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Linklater, W. L.; Cameron, E. Z.; Stafford, K. J.; Minot, E. O. |
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Estimating Kaimanawa feral horse population size and growth |
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SCIENCE & RESEARCH INTERNAL REPORT 185 |
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Animal flight behaviour in response to aircraft could have a profound influence
on the accuracy and precision of aerial estimates of population size but is rarely
investigated. Using independent observers on the ground and in the air we
recorded the presence and behaviour of 17 groups, including 136 individually
marked horses, during a helicopter count in New Zealand’s Kaimanawa
Mountains. We also compared the helicopter count with ground-based
estimates using mark-resight and line-transect methods in areas ranging from
20.5 to 176 km2. Helicopter counts were from 16% smaller to 54% larger than
ground-based estimates. The helicopter induced a flight response in all horse
groups monitored. During flight, horse groups traveled from 0.1 up to 2.75 km
before leaving the ground observer’s view and temporarily changed in size and
composition. A tenth of the horses were not counted and a quarter counted
twice. A further 23 (17%) may have been counted twice but only two of the
three observers’ records concurred. Thus, the helicopter count over-estimated
the marked sub-population by at least 15% and possibly by up to 32%. The net
over-estimate of the marked sub-population corresponded to the 17% and 13%
difference between helicopter counts and ground-based estimates in the central
study area and for the largest area sampled, respectively. Feral horse flight
behaviour should be considered when designing methods for population
monitoring using aircraft. We identify the characteristics of the helicopter
count that motivated horse flight behaviour. We compared our own recent
estimate of population growth from measures of fecundity and mortality (λ =
1.096 with an earlier-published one (λ = 1.182, where r = 0.167) that had been
derived by interpolating between the available history of single counts. Our
model of population growth, standardised aerial counts, and historical estimates
of annual reproduction suggest that the historical sequence of counts since
1979 probably over-estimated growth because count techniques improved and
greater effort was expended in successive counts. We used line-transect, markresight
and dung density sampling methods for population monitoring and
discuss their advantages and limitations over helicopter counts. |
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refbase @ user @ |
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515 |
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Author |
Linklater, W.L.; Cameron, E.Z.; Minot, E.O.; Stafford, K.J. |
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Title |
Feral horse demography and population growth in the Kaimanawa Ranges, New Zealand |
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Journal Article |
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2004 |
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Wildl. Res. |
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31 |
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2 |
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119-128 |
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Although feral horses are a common management problem in numerous countries, detailed and long-term demographic studies are rare. We measured the age and sex structure, and pregnancy, birth and death rates in a population of 413 feral horses in New Zealand during 1994–98 and used them to construct a model simulating population growth. Survivorship increased with age (0–1 years old = 86.8%, 1–2 = 92.3%, 2–4 = 92.4%, ≥? 4 years old = females 94%, males 97% per annum). Birth sex ratio parity, a slight female bias in the adult sex ratio (92 males per 100 females) and higher adult male survivorship indicated lower average survivorship for young males than females that was not detectable in mortality statistics. Pregnancy and foaling rates for mares ≥? 2 years old averaged 79 and 49%, respectively. Foaling rates increased as mares matured (2–3-year-old mares = 1.9%, 3–4 = 20.0%, 4–5 = 42.1%, ≥? 5 = 61.5% per annum). Young mares had higher rates of foetal and neonatal mortality (95% of pregnancies failed and/or were lost as neonatal foals in 2–3-year-old mares, 70.6% in 3–4, 43.2% in 4–5, and 31% in mares ≥? 5 years old). Population growth was 9.6% per annum (9.5–9.8, 95% CI) without human-induced mortalities (i.e. r = 0.092). Our model, standardised aerial counts, and historical estimates of annual reproduction suggest that the historical sequence of counts since 1979 has overestimated growth by ~50% probably because of improvements in count effort and technique.</p> |
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Equine Behaviour @ team @ |
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3695 |
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