Impact of Allee effects on the establishment of reintroduction populations of endangered species: The case of the Crested Ibis

Abstract

Allee effects, defined as the positive relationships between component fitness and population density (or size), play an important role in the dynamics of small or lowdensity populations. Reintroduction has become a widely applied tool with the continuous loss of biodiversity. Since reintroduced populations are initially small, Allee effects commonly exist when a species is colonizing new habitat. However, direct evidence of positive density-dependence acting in reintroduced populations is rare. To understand the role of Allee effects in regulating post-release population dynamics of reintroduced species, we analyzed time-series data collected from two spatially isolated populations of the reintroduced Crested Ibis (Nipponia nippon) in Shaanxi Province, China (Ningshan and Qianyang Counties). We examined the potential relationships between population size and (1) survival and reproductive rates, (2) per capita population growth rates for the existence of Allee effects in the reintroduced ibis populations. The results showed that the simultaneous occurrence of component Allee effects in survival and reproduction have been detected, while the reduction of adult survival and per female breeding probability led to a demographic Allee effect in the Qianyang ibis population, which may have contributed to the population decline. In parallel, mate-limitation and predation as the possible initiation mechanisms of Allee effects were presented. Our findings provided the evidence of multiple Allee effects in reintroduced populations and conservation management strategies to eliminate or reduce the strength of Allee effects in future reintroductions of endangered species were proposed, including release of large number of individuals, food supplementation, and predator control.