The Effects Of Urbanization On Niche Dynamics And Stress Responses In Birds

In the Anthropocene,human-made habitat transformation is increasingly evident around the world.Such environmental change is usually associated with a considerable reduction in species richness and in functional diversity across several taxa,from which the case of avian species is particularly well-known.Niche was commonly defined as n-dimensional hypervolumes of environmental factors and resources,and it can be divided into the fundamental and the realized niche.In human-made habitats completely novel niches are available,so that species' low niche specificity may drive drastic changes in niche characteristics.Still,it is unclear how this process influences niche dynamics,and how individuals and species cope with the different habitats with considerable anthropogenic disturbances.To answer these questions,in this thesis,we compare stress response,isotope niche,individual specialization,morphological variation of songbirds from natural to urban habitats.We did this in Southwest China,an area is experiencing one of the highest urbanization rates in the world and has very high biodiversity1.Breath rate showed no differences among habitats,but it can be diet-mediatedThe pace-of-life hypothesis predicts no impact of urbanization on stress responses Accordingly,several studies have been inconsistent in showing differences in breath rate(BR),a proxy of acute stress responses to handling in passerines,between rural and urban areas.However,this evidence is limited to a single bird species and a limited geographic region(SW Europe).In Chapter 2,we tested these hypotheses by comparing breath rates of species in eight highly diverse passerine assemblages in natural,rural,and urban habitats in SW China.BR was a moderately repeatable trait,which showed a negative relationship with body mass and a positive relationship with the time of the day.There were no differences in BR among natural,rural,and urban locations.Similarly,within species,there were no differences in BR between rural and urban locations.However,herbivores showed higher BR than omnivores.Our results provide support to the pace-of-life hypothesis,which suggests that species are generally well adapted to the places where they are found.The fact that acute stress responses can be diet-mediated may help to explain the marked decline of specialized trophic guilds around the world in response to anthropogenic disturbance2.Isotope niches of passerines showed niche expansion from natural to human-made habitatsThe niche overlap hypothesis predicts an increase in niche width and overlap would be expected from natural to human-made habitats as a consequence of species' low resource selectivity and increasing between-species similarity.On the other hand,the niche opportunity hypothesis predicts that species colonizing a new habitat might use empty niches rather than engaging in strong competition with other species.If assemblages are similarly diverse and species niches are added incrementally,but do not experience increased niche overlap,there should be niche expansion from natural to human-made habitats.In Chapter 3,using a stable isotopes approach,we tested these predictions in eight highly diverse passerine assemblages showing similar species richness in natural,rural and urban habitats in SW China.Species' niche width drastically increased from natural to human-made habitats,while there were no differences in niche overlap among habitats.Additionally,species richness was inversely correlated to assemblages' niche width and species' niche overlap.Taken together,these results suggest that increasingly generalist species thriving in human-made habitats made use of broad empty or available niches,but also that interspecific competition was the main factor constraining species' niche expansion3.Divergent relationships between niche width and individual specialization in natural and human-made habitatsThe niche variation hypothesis predicts that population niche width is positively correlated to individual specialization.Urban and rural areas are heterogeneous habitats,so that having a high level of individual specialization likely benefits fitness.Yet,at the same time,species in human-made habitats are generalist species with low resource selectivity Thus,it is unclear if broader species' niches,characteristic of species in human-made habitats,are achieved by means of increased resource specialization or generalization at the individual level.In Chapter four,we answered this question using a stable isotopes approach,by computing niche width and individual specialization in 108 individuals of five common passerine species from natural to human-made habitats.We confirmed that species had broader niche width in human-made than in natural habitats,but they showed similar levels of individual specialization across this gradient.Overall,we also recorded a weak relationship between niche width and individual specialization.However,we found divergent relationships between niche width and individual specialization in natural and human-made habitats.Niche width and individual specialization were positively correlated in human-made habitats,but not in natural ones.This means that in natural areas broad population niches were achieved by increasingly generalist individuals,while in human-made habitats broad population niches were achieved by increasingly specialist individuals.Therefore,our results confirm that habitat transformation plays an important role in shaping individual specialization.4.Urban species are ecological generalists and morphological specialistsThe niche variation hypothesis predicts that species with broader niches may also have more morphological variation,whereas Liem's paradox predict that ecological generalists may be morphological specialists.However,it is unclear if species with broader niches,in human-made habitats,are morphological generalists or morphological specialists.In Chapter five,we test these hypotheses,by computing isotope niches and individual morphological variation of passerines from natural to urban habitats in SW China.Our results showed that,species in urban habitats showed broader niches but smallest morphological variation,than species in natural,and rural habitats.We also recorded a week positive relationship between niche width and morphological variation.However,the significant relationship between morphological variation and niche width is only recorded in rural,but not natural,and urban habitats.Thus,our results suggest that,urban species are ecological generalists but also morphological specialists,and that habitat transformation plays an important role in shaping morphological variationOverall,the results of this thesis demonstrate that species seems well adapted to the habitats where they are found and habitat transformation dramatically altered niche dimensions.This thesis has begun to disentangle the complex differences in niche width,niche overlap,individual specialization,and morphological variation among habitats.The results of this thesis suggested niche expansion of species in novel habitats seems mostly driven by the differences in intra-and interspecific competition among habitats.Moreover,the morphological specialists in urban habitats may be able to use alternative recourses.Future research may focus on examining the relationships between isotopic,functional and phylogenetic diversity at community level,the main drivers of individual specialization,and the experimental evidences of effects of intra-and interspecific competition on niches among habitats.