Poor horse traders: large mammals trade survival for reproduction during the process of feralization

Grange, Sophie; Duncan, Patrick; Gaillard, Jean‐Michel

doi:10.1098/rspb.2008.1828

Cited by 37 publications

(88 citation statements)

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“…They concluded that feral ungulates including horses, cattle, and sheep respond differently from wild ungulates to increases in density by trading adult survival for reproduction. Our data do not agree with this, as no 2‐year‐olds ever produced foals, and we observed adult foaling rates (note that ours are for female production only, multiply by 2 if assuming a 50:50 sex ratio) at the low end of the range identified by Grange et al (). Unlike many populations of feral horses, we think that the Sable Island horse has experienced a remarkably long period of feralization (e.g., 250 years), which may have allowed for more time (approx.…”

Section: Discussioncontrasting

confidence: 85%

Identifying hidden sinks in growing populations from individual fates and movements: The feral horses of Sable Island

et al. 2013

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Identifying the existence of population sinks is critical for conservation and management. However, because of density‐dependent dispersal, sinks can sometimes be masked by immigration events, especially during phases of population growth. We present a large‐scale, empirical demonstration of within‐population source‐sink dynamics using the feral horses (Equus ferus caballus) of Sable Island National Park Reserve, Nova Scotia, Canada, as a model. We tracked the fates and movements of 98.7% of the female population (n = 190–237) across 3 demographic clusters (subunits) during a period of rapid population growth (2008–2010; 24.7% increase in density). All subunits experienced increases in population size each year (λ > 1.0). Our individual‐based analysis showed that western Sable Island, where water availability was greatest, behaved as a source and would have grown with or without immigration in all years. However, the central (and fastest growing subunit) would have declined from 2008–2009 (λ = 0.951) without immigration. Further, the eastern subunit would have declined in 2 intervals (λ = 0.932, 0.999) without immigration. Our study demonstrates that the propensity of habitat to act as a sink can be masked during a period of population growth because of density‐dependent immigration from adjacent habitats. These findings present a caution to managers charged with conserving wide‐ranging species with long population cycles for which effects of immigration on local population growth rate can be difficult to isolate using standard methods of enumeration. © 2013 The Wildlife Society.

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Section: Discussioncontrasting

confidence: 85%

Identifying hidden sinks in growing populations from individual fates and movements: The feral horses of Sable Island

et al. 2013

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“…The highest rates of population increase have been reported where food was abundant and disease and predation were minimal (Eberhardt et al 1982;Garrott et al 1991;Duncan 1992;Grange et al 2009). …”

Section: Discussionmentioning

confidence: 99%

“…Despite these differences in definition, usually 50 -66% of an unmanaged feral horse population are adults (Welsh 1973;Feist and McCullough 1976;Garrott and Taylor 1990;. A preponderance of adults in long-lived large herbivore populations has been attributed to moderate to high fecundity and high survival (Gaillard et al 2000a;Grange et al 2009). As the fecundity of the TTSF population was low (23%) and survival was high (95%), it can be concluded that long term survival was most likely responsible for the preponderance of adult horses in the studied population.…”

Section: Discussionmentioning

confidence: 99%

“…In some studies, birth dates were available, which obviously provided an exact age and, as there is a close correlation between dental characteristics and age, this allowed observers (where the mouth could be examined) to determine with considerable accuracy the animal's age Garrott 1991b;Grange et al 2009). Those studies that visually estimated age based on size and behaviour were limited to broad age classifications such as foals, yearlings, adults, and occasionally subadults (Feh et al 2001;Scorolli 2007).…”

Section: Social Organisation Of Feral Horsesmentioning

confidence: 99%

“…Seasonal breeders adjust their reproductive activity to the most favourable conditions thereby maximising survival of their offspring (Gaillard et al 2000a;Grange et al 2009). …”

Section: Introductionmentioning

confidence: 99%

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Understanding population dynamics of feral horses in the Tuan and Toolara State Forest for successful long-term population management

Zabek¹

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Feral horses (Equus caballus) in Australia are a growing problem despite implementation of management strategies. The increasing number of feral horses within the Tuan and Toolara State Forest (TTSF), a coniferous plantation on the Sunshine Coast, Queensland, and particularly near major public roads, has been recognised as a problem in the last decade. Desensitised to road traffic, horses are a serious distraction for motorists, and have potential to cause serious animal-vehicle accidents.The primary objective of the study was to characterise the essential demographics of the TTSF feral horses, so that the most suitable methods for long-term population management could be determined. The study was conducted from 2012 -2014 and determined social structure, reproductive performance and survival, and population growth rate. The estimation of population distribution, abundance and density together with assessment of habitat use and home range size were also addressed.The studied population did not show significant annual variation in demographics, which is consistent with measurements in other unmanaged feral horse populations in Australia and abroad. The overall size and age composition of social groups remained stable during the study period. The average harem size was estimated as 4.95 (95%CI 4.53-5.41), and usually consisted of 1 stallion, 2 -3 adult females, and 2 immature offspring (≤ 3 years of age). Males not associated with harems consisted of adult (≥ 3 years old) and sub-adult (2 years old) individuals, either forming groups of 2 -6 or living by themselves. The population showed a stable age distribution, with adult horses constituting the largest group (68%; n = 247). The sex ratio of adult female to male horses was nearly equal (0.99:1.00).The mean annual fecundity was 0.23 ± 0.07 SD and was comparable to those obtained for other feral horse populations where the environment imposed nutritional limitations. Adult females were observed to foal on average every second year. The overall nutritional status of the population expressed as body condition score (BCS; 0 -5) was 2.55 ± 0.51 SD with adult females having poorer scores than other age and gender groups. Survival estimates were consistently high (0.92 -0.95) across all age groups. The average annual finite rate of population increase (λ) for the three years of the study was 1.088 which was lower than the maximum reported for populations living in the most favourable conditions. Elasticity iii analysis demonstrated that the TTSF population growth rate was almost seven times more sensitive to changes in adult survival compared with juvenile survival, and almost twice more sensitive than changes in fecundity.Assessment of the population distribution based on a strip transect survey using horse dung on forest tracks indicated that the majority of the forestry was occupied by horses, with the highest population density being located in the central region of the plantation.Abundance and density of horses were estimated by distance line transect surv...

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Interacting effects of age, density, and weather on survival and current reproduction for a large mammal

Richard

Simpson

Medill

et al. 2014

Ecology and Evolution

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Individual-based study of natural populations allows for accurate and precise estimation of fitness components and the extent to which they might vary with ecological conditions. By tracking the fates of all 701 horses known to have lived on Sable Island, Canada, from 2009 to 2013 (where there is no predation, human interference, or interspecific competition for food), we present a detailed analysis of structured population dynamics with focus on interacting effects of intraspecific competition and weather on reproduction and survival. Annual survival of adult females (0.866 ± 0.107 [ ± SE]) was lower than that of 3-year-olds (0.955 ± 0.051), although annual fecundity (producing a foal in a year that was observed during our census) was higher in adults (0.616 ± 0.023) compared to 3-year-olds (0.402 ± 0.054). Milder winters and lower densities during gestation increased fecundity. Density negatively impacted survival for all age and sex categories; however, highest adult female survival was observed during high-density years coupled with a harsh winter, the result expected if pregnancy loss during winter or loss of foals in spring improved survival. Three-year-old females, which reproduced at lower rates, experienced higher survival than adults. Our results contrast with a previous study of feral horses that suggested recently feral ungulates might be artificially selected to reproduce even when costs to survival are high. In part, this may be because of the comparably long history of feralization (250 years; at least 25 generations) for Sable Island horses.

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Poor horse traders: large mammals trade survival for reproduction during the process of feralization

Cited by 37 publications

References 36 publications

Identifying hidden sinks in growing populations from individual fates and movements: The feral horses of Sable Island

Identifying hidden sinks in growing populations from individual fates and movements: The feral horses of Sable Island

Understanding population dynamics of feral horses in the Tuan and Toolara State Forest for successful long-term population management

Interacting effects of age, density, and weather on survival and current reproduction for a large mammal

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