Elevational cline in herbivore abundance driven by a monotonic increase in trophic level sensitivity to aridity

1. The abiotic environment drives species abundances and distributions both directly and indirectly through effects on multi-trophic species interactions. However, few studies have documented the individual and combined consequences of these direct and indirect effects. 2. We studied an ant-tended aphid along an elevational gradient, where lower elevations were more arid. Hypotheses of stronger species interactions at lower elevations and a greater sensitivity of higher trophic levels to climate led us to predict increased top-down control of aphids by natural enemies (third trophic level) but even stronger protection from mutualist ants (fourth trophic level) with increasing aridity. As a result, we predicted that mutualism strength and aphid abundance would increase with aridity. 3. We documented patterns of aphid abundance and tested for both the direct and multi-trophic indirect effects of aridity on aphid performance. To do so, we used both observational and manipulative methods ac...

1. 非生物环境（abiotic environment）可通过调控多营养级物种间的互作，直接或间接影响物种种群丰度与分布格局。然而目前鲜有研究同时阐明这类直接与间接效应各自及联合产生的生态后果。
2. 我们沿海拔梯度开展了蚁伴蚜虫（ant-tended aphid）的相关研究，该梯度内海拔越低的区域干旱程度越高。基于“低海拔区域物种互作强度更高”以及“高营养级类群对气候波动更为敏感”两项假说，我们提出预测：随着干旱程度加剧，蚜虫天敌（第三营养级）对其的下行控制（top-down control）作用会增强，而互利共生蚂蚁（第四营养级）为其提供的保护效应会更为显著。据此我们进一步推测，互利共生强度与蚜虫种群丰度均会随干旱程度提升而增加。
3. 我们记录了蚜虫种群丰度的分布格局，并检验了干旱对蚜虫适合度的直接效应与多营养级间接效应。为此，我们同时采用了观测与操控性实验两类研究方法……