Effects of Sexual Dimorphism and Landscape Composition on the Trophic Behavior of Greater Prairie-Chicken

Partitioning of ecological niche is expected in lekking species that show marked sexual size dimorphism as a consequence of sex-specific ecological constraints. However, niche partitioning is uncertain in species with moderate sexual dimorphism. In addition, the ecological niche of a species may also be affected by landscape composition; particularly, agricultural fragmentation may greatly influence the trophic behavior of herbivores. We studied trophic niche variation in Greater Prairie-Chickens (Tympanuchus cupido), a grouse species that shows moderate sex-dimorphism. Greater Prairie-Chickens are native to tallgrass prairies of North America, although populations persist in less natural mosaics of cropland and native habitats. We used stable isotope analysis of carbon and nitrogen in blood, claws and feathers to assess seasonal differences in trophic niche breadth and individual specialization between male and female Greater Prairie-Chickens, and between birds living in continuous and fragmented landscapes. We found that females showed broader niches and higher individual specialization than males, especially in winter and autumn. However, differences between females and males were smaller in spring when birds converge at leks, suggesting that females and males may exhibit similar feeding behaviors during the lekking period. In addition, we found that birds living in native prairies showed greater annual trophic variability than conspecifics in agricultural mosaic landscapes. Native habitats may provide greater dietary diversity, resulting in greater diversity of feeding strategies.由于存在性别特异性的生态约束，具有显著体型性二态（sexual size dimorphism）的求偶场（lekking）物种，其生态位（ecological niche）分化现象被学界普遍认为是存在的。然而，对于体型性二态程度适中的物种而言，其生态位分化情况尚不明确。此外，物种的生态位还可能受景观组成（landscape composition）的影响；其中农业生境破碎化（agricultural fragmentation）会对植食性动物的营养行为产生显著影响。本研究以体型性二态程度适中的松鸡科鸟类——大草原榛鸡（Greater Prairie-Chicken，学名：*Tympanuchus cupido*）为研究对象，探讨其营养生态位的变异情况。大草原榛鸡原产于北美高草草原，目前部分种群仍存活于以农田与原生栖息地构成的半自然镶嵌生境中。本研究通过对血液、爪部及羽毛中的碳、氮稳定同位素分析（stable isotope analysis），评估大草原榛鸡雌雄个体间，以及栖息于连续景观与破碎化景观中的种群间，营养生态位宽度与个体特化程度的季节差异。研究结果显示，雌性个体的营养生态位宽度与个体特化程度均高于雄性，该差异在冬季与秋季尤为显著。但在春季个体聚集于求偶场时，雌雄个体间的差异有所缩小，表明求偶期雌雄个体的取食行为可能趋于一致。此外，本研究发现栖息于原生草原的个体，其营养生态位的年度变异性高于农业镶嵌生境中的同种个体。原生栖息地能够提供更为丰富的食物资源，进而催生更多样化的取食策略。