Long-term livestock grazing increases the recruitment success of epigeal termites: insights from a >75-year grazing experiment in the Karoo, South Africa

Epigeal termites alter ecosystem-level processes through foraging and mound building. These effects are proportional to density. Whilst much is known about the factors influencing mound density in natural settings, how termites respond to livestock grazing has been overlooked. We compared the characteristics and density of Trinervitermes trinervoides (Isoptera: Termitidae) mounds in the Karoo (South Africa) within an exclosure and continuously-grazed plot from a >75-year grazing experiment. Specifically, we asked whether long-term livestock grazing impacted (1) termite mound density, volume, basal area and size, (2) mound occupancy and age, (3) vegetation characteristics and (4) soil properties. Long-term grazing increased mound density two-fold. This was coupled with a decrease in mean mound size, an increase in the size of the largest mounds and range of mound sizes, and an increase in the proportion and numbers of young, active mounds. However, these changes did not alter landscape-scale mound basal area or volume. Vegetation was unaffected by long-term grazing but it did cause soil phosphorus, magnesium and calcium depletion. We suggest that livestock grazing provides additional forage resources for termites through litter breakup and dung production, leading to greater mound recruitment and thus densities, whilst allowing mounds to achieve greater maximum size.

地表白蚁通过觅食与筑丘活动改变生态系统尺度的过程，且此类影响与白蚁种群密度呈正相关。尽管学界已对自然生境中影响蚁丘密度的诸多因素已有充分认知，但白蚁对家畜放牧的响应机制仍未得到足够关注。
本研究依托一项已开展超75年的放牧定位实验，对比了南非卡鲁（Karoo）地区围栏禁牧区与连续放牧样地内的三叉白蚁（Trinervitermes trinervoides，等翅目：白蚁科）蚁丘的特征与密度，并针对性探讨四个科学问题：长期家畜放牧是否会(1) 影响白蚁蚁丘的密度、体积、基面积与尺寸；(2) 改变蚁丘占用率与龄期；(3) 影响植被特征；(4) 改变土壤理化性质。
研究结果表明，长期放牧使蚁丘密度提升了一倍；伴随这一变化的是平均蚁丘尺寸的减小，以及最大蚁丘尺寸与蚁丘尺寸分布范围的扩大，同时年轻活跃蚁丘的占比与数量均有所增加。不过上述变化并未对景观尺度下蚁丘的总基面积与总体积产生影响。
长期放牧对研究区域的植被无显著影响，但会造成土壤中磷、镁与钙元素的耗竭。
本研究推测，家畜放牧可通过破碎枯落物与产生粪便为白蚁提供额外的觅食资源，进而促进蚁丘的新生与种群密度提升，同时也使得蚁丘能够达到更大的最大尺寸。