Changes in bodyweight and productivity in resource-restricted populations of red deer (Cervus elaphus) in response to deliberate reductions in density

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.