The Effect Of The Arley Effect Based On Pairing Restriction On The Reintroduction Of Nipponia Nippon Population In Shaanxi Province

The Crested Ibis,one of the rare Ciconiiformes species,once was in the brink of extinction.It was listed as the national first-grade protected animal and endangered species in China Species Red List.The first reintroduction project of the species was implemented in Ningshan County,Shaanxi in May 2007.Since the release of 56 captive-reared individuals,extensive post-release monitoring has been conducted for 10 years.Thirty breeding pairs have formed and 112 yearlings fledged successfully by the end of 2016.Although a self-sustaining population has formed,researchers found that the population grew slowly with the negative growth rate in some specific years.The population size has been in a low level in recent years.As a result,we speculate that Allee effect may be occur in the reintroduced population.Banding,radio-telemetry and field motoring were used to research Allee effects relevant to mate limitation in our focused population.The study birds include 56 released individuals and their offspring produced in the wild from 2007 to 2016.The results are summarized as below:From 2007 to 2016,the population size increased from 12 to 75.The average population growth rate was 1.108±0.408(range 0 to 1.400,n=10).Population growth rate which sightly fluctuated among years(hump-shaped curve)was not significantly related to population size(Pearson r=0.286,P>0.05,n=10).As a result,it might be possible that demographic Allee effect occur in the population.The number of mated females and males increased from 3 to 20 and 9 to 34 in a 10-year period(2007-2016);The annual average sex ratio was 0.688±0.193(ranged 0.333 to 1.0,n=10).The unbalanced sex ratio shows that it is more difficult for a sexually mature female/male to find a potential mate,indicating there are mate limitations in the population.The number of females and males were both significantly positive related to population size(Pearson r=0.984,P<0.01;Pearson r=0.933,P<0.01,n=10),indicating that the number of mated females increased with population size and mated females fluctuated with the increase of population size;The sex ratios was also positive related to population size(Pearson r=0.363,P>0.05,n=10),but it fluctuated with the increase of population size.The relationship between the number of females and males,sex ratios and population size reflect the occurrence of mate-based Allee effect.The number of both sexes and sex ratios may cause the fluctuations of population size,which may affect the relationship between population size and the population growth rate.Therefore,sex ratios could be one of the reasons for the demographic Allee effect.The number of breeding pairs increased from 0 of 2007 to 11 of 2016.The annual average mating rate was only 24.63%,which indicated that there may be mate limitation resulting from low availability of potential mates in the population.The number of breeding pairs increasing with population size with a slight fluctuation among years was significantly positive related to population size(Pearson r=0.900,P<0.01,n=10).The relationship between the number of breeding pairs and population size may be contributable to the mate-based Allee effect.The number of breeding pairs may affect the relationship between population size and the population growth rate.Therefore,the number of breeding pairs could be one of the reasons for the demographic Allee effect.From 2007 to 2016,the low annual female mating rate averaged at 44.02%±22.14%(ranged 0 to 83.33%,n=10)may reflect the occurrence of mate limitation in the population.The weak negative relationship as well as the fluctuation between female mating rate and population size(Pearson r=-0.191,P>0.05,n=10)may affect the relationship between population size and the population growth rate and be one of the contributors to the demographic Allee effect.From 2007 to 2016,the number of yearlings,juveniles and adult averaged at 6.80±4.37(ranged 0 to 16,n=10),5.20±3.46(ranged 0 to 10,n=10)and 28.90±16.60(ranged 12 to 54,n=10),respectively.The percentage of the there age group was account for 16.60%,12.70%,and 70.70%,respectively.The lowest percentage of the juveniles means the lower number of potential mate,indicating the mate limitation in the population.The significant positive relationships as well as the fluctuations between the number of the three age groups and population size(Pearson r=0.713,P<0.05;Pearson r=0.687,P<0.05;Pearson r=0.971,P<0.01,n=10)may be attributable to the fluctuation of the population,which may affect the relationship between population size and the population growth rate.Therefore,age structure could be one of the reasons for the demographic Allee effect.The mate-based Allee effects resulting from mate limitation,that is,weak component Allee effects can lead to the mate-finding difficulty,lower mating rate and lower productivity.Mate limitation may cause the weak demographic Allee effect,which may lead to the decrease of population growth rate.If the weak demographic Allee effect converts to strong Allee effect,the population size will decrease.The extinction may be occur when the population size was lower than Allee threshold.The factors resulting in mate limitation Allee effects were analyzed in the present study.The management suggestions and minimum strategies of Allee effect were also proposed.