Data from: Network reorganization and breakdown of an ant–plant protection mutualism with elevation

Both the abiotic environment and the composition of animal and plant communities change with elevation. For mutualistic species, these changes are expected to result in altered partner availability, and shifts in context-dependent benefits for partners. To test these predictions, we assessed the network structure of terrestrial ant-plant mutualists and how the benefits to plants of ant inhabitation changed with elevation in tropical forest in Papua New Guinea. At higher elevations, ant-plants were rarer, species richness of both ants and plants decreased, and the average ant or plant species interacted with fewer partners. However, networks became increasingly connected and less specialized, more than could be accounted for by reductions in ant-plant abundance. On the most common ant-plant, ants recruited less and spent less time attacking a surrogate herbivore at higher elevations, and herbivory damage increased. These changes were driven by turnover of ant species rather than by within-species shifts in protective behaviour. We speculate that reduced partner availability at higher elevations results in less specialized networks, while lower temperatures mean that even for ant-inhabited plants, benefits are reduced. Under increased abiotic stress, mutualistic networks can break down, owing to a combination of lower population sizes, and a reduction in context-dependent mutualistic benefits.

非生物环境与动植物群落的组成均会随海拔梯度发生变化。对于互利共生物种来说，此类变化将导致共生伙伴的可获得性发生改变，并引发伙伴间情境依赖收益的偏移。为验证上述预测，我们以巴布亚新几内亚的热带森林为研究样区，对陆生蚁植互利共生生物（terrestrial ant-plant mutualists）的网络结构，以及蚁类栖息为植物带来的收益随海拔梯度的变化情况展开了评估。
海拔越高，蚁栖植物（ant-plant）的丰度越低；蚁类与植物的物种丰富度均出现下降，且单个蚂蚁或植物物种的平均交互伙伴数量也随之减少。但共生网络的连通性却不断提升，特化程度逐渐降低，且该变化幅度远超蚁栖植物丰度下降所能解释的范围。
在最为常见的蚁栖植物上，海拔越高，蚂蚁的招募行为越少，攻击替代植食者（surrogate herbivore）的时长也越短，同时植食损伤程度随之上升。此类变化由蚁类物种的更替所驱动，而非同一蚁类物种内保护行为的改变。
我们推测，高海拔区域共生伙伴可获得性的降低会导致共生网络特化程度下降，而低温环境则意味着即便对于蚁栖植物而言，其获得的共生收益也会有所减少。在非生物胁迫加剧的背景下，受种群规模缩小与情境依赖的共生收益下降双重因素共同作用，互利共生网络可能会发生解体。