Pulsed resource availability changes dietary niche breadth and partitioning between generalist rodent consumers

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast-fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a two-year period using stable isotope analysis (δ13C, δ15N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beech nuts (a high-quality food resource) and having a narrower niche breadth during masting seasons compared to non-masting seasons when dietary niches expanded and more fungi (a low quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during non-masting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.

阐明调控物种种间生态位宽度（niche breadth）与生态位重叠（niche overlap）的机制，对于明确物种间相互作用模式、评估其在生态系统中的功能角色具有重要意义。若不开展操控性实验（manipulative experiments），评估觅食生态学（foraging ecology）与种间竞争（interspecific competition）在塑造物种食谱结构中的作用颇具挑战。具有规律性资源脉冲（resource pulses）的生态系统可作为天然实验（natural experiment），用以探究影响消费者膳食生态位（dietary niches）的调控因子。我们以美洲山毛榉（Fagus grandifolia）的天然结实脉冲为研究对象，检验最优觅食理论（optimal foraging theory）与种间竞争是否调控了两种同属啮齿动物——白足鼠（Peromyscus leucopus）和北美鹿鼠（Peromyscus maniculatus）——的膳食生态位宽度与生态位重叠，二者均为广食性消费者。我们通过为期两年的季节性采样，利用毛发与潜在食物源的稳定同位素分析（stable isotope analysis，δ¹³C、δ¹⁵N）重建了两种鼠类的食谱，并基于贝叶斯框架（Bayesian framework）内计算得到的标准椭圆面积（standard ellipse area）量化了生态位动态变化。生态位宽度的变化总体上符合最优觅食理论的预测：在结实大年（masting seasons），两种鼠类会取食更多山毛榉坚果（优质食物资源），生态位宽度更窄；而在非结实大年（non-masting seasons），其膳食生态位会扩张，更多取食真菌（低质量食物来源）。与之相反，膳食生态位重叠的变化则符合种间竞争理论：结实大年的食谱重叠度高于非结实大年。总体而言，膳食生态位动态与美洲山毛榉结实大年现象紧密相关，这凸显了食物可获得性对种间竞争的调控作用。两种鹿鼠属（Peromyscus）物种的食谱可塑性（diet plasticity）与生态位分化（niche partitioning），可能反映了其觅食策略的差异，从而在食物可获得性较低时降低种间竞争。此类食谱变化可能对生态系统功能的改变产生重要影响，其中包括真菌孢子的传播过程。