juras, R., Cothran, E.G., Buchanan, F.C., and Plante, Y. 2012a. Genetic diversity and admixture among Canadian, Mountain and Moorland and Nordic pony populations. Animal 6:19–30.

Rubenstein, D.I., and M. E. Hohmann. 1989. Parasites and social behavior of island feral horses. Oikos 55:312–320.

Shaw, R.G. and F.H. Shaw (2014) Quantitative genetic study of the adaptive process. Heredity 112:13–20.

Slate, J., A.W. Santure, P.G.D. Feulner, E.A. Brown, A.D. Ball, S.E. Johnston, and J. Gratten 2010. Genome mapping in intensively studied wild vertebrate populations. Trends in Genetics 26: 275­–284.

Tissier, E. 2011. Vegetation associations along disturbance gradients on the sand dunes of Sable Island, Nova Scotia. M.Sc. University of Saskatchewan, Saskatoon.

Tissier, E., P.D. McLoughlin, J. Sheard, and J.F. Johnstone. 2013. Distribution of vegetation along environmental gradients on Sable Island, Nova Scotia. Écoscience 20:361–372.

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