Effects Of Habitat And Landscape Changes On The Dynamics And Genetic Structure Of Amphibians During Urbanization

Globally,amphibians have suffered massive,widespread,often unexplained,and probably irreversible declines over the last several decades.An estimated half of the world's amphibian species are currently experiencing population declines,far more than birds or mammals.Habitat loss,fragmentation and degradation,which often result from urbanization,currently impact 88%of threatened amphibians,and are therefore among the greatest threats to amphibian populations.The rising human population is driving the expansion of urban areas and increasing infrastructure development,resulting the threat of urbanization to amphibians continues to increase.Amphibians often comprise a significant proportion of the vertebrate biomass in forest and wetland ecosystems,as well as being important carnivores and prey species.Despite their importance to ecosystem function and the widespread declines observed for many species,however,amphibians are among the least studied taxonomic groups in urban and suburban areas.The ponds investigated in this study were located in Shanghai,a region experiencing intensive urbanization.We assessed the breeding distribution of amphibians along an urban-rural gradient,and examined how local-landscape configuration and microhabitat variables interact in influencing amphibian community composition.We specifically investigated how the abundance and species richness of anurans in ponds was related to the age of ponds.Using generalized linear models to quantify independent effects,we examined the relationship between landscape structure variables and amphibian communities at multi-scale.In this paper,we examined the genetic structure of Pelophylax plancyi using mitochondrial cytochrome b gene and compared the genetic differences between different populations in a metropolitan area.Our results showed that:1)Overall,anuran abundance and species richness in ponds steadily declined with increasing urban density.Relative abundance of each study species exhibited a negative relationship with the index of urban density,and the effect of urbanization for each species was significant(P.nigromaculatus,R2 = 0.246,P = 0.002;P.plancyi,R2= 0.247,P = 0.007;F.multistriata,R2= 0.437,P = 0.000;B.gargarizans,R2= 0.165,P = 0.031)except for M.fissipes(R2= 0.131,P = 0.059).There was a significant negative relationship between urbanization and total relative abundance(R2= 0.523,P = 0.000).Urban density also demonstrated a significant negative relationship with species richness(R2= 0.321,P = 0.001).Our results showed that species was differentially affected by changes in land habitats,but all species exhibited a decline with increasing urbanization.The regression modelling provided strong support for an important positive effect of pond age on abundance of P.nigromaculatus,F.multistriata,M.fissipes,and total anuran abundance.The proportion of aquatic vegetation in a pond appeared to havea relatively small positive effect on total anuran abundance,and a moderately strong effect on B.gargarizans and P.plancyi.Based on our results,urban density was the most influential landscape feature for predicting the presence of B.gargarizans,total anuran abundance,and anuran richness.The larger scale landscape variables surrounding breeding habitat such as farmland,forests and wetlands also affected amphibian communities,reflecting differences in ecological requirements.Our regression models also explained the "dispersal-dependent-declining" hypothesis,mobile species are more susceptible to habitat fragmentation;non-connectivity;and frag-mentation of populations by roads,concrete levees,and urban land;while sedentary species were threatened more seriously by habitat loss.Amphibian species regarded to have the best dispersal ability may be the ones most sensitive to habitat fragmentation.2)Our results confirmed that the distribution of amphibians was affected by landscape configuration variables of different spatial scales.According to our regression models,amphibians were usually more sensitive to landscape configuration metric variables over a larger geographic scale in Shanghai.And most of models assessing the relationship between anuran community and landscape configuration had good support at 1500m and 2000m radii around breeding site.Different species,within the same community,responded in different ways to the same landscape heterogeneity,which dominated by different trait attributes.The models predicting P.nigromaculatus included patch richness(PR)significantly increased at 1500m and 2000 m scale.In the best F.multistriata model,patch area distribution(Area_MN)and patch richness(PR),at 1500m scale were incorporated as explanatory variables.The abundance of F.multistriata significantly increased with patch area distribution(Area_MN)and patch richness(PR)at 1500 m scale.While F.multistriata were positively correlated with patch richness(PR)and connectance index(CONNECT)and strongly negatively correlated with largest patch index(LPI)at 2000 m scale.All models that predicted abundance of M.fissipes had weak relative fit at four different spatial scales.The models explaining variation in B.gargarizans included the patch richness(PR)at four different spatial scales,and only had good fit at 1500 m and 2000 m scales.The model selection results of(P.plancyi)revealed that we identified the model included Shannon's evenness index(SHEI)at 500 m scale had good support,abundance was strongly positively correlated to Shannon's evenness index(SHEI).And the model included largest patch index(LPI)had good fit at 2000 m scale,strongly negatively correlated with P.plancyi.The best model explaining variation in total anuran abundance included largest patch index(LPI)at 1000 m scale.The best model predicting species richness included interspersion and juxtaposition index(IJI)at the range of 1000 m radii,and was significantly negatively related to interspersion and juxtaposition index(IJI)at different scales.Our results suggested that configurational heterogeneity acts a strong environmental filter,creating different amphibian communities dominated by different trait attributes.Configurational landscape heterogeneity strongly affected traits reflecting dispersal capacity,and led to different amphibian communities.3)In this study,10 population were sampled across the range of P.plancyi in Shanghai.In total,717bp of the mitochondrial Cyt b gene were sequenced in 240 individuals,and 69 haplotypes were identified.The haplotype diversity(Hd)were 0.856 ± 0.021,the nucleotide diversity(Pi)were 0.0043 ± 0.0003,the average nucleotide difference(k)were 2.948.The Changxing(CX)population exhibited the highest nucleotide and haplotype diversity,and Nanhui(NH)was the lowest.There was a strong correlation between the nucleotide and haplotype diversity among different populations.Urban populations showed lower nucleotide and haplotype diversity than rural populations and island populations.We found small to moderate(Max FST=0.296;Min FST=0.004)population differentiation in Shanghai.Relatively low values of FST reflect the limited differentiation of these populations at the small geographic scale.FST values among urban,rural and island populations revealed significant genetic differentiation.Relatively low genetic differentiation was observed between urban populations near the Huangpu River,and no significant differentiation was found between urban populations.The value of Nm suggested that gene exchange between various urban populations made genetic differentiation relatively low among populations.Urbanization affects genetic differentiation of P.plancyi through landscape structure,habitat loss and fragmentation,and urban areas and busy highways represent barriers to gene flow.But water corridors may facilitate dispersal and gene flow between urban populations.The 360-m-wide Huangpu river with its strong current is definitely but not a suitable habitat for P.plancyi dispersal,but passive transport during floods may explain the low genetic differentiation of breeding sites lying near the river.Our results suggested Huangpu river may act as a natural corridor for P.plancyi dispersal.The AMOVA revealed that 88.35%of the molecular variance is explained by within-population variation.The ?CT value also indicated that it is a possibility divided P.plancyi of Shanghai into three groups.Analyses on phylogeny showed that the haplotypes of P.plancyi samples did not form a monophyletic or more which geographically correspond to urban,rural and island populations.We did not found that there are different genetic lineages which corresponding to different urban areas.