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

Abstract

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.

Environmental and demographic drivers of male mating success vary across sequential reproductive episodes in a polygynous breeder

“…Each record consisted of the date of observation, geographic coordinates, sex, age, reproductive status, and group affiliation. High‐resolution aerial photography from January 2010 was used to confirm that population surveys accounted for >99% of the horses found on the island (Richard et al, ). Although feral, horses were neither fearful of humans nor accustomed to rewards, allowing us to approach each individual and record data without disturbing or altering behaviors.…”

“…We expected density effects on male mating inequality in both selection episodes, as density has been linked to ASRs in this system (Manning et al, ). We predicted that limited freshwater availability would increase male mating inequality in the second episode because increased water requirements during reproduction, pregnancy, and lactation would heighten female mate choice under these conditions (Contasti, Tissier, Johnstone, & McLoughlin, ; Richard, Simpson, Medill, & McLoughlin, ). Lastly, in line with the EPP hypothesis, we predicted that a freshwater availability effect on male mating inequality would vary among years, as the annual amount of rainfall that recharges the island's water table and influences freshwater availability has been shown to underlie spatially dynamic polygyny thresholds in this system (Manning & McLoughlin, ).…”

Density‐dependent resource selection by a terrestrial herbivore in response to sea‐to‐land nutrient transfer by seals

“…The island supports 3,300-4,200 and 2,400-3,960 breeding pairs of gulls and terns, respectively (Toms et al 2006). Horses are identified by their unique markings using digital photography, with markresight detection probability for years 2008-2013 averaging 0.992 (sampling protocols in Contasti et al 2012, van Beest et al 2014, and Richard et al 2014. Transfer of nutrients occurs primarily through either defecation or decay of carcasses from dead seals.…”

Island tameness and the repeatability of flight initiation distance in a large herbivore

“…The population, which has been unmanaged and protected from human interference since 1961 (Christie 1995), has been the subject of a long-term, individual-based monitoring program since 2007 (see, e.g., Contasti et al 2012;Marjamäki et al 2013;van Beest et al 2014). Each year between July and September, all individuals are censused and most are measured for a variety of phenotypic traits including dispersal (Debeffe et al 2015a), morphology (Weisgerber et al 2015), parasite burden (Debeffe et al 2016), and life history (Richard et al 2014). The relatively small size of the island, combined with an absence of trees, makes locating individuals and obtaining repeated FID measurements rather easy.…”