Abstract
In resource-restricted populations of red deer, it is well-established that body size at maturity, female fecundity and calf survivorship are reduced, yet there are few formal studies documenting a reversal of these effects with reduction of density. In this paper, we consider changes in adult bodyweight and fecundity in populations of red deer in upland Scotland, before, during and after substantial reductions in population. Using generalised linear mixed models, we analysed changes in bodyweight and fecundity for 15,401 male and 21,053 female red deer culled from 9 different properties over periods from 9 to 35 years. After controlling for the effects of age, bodyweight in males showed a significant negative relationship with density and a significant positive relationship with the magnitude of reduction in density from that of the previous year as well as from population density recorded 2 and 3 years previously, implying that although main effects may relate to immediate reductions in density from that of the preceding year, increases in male bodyweight may be responding to cumulative reductions in densities as much as 3 years later. Analyses of female bodyweight yielded similar results with a direct and measurable effect of current density on age-related female bodyweight and significant effects of reductions in density in immediately preceding years. In this case, the model of best fit (lowest AIC score) is that incorporating a 3-year time lag, implying that bodyweight may be responding to the effects of cumulative culls over a preceding period of up to 3 years. Pregnancy rates among females were strongly influenced by bodyweight and prevailing (current) density (with females of higher bodyweight more likely to be pregnant), but there was no consistent effect of reduction of density on the probability of pregnancy of young females (aged < 4 years) or older animals (4 years or older). The probability of a lactating female becoming pregnant again in the same year was however significantly higher 1 year after a reduction in population density. While we focus in this paper on red deer, the results are applicable to other mammalian species where bodyweight is a major driver of fecundity. Our analyses suggest that some improvement in fertility and individual animal quality (bodyweight) may be expected where population densities of resource-limited populations are sufficiently reduced.
Highlights
Management of red deer in ScotlandAs elsewhere within Europe, management of red deer (Cervus elaphus) in the Highlands of Scotland offers a delicate balance of costs and benefits (Putman and Watson 2010)
We have investigated the effects of age, density and reduction in population density on: 1) male and female bodyweight 2) pregnancy rates of females aged ≤ 3 years
Bodyweight showed a similar pattern of increase followed by decline, with lower recorded weights at any given age than those recorded for males and with, in this case, maximum weights reached at approximately 8–9 years of age (Fig. 1 b)
Summary
As elsewhere within Europe (see for example Apollonio et al 2010), management of red deer (Cervus elaphus) in the Highlands of Scotland offers a delicate balance of costs and benefits (Putman and Watson 2010). Red deer may be implicated in significant potential damage to commercial forestry (see for example Perez-Espona et al 2009). Where they occur at high densities, there have in addition been increasing concerns expressed about suppression of regeneration of native woodlands and damaging impacts of grazing and browsing on heathland and other open moorland communities (Scottish Natural Heritage/Deer Commission for Scotland 2002; and see for example Reimoser and Putman 2011). In response to these concerns in different areas, some management intervention is often indicated
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