| Biological invasion severely threatens the global biodiversity,and negatively influences the existence of native species and original ecosystem.The smooth cordgrass(Spartina alterniflora)native to North America has been introduced to improve the coastal ecological environment all over the world.However,this rapidly growing invasive plant tends to replace native vegetation and alters the structure and function of the original ecosystem,greatly influencing the native biological groups including birds.Since in the 1970 s,the rapidly increased S.alterniflora has been brought into China,especially in the eastern coastal wetlands of China,and become the dominant vegetation communities in several natural reserves,leading to the degradation of native bird habitats and the decline of community diversity.Considering that the passerine birds harboring the most extensive radiation distribution have strong behavioral adaptability and can quickly occupy diverse habitat types,after experiencing long-termed coexistence of the invasive S.alterniflora,whether they could use the invaded habitats to reproduce and what behavioral response strategies they adopted are still to be unknown.In this study,we investigated the native passerine birds in Yancheng coastal wetland during 2018 and 2019 breeding seasons.Through comparing the avian distribution and population dynamics,courtship singing behaviors,nest site selection and nest survival rates,nest predator groups and nest predation rates between the native and invasive vegetation habitats,we explored the utilizing tactics and adaptive behavior responses of native passerine birds to invasive S.alterniflora habitat during the breeding season.Main reults were as follows:1)Many native passerine birds could use invasive S.alterniflora habitats during the breeding season,but there were significant differences in avian population and nest distribution between native and invasive habitats.To assess the utilization of invasive habitats by native passerine birds,we compared the avian abundance and diversity in distinct habitats separately created by native Phragmites australis and Imperata cylindrica,invasive S.alterniflora,and mixed PhragmitesSpartina.Of 10 recorded bird species,nine and eight were separately found in native and invaded habitats,and seven were found in both habitats with three(Paradoxornis webbianus,Cisticola juncidis and Prinia inornata)having the entire nesting and breeding behaviors and the other four having courtship singing behavior.For native habitats,the population density of passerine birds was significantly higher than that in invaded and mixed habitats,and the community diversity was higher than that in mixed habitats,but had no significant difference from that in invaded habitats.Moreover,the nest density in native habitats was significantly higher than that in invaded and mixed habitats,and the aggregation of nest sites gradually decreased from native to invaded habitats.We found that P.inornata only utilize invaded habitats as nesting grounds in the early breeding season,while P.webbianus utilize both habitats throughout the breeding season,suggesting that various birds have different utilization degrees of invaded habitats.We speculated that the loss of native habitat area caused by the invasion might exacerbate the competition for resources,and force the uncompetitive species to choose suboptimal habitats.2)The courtship songs of male P.inornatas had significant differences between native and invasive habitats.We analyzed the courtship songs of 44 male P.inornatas,including 19 in native habitats and 25 in invaded habitats,and found that P.inornata in native habitats sang with a longer song length,longer syllable length,higher syllable number,higher minimum frequency and lower frequency bandwidth than those in invaded habitats.Meanwhile,P.inornata in invaded habitats had lower diversity of songs and unitary song types.The predominance of monotonous song type in invaded habitats might explain the main song differences between two habitat types.We suggested that the short soft stems and strong wind in invaded habitats are difficult for males to maintain standing as well as singing for a long time,and extra energy consumption might lead to a shorter song of birds in invaded habitats.Furthermore,many males in invaded habitats possess much complex courtship display,such as dancing while singing,which might be a behavioral compensation mechanism that increase exposure to females in response to the single dense vegetation structure.In sum,the invasive S.alterniflora might have driven the short-term adaptive evolution of native avian courtship song.3)The native P.webbianus could successfully reproduce in invaded habitats and raise nest height to avoid flooding.We compared parameters of nest-site selection and nest survival of P.webbianus between native(37 nests)and invaded(16 nests)habitats.Results showed that except the lower nest height and attached vegetation height in native habitats,nest characteristics had no significant difference between two habitats,such as a shared preference to the dense bushes or grass near water for nesting.Similarly,the clutch size,egg color type,hatching period and nestling period of P.webbianus had no significant difference between two habitats.We found that the nest failure in native habitats was probably caused by nest predation,wheras in invaded habitats the flowing tide became the major threat.The nest daily survival rate of native habitats was significantly lower than that of invaded habitats.Allowing for the nest height and distance to water being the optimal explanatory variables for nest survival model and the higher mean nest height in invaded habitats,we believed that although tidal inundation in invaded habitats serves as a potential “ecological trap”,P.webbianus could successfully adapt to the new breeding habitats by raising nesting height.4)There were significant differences in nest predator groups and nest predation rates of passerine birds between native and invaded habitats.By monitoring 61 natural nests and 304 artificial nests,we identified 12 species of nest predators in both habitats,including mammals,birds,snakes,crabs and ants.Compared with the main nesting predators(snakes and rodents)in native habitats,rodents,birds and crabs were major predators in invaded habitats.Meanwhile,the predation rate of natural nests in native habitats was significantly higher than that in invaded habitats,but the difference of artificial nest experiment was not significant.For natural nests,the daily predation rate in native habitats was closely related to the nest height,while in invaded habitats it correlated to the nest height and the distance to water.For artificial nests,the daily predation rate in native habitats was closely related to the density of vegetation and the distance to road,while in invaded habitats it correlated to the distance to road and the experimental date.We suggested that the absence of snakes and lower nest predation rates in invaded habitats may contribute to the utilization of S.alterniflora habitats by native birds.In conclusion,this study suggests that alonging with the long-term S.alterniflora invasion to the wetlands of China's east coast,the native passerine birds are trying to utilize and gradually adapt to the invaded habitats,and some native birds can complete reproduction in invaded habitats and achieve better reproductive benefits.However,altered habitats also accompanied by new reproductive risk,which brought a series of behavioral responses and adaptive evolution of native passerine birds,such as songs,nesting,and anti-predation behavior.In future,during the management of the invasive S.alterniflora,its impact on native species that have adapted to invaded habitats should be considered. |
PDF Full Text Request