Partial migration alters population ecology and food chain length: evidence from a salmonid fish

Many migratory species, from monarch butterflies to wildebeest, express partial migration, where only a subset of a population migrates. This intraspecific variation is likely to have large ecological consequences. We studied the ecological consequences of partial migration in a salmonid fish, Oncorhynchus mykiss, in coastal streams in California, USA. One ecotype, steelhead trout, migrates to the ocean, whereas the other, rainbow trout, completes its lifecycle in freshwater. Migration has a strong genetic basis in O. mykiss. In one stream, we found differences in the frequency of migration-linked genotypes below and above a waterfall barrier (migratory allele frequency of 60% below vs. 31% above). Below the waterfall, in the migratory-dominated region, the density of young fish (<1 year old) was approximately twice that in the resident-dominated region above the waterfall (0.46 vs 0.26 individuals/m2, respectively), presumably reflecting the higher fecundity of migratory females. Additionally, there were half as many older fish (>1 year old) in pools downstream of the waterfall (0.05 vs. 0.13 individuals/m2). In a second stream, between-year variation in the dominance of migratory vs. resident fish allowed us to explore differences in fish density and size structure through time, and we found a consistent pattern. In brief, when migratory genotypes dominated, we found higher densities of young fish and lower densities of older fish, resulting in a simpler size structure, compared to when resident genotypes dominated. Moreover, large resident trout had a slightly higher trophic position than young fish (3.92 vs. 3.42 in one creek and 3.77 vs. 3.17 in the other), quantified with stable isotope data. The difference in fish size structure did not generate trophic cascades. Partial migration is widespread among migratory populations, as is phenotypic divergence between resident and migratory forms, suggesting the potential for widespread ecological effects arising from this common form of intraspecific variation.从帝王蝶到牛羚，诸多迁徙物种均存在部分迁徙现象——即种群中仅有部分个体发生迁徙。这种种内变异大概率会对生态系统产生显著影响。我们以美国加利福尼亚州沿海溪流中的鲑科鱼类虹鳟（Oncorhynchus mykiss）为研究对象，探讨了部分迁徙现象的生态效应。该物种存在两种生态型：其一为硬头鳟（steelhead trout），会向海洋洄游；其二为虹鳟（rainbow trout），其完整生活史均在淡水环境中完成。虹鳟的迁徙行为具有较强的遗传基础。在其中一条溪流中，我们发现瀑布屏障上下游与迁徙相关的基因型频率存在显著差异：瀑布下游的迁徙等位基因频率为60%，上游则为31%。瀑布下游为迁徙型主导区域，其幼鱼（年龄<1年）密度约为上游陆封型主导区域的2倍（分别为0.46与0.26尾/平方米），这大概率反映了迁徙型雌性个体更高的繁殖力。此外，瀑布下游池水区的成鱼（年龄>1年）密度仅为上游的一半（分别为0.05与0.13尾/平方米）。在第二条溪流中，由于不同年份间洄游型与陆封型个体的优势地位存在差异，我们得以探究鱼类密度与体型结构随时间的变化规律，并发现了一致的模式。简言之，与陆封型基因型占主导的情况相比，当迁徙型基因型占主导时，幼鱼密度更高而成鱼密度更低，最终形成更为简单的体型结构。此外，通过稳定同位素数据量化发现，大型陆封型虹鳟的营养级略高于幼鱼：在第一条溪流中分别为3.92与3.42，第二条溪流中则为3.77与3.17。鱼类体型结构的差异并未引发营养级联效应。部分迁徙现象在迁徙类群中广泛存在，陆封型与迁徙型个体间的表型分化亦是如此，这表明这种常见的种内变异可能会产生广泛的生态效应。