Ecology, Evolution, and Conservation of the Sable Island Horses

Ecology, Evolution, and Conservation of the Sable Island Horses

Contributed by Philip D. McLoughlin, Department of Biology, University of Saskatchewan. Philip is an Associate Professor of Animal Population Ecology and is leading a long-term (30+ year), collaborative research program into the ecology, evolution, and conservation of the Sable Island horse. The project was initiated in 2007, and annually since then Philip, his students, and colleagues have been conducting field work on the island to study the population dynamics, behaviour, and genetics of the horses. Please see Philip’s lab’s website at for more information. His research interests include both fundamental and applied questions of the ecology and evolution of North American and European ungulates (horses, caribou, moose, red deer, roe deer) and carnivores (wolves, polar bears, grizzly bears). Philip is a strong supporter of the mandate of the Friends of Sable Island “to promote Sable Island as a platform for scientific and historical research of provincial, national and international significance.”

Figure 1. Rocky (horse ID #75 in the University of Saskatchewan database), a stallion living near the west ponds on Sable Island as photographed by P.D. McLoughlin in 2007.

Canadians have come to recognize that Sable Island deserves greater protection than what it was afforded under the Sable Island Regulations of the Canada Shipping Act. Designating Sable Island as a National Park Reserve under the National Parks Act is a step in the right direction. But conservation of the island and management of its species and resources will require more than its designation as a protected space. 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 on Sable Island by developing a complete and thorough understanding of the role of the wild (feral) horses in the ecosystem, what the horses mean (for better or worse) for other species that call the island home, and what we can expect for long-term population viability of the Sable Island horses. We are not shy about asking both ‘blue sky’ questions about the system and hard-hitting questions of 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.

The Sable Island horse, with its long and storied history on Sable Island (for background see Christie [1995] and the 1975 dissertation of Daniel Welsh [references below]), is treated as ‘naturalized wildlife’ by Parks Canada Agency; that is, the horses are being managed as a taxon equal to other species living on the island and not as an invasive nor second-class species. There is functionally only one population of Sable Island horses; they exist nowhere else but on the island (aside from a few individuals descended from horses shipped from the island in the 1950s, now living at Shubenacadie Wildlife Park [mainland Nova Scotia]). Further to their cultural significance to Canada and Nova Scotia, the Sable Island horses are of special conservation interest because of their unique genetic heritage: genetically they are very different from any other breed or population of horses (see Prystupa et al. [2012a,b], Lucas et al. [2009], and Plante et al. [2007] for analyses). These differences were forged, in part, by 250 years of natural selection as the horses adapted to living free on the island with only periodic interference by humans (until protections of the Canada Shipping Act were enacted in 1961).

Driving our research program is the logic that if the Sable Island horse was formally classed as a distinct taxon—a notion 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 there being fewer than 250 mature individuals in the wild (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-Species Survival Commission to identify taxa (species or designated population units) as Endangered and thus at ‘very high risk’ of extinction over the long-term (see criteria D1 at: Note that there are also additional criteria that the Sable Island horse meets to identify it as a species 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 strongly 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 this functionally endangered 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. 2013]).

Our research program currently includes collaborations with Parks Canada Agency (e.g., Dr. Todd Shury, veterinarian and wildlife health specialist of the Office of the Chief Ecosystem Scientist), the University of Sheffield (Dr. Jocelyn Poissant), the Bedford Institute of Oceanography (Fisheries and Oceans, DFO Science, Dr. Don Bowen, Mr. Jim McMillan), and now Memorial University of Newfoundland (recently hired professor and former graduate student in the McLoughlin lab, Dr. Eric Vander Wal). Our program is designed to be complementary to other ongoing research projects on Sable Island. We are funded primarily by agencies such as the Natural Sciences and Engineering Research Council (NSERC), the Canada Foundation for Innovation, and the Royal Society, and by the generous donations of individuals and groups including the Friends of Sable Island. Our program currently supports five graduate students and several undergraduate students working on three inter-related themes of research. These themes address:

Theme I. Population ecology of the Sable Island horses. 

Long-term, individual-based study of free-living populations provide 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, like Jane Goodall’s Chimpanzee project at Gombe River in Africa, and the equally famous projects on the Soay sheep of St. Kilda (Scotland), red deer of the Isle of Rum (Scotland), and Great tits at Wytham Woods (Oxford, England). (For an excellent perspective paper, see Tim Clutton-Brock’s and Ben Sheldon’s 2010 piece in the journal Science, “The Seven Ages of Pan.”) 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 (Alberta). To this list we can add our research program on the population ecology of the Sable Island horses.

The horses of Sable Island are particularly amenable to individual-based research: each horse is recognizable from photographs, and the lack of trees on the island makes it easy to relocate each animal several times a year. The horses are wild but approachable, as they do not fear humans since we stopped attempting to manage the population in the 1960s (horses are not ‘habituated’ to people but rather tend to ignore humans, which they do not view as a threat nor source of benefit). Our research program is based on a monitoring program in summer that records 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 have life history data for all 720 individuals that have lived on the island since 2008 to now, 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 (see our reference list below, and feel free to contact me for reprints and preprints of papers at We have documented, e.g., high population growth in the feral horses over the past five years (>42% increase in size) and densities that are now higher than has been known on the island historically. So on the one hand this is good news for the Sable Island horse; however, the population will not increase forever and our best data indicate that it is at or has exceeded local carrying capacity in some places on Sable Island. We are expecting a decline for 2013, and the horse population has been noted to ‘crash’ before (e.g., as in the winters of 1964 and 1971 [Welsh 1975]).

Figure 2. From 1900 to 1973 the Sable Island horses have been estimated to number from 120 to 320 individuals (Welsh 1975). Accurate air and ground surveys are available from 1961 to 1973 (also documented in Welsh [1975]). Since commencing our whole-island censuses in 2008, we have recorded an approximately 42% increase in population size from 375 individuals in 2008 to 533 horses observed in 2012. From 2008 to 2011 the number of adults (aged 4+) has ranged from 158 to 209, and only last year did the island support anything close to 250 adults (fully mature individuals).

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]).

Figure 3. By tracking every horse on the island, we can demonstrate how each individual is differently exposed to unique environmental stresses, such as the density of breeding adults that each stallion on Sable Island is exposed to (as shown over an 8000 m radius in 2009, above). There is a west-east gradient in density associated with a gradient in habitat quality (forage and availability of fresh water) on Sable Island, which affects the structure and dynamics of the horse population. See Contasti et al. (2012) for more details.

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 cautiously 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). We measure our horse body sizes using a camera mounted with a set of parallel-lasers (laser pointers) to use as calipers, and software to measure relative distances for comparing morphologies.

Figure 4. This map of horse colouration patterns was created by Ph.D. student Sarah Medill. We keep track of all of the horses on Sable Island by documenting their unique individual features using digital photography.

II. Interactions between the Sable Island horses and other species on Sable Island

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). 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. 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 (see Kenton Lysak’s newly drafted [2013] M.Sc. thesis for an introduction, which is summarized immediately below).

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 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 (r2 = 0.41). 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.

We also then identified individual contributions of seal-, tern- and horse-mediated transfer of marine-derived nutrients inland. 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.

Figure 5. Emily Tissier, M.Sc., conducting vegetation sampling on Sable Island in 2010.

What are the long-term implications of this? 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.

On a related theme, we have also been interested in how disturbance (wind, salt spray, horse browsing) governs plant community assemblages on Sable Island (see the 2011 M.Sc. thesis of Emily Tissier, and complementary research by Freedman et al. [2012]). In 2009 we sampled the composition of vegetation across the island using a stratified random design that captured a range of environmental predictors associated with substrate conditions and disturbance from coastal processes, as well as grazing by the horses. We identified three different vegetation assemblages using hierarchical cluster analysis and non-metric multidimensional scaling that were associated with predictor variables. Distance from shore (both north and south shore) and slope angle were strongly related to both vegetation distribution and community composition. Areas farther from shore (subject to less wind and wave disturbance) contained greater amounts of shrub and heath vegetation. Patterns of vegetation succession inferred for Sable Island were not linear and are better described as responses to repeated environmental disturbance rather than a gradual process of soil development and competitive displacement.

Figure 6. Grazing and disturbance by the horses are likely to have an effect on plant community dynamics on Sable Island, as this photograph of the BIO house exclosure suggests (see also Freedman et al. [2012]). However, the effects may not be as supposed and entirely negative. Recycling of nutrients by the horses and addition of nitrogen to the system from seaward inland by grazing horses (through their feces) may contribute to soil development and successional trajectories that are, at this time, relatively unknown.

III. Population genetics, conservation genetics, and evolutionary genomics of the 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 (in the mid-1700s) 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 very 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 collected hairs and possibly feces to extract DNA). 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 how selection and genes interact to shape evolution in the horses, and how deleterious genes that could related to inbreeding depression may accumulate in the population and pose a conservation risk.

The approaches described above, however, have one major drawback—they cannot identify the actual genes responsible for genetic variation. Therefore, it is impossible to link such evolutionary studies to molecular genetic variation, which greatly restricts our understanding of the processes involved. A major incentive for pursuing new research on the genetics of the Sable Island horses is the availability of a full genome horse sequence (the horse genome was the fifth mammal to be sequenced [in 2009]) as well as a wealth of information on functional variants and quantitative trait loci (QTL) for that species. This is very important as it means that linking molecular, phenotypic and fitness variation will be a realistic feat in our system. For example, as most genes influencing coat colour variation in horses are known, it will be possible to quickly identify variants influencing colour polymorphism in the Sable Island population, and then track evolution in real time by assessing the fitness effect of different alleles and document their temporal dynamics—one of the fundamental but until now inaccessible objectives of evolutionary biology research. Coat colouration is one example of a phenotype that we can track using these methods, but we can also track known ailments that may have a genetic basis such as hoof overgrowth leading to laminitis, and a host of genetic diseases known to horses that may accumulate in the population due to inbreeding.

The prospects of conducting whole-genome analyses to understand the interplay between population dynamics and population genetics will be a major new undertaking for our collaborative research group, which will bring together an international consortium of researchers at the University of Saskatchewan and its vet college, the University of Sheffield, and University of Exeter. We are now pursuing methods, options, and funding opportunities to conduct advanced genetics research in the Sable Island horses to understand the risks posed to the Sable Island horses by the very protections we have placed on them (principally isolation) by the Canada Shipping Act and National Parks Act.

If anyone is interested in contacting our lab for more information, please feel free to email me at

The below is a list of some of the very recent papers and theses that apply to our research (including submitted papers, contact me for drafts and pre-prints):

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. thesis, 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 accepted JWM-12-0377. (in press)

Lucas, Z., McLoughlin, P.D., Coltman, D.W., and Barber, S. 2009. Multi-scale analysis reveals restricted gene flow and a linear gradient in heterozygosity for an 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. University of Saskatchewan, thesis defense will be July 9, 2013.

Marjamäki, P.H., Contasti, A.L., Coulson, T.N., and McLoughlin, P.D. 2013. Scale at which density is assessed interacts with age and sex to determine direction and rate of density-dependent dispersal in a large mammal. Ecology and Evolution MS# ECE-2013-04-0192. (submitted)

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

van Beest, F.M., Uzal, A., Vander Wal, E., Laforge, M.P., Contasti, A.L., Colville, D., and McLoughlin, P.D. 2013. Increasing density leads to generalization in both coarse-grained habitat selection and fine-grained resource selection in a large mammal. Journal of Animal Ecology JAE-2012-00620. (in press)

Other references of interest (some cited above) include:

Beson, K. 1998. Towards a conservation strategy for Sable Island. Environment Canada, Canadian Wildlife Service, Atlantic Region.

Bowen, W.D., McMillan, J., 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.

Catling, P.M., Freeman, B., Lucas, Z. 1984. The vegetation and phytogeography of Sable Island, Nova Scotia. Proceedings of Nova Scotian Institute of Science. 34:181–247.

Catling, P., Z. Lucas, and B. Freedman. 2009. Plants and insects new to Sable Island, Nova Scotia. Canadian Field-Naturalist 123:141–145.

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

Clutton-Brock, T., and Sheldon, B.C. 2010. The seven ages of Pan. Science 327:1207–1208.

Freedman, W., Catling, P.M., and Lucas, Z. 2012. Effects of feral horses on vegetation of Sable Island, Nova Scotia. Canadian Field-Naturalist 125:200–212.

Lucas, Z., Raeside, J.I., and Betteridge, K.J. 1991. Non-invasive assessment of the incidences of pregnancy and pregnancy loss in the feral horses of Sable Island. Journal of Reproduction and Fertility, Supplement 44:479–488.

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., 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.

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.

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

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.

Welsh, D.A. 1975. Population, behavioral and grazing ecology of the horses of Sable Island, Nova Scotia. Ph.D Thesis, Dalhousie University, Canada.