Density‐dependent population growth in a reintroduced population of North Island saddlebacks

Abstract

Summary 1.Reintroductions provide a good opportunity to study density-dependent population growth, as populations can be studied at a range of densities and the change in density is not confounded with environmental conditions. An understanding of density dependence is also necessary to predict dynamics of reintroduced populations under different management regimens, and assess the extent to which they can be harvested for further reintroductions. 2. We monitored a North Island saddleback ( Philesturnus rufusater ) population for 6 years after reintroduction to Mokoia, a 135 ha island in New Zealand that was made suitable for saddlebacks by eradicating introduced Norway rats ( Rattus norvegicus ). We modelled adult and juvenile survival using Program  , and modelled numbers of young fledged per pair using Proc Mixed in SAS with individual female as a random factor. 3. Juvenile survival clearly declined as the population increased, and the decline was closely correlated with the number of breeding pairs. Reproduction also showed a clear decline that was explained by two factors: a difference in quality between territories occupied immediately after reintroduction and those occupied later, and an overall decline as the number of pairs increased. Reproduction was also strongly affected by age, and this needed to be accounted for when modelling density dependence. 4. A stochastic simulation model incorporating these dynamics closely predicted the observed population growth. The equilibrium population size was insensitive to density dependence in reproduction, but highly sensitive to density dependence in juvenile survival. 5. The model is being used to plan management strategies for potential reintroductions of saddlebacks to mainland areas with predator control. The species is currently confined to predator-free islands and one fenced mainland sanctuary.

Successful island reintroductions of New Zealand robins and saddlebacks with small numbers of founders

“…Saddleback and robin populations can still grow even when inbreeding depression in the form of egg failure rates is severe. This is partly the consequence of relatively high survival and reproduction rates associated with low population densities in newly established populations (Armstrong et al, , 2005. Therefore any negative effects of inbreeding depression on fitness traits such as egg fertility and hatchability can still be overcome through rapid population growth, although the greater the inbreeding depression the longer it takes to reach carrying capacity and the slower the recovery from any subsequent population catastrophes.…”

“…Furthermore, relatively small numbers of released birds do not appear to prevent population growth recovery to carrying capacity as long .5 percentile (upper broken line) and 2.5 percentile (lower broken line) from 500 simulations incorporating demographic stochasticity in survival, reproduction and sex of offspring. Parameters were estimated from 6 years of data collected on Mokoia and they incorporate density dependence in reproduction and juvenile survival (Armstrong et al, 2005). (a) Incorporates the observed egg failure rate (22%), while (b) and (c) show the projected effects of hypothetical increases in that rate.…”

“…We also assessed extinction probability using stochastic matrix models based on and Armstrong et al's (2002) analysis of the reintroduced robins on Tiritiri Matangi Island and Armstrong et al's (2005) analysis of the reintroduced saddleback population on Mokoia Island. Both models included demographic stochasticity in births, deaths and sex ratio of offspring, with births sampled from the Poisson distribution and deaths and sex sampled from the binomial distribution.…”

Evolutionary conservation advice for despotic populations: habitat heterogeneity favours conflict and reduces productivity in Seychelles magpie robins

“…However, we argue that these should be viewed as two ends of a single continuum. Indeed, studies on saddlebacks Philesturnus rufusater in New Zealand (Armstrong et al 2005) and bearded vultures Gypaetus barbatus in Spain (Carrete et al 2006) show patterns associated with both processes. Our study adds to the evidence for such a continuum by providing a mechanistic understanding of the processes involved.…”

Non-breeding season habitat quality mediates the strength of density-dependence for a migratory bird

“…Much of this work has investigated the relationship between density and reproductive success [8,17]. Studies designed to examine density-dependent effects on adult or juvenile survival have typically been conducted with relatively sedentary, small or recovering populations [18][19][20]. Investigations of migratory birds have considered the effects of density-dependent annual survival in large-bodied species with relatively slow life histories, such as great cormorant (Phalacrocorax carbo) [21] and black-tailed godwit (Limosa limosa) [22].…”