You eat what you find – local patterns in vegetation structure control diets of African fungus-growing termites

Fungus-growing termites and their symbiotic Termitomyces fungi are critically important carbon and nutrient recyclers in arid and semiarid environments of sub-Saharan Africa. A major proportion of plant litter produced in these ecosystems is decomposed within nest chambers of termite mounds, where temperature and humidity are kept optimal for the fungal symbionts. While fungus-growing termites are generally believed to exploit a wide range of different plant substrates, the actual diets of most species remain elusive. We studied dietary niches of two Macrotermes species across the semiarid savanna landscape in the Tsavo Ecosystem, southern Kenya, based on carbon (C) and nitrogen (N) stable isotopes in Termitomyces fungus combs. We applied Bayesian mixing models to determine the proportion of grass and woody plant matter in the combs, these being the two major food sources available for Macrotermes species in the region. Our results showed that both termite species, and colonies cultivating different Termitomyces fungi, occupied broad and largely overlapping isotopic niches, indicating no dietary specialization. Including laser scanning derived vegetation cover estimates to the dietary mixing model revealed that the proportion of woody plant matter in fungus combs increased with increasing woody plant cover in the nest surroundings. Nitrogen content of fungus combs was positively correlated with woody plant cover around the mounds and negatively correlated with the proportion of grass matter in the comb. Considering the high N demand of large Macrotermes colonies, woody plant matter seems to thus represent a more profitable food source than grass. As grass is also utilized by grazing mammals, and the availability of grass matter typically fluctuates over the year, mixed woodland-grasslands and bushlands seem to represent more favorable habitats for large Macrotermes colonies than open grasslands.

Methods
Data originates from 59 termite mounds representing two Macrotermes species occurring in the Tsavo Ecosystem, Kenya. Each termite mound was georeferenced using a handheld GPS and the inhabitant termite species was identified based on species-specific mound morphology. The results of stable isotope (δ13C, δ15N) and elemental (C%, N%) analysis originate from a single fungus comb specimen collected from each studied mound. The analysis were performed at the Laboratory of Chronology, Finnish Museum of Natural History (Helsinki, Finland) using an NC2500 elemental analyzer coupled to a Thermo Scientific Delta V Plus isotope ratio mass spectrometer and the raw isotope data was normalized with a multi-point calibration using certified isotopic reference materials (USGS-40, USGS-41, IAEA-N1, IAEA-N2, IAEA-CH3 and IAEA-CH7). The species identity of the cultivated Termitomyces fungi (unnamed species "A", "B" or "C") is based on DNA barcoding of the ribosomal ITS1–5.8S–ITS2 region extracted from the fungal nodules growing on the comb surface (Vesala et al. 2017, 2019). Canopy cover was assessed around each termite mound using LiDAR (Light Detection and Ranging) point clouds produced during a flight campaign in March 2014. Lidar point clouds were classified and converted to topographic and surface models representing either ground or vegetation canopy, respectively, using Lastools processing software (https://rapidlasso.com/lastools/). A Canopy Height Model at 1 m spatial resolution was derived from topographic and surface models and was used to calculate precentage canopy cover using QGIS 3.6. Canopy cover around each studied mound was calculated separately for three different tree height classes (> 1, 3 and 6 meters) within three different circular buffers (radiuses 35, 50 and 100 m) centered at mound centrum, leading to a total of nine different canopy cover estimates.

培菌白蚁与其共生的白蚁伞属（Termitomyces）真菌是撒哈拉以南非洲干旱与半干旱环境中至关重要的碳与养分循环者。这些生态系统中产生的绝大部分植物凋落物，会在白蚁丘的巢室内被分解，而巢室内的温度与湿度被维持在最适配共生真菌的最优状态。尽管普遍认为培菌白蚁可利用多种不同的植物基质，但多数物种的实际食性仍有待明晰。本研究针对肯尼亚南部察沃生态系统的半干旱稀树草原景观中的两种大白蚁属（Macrotermes）物种，基于其共生白蚁伞属真菌的菌圃中的碳（C）与氮（N）稳定同位素（stable isotope），探究了二者的食性生态位。我们应用贝叶斯混合模型（Bayesian mixing models），以确定菌圃中草本植物与木本植物物质的占比——这两类是该区域大白蚁属物种可获取的两大主要食物来源。

研究结果显示，这两种白蚁以及培育不同白蚁伞属真菌的蚁群，均拥有宽泛且高度重叠的同位素生态位，表明其不存在食性特化。将激光扫描获取的植被覆盖估算值纳入食性混合模型后发现，菌圃中的木本植物物质占比，会随着蚁巢周边木本植物覆盖度的提升而增加。菌圃的氮含量与蚁丘周边的木本植物覆盖度呈正相关，与菌圃中草本物质的占比呈负相关。考虑到大型大白蚁属蚁群对氮的高需求，木本植物物质似乎是比草本植物更具收益的食物来源。由于草本植物也会被放牧哺乳动物利用，且草本物质的可获得性通常随年度波动，因此混生林地-草原与灌丛地带，相比开阔草原更适合大型大白蚁属蚁群生存。

## 方法
本研究的数据源自肯尼亚察沃生态系统中代表两个大白蚁属物种的59个白蚁丘。每个白蚁丘均通过手持GPS完成地理定位，栖息的白蚁物种则依据物种特有的蚁丘形态进行鉴定。稳定同位素（δ¹³C、δ¹⁵N）与元素（C%、N%）分析的结果，取自每个研究蚁丘采集的单份菌圃样本。分析工作在芬兰自然历史博物馆年代学实验室（芬兰赫尔辛基）完成，使用的是与赛默飞世尔科技Delta V Plus同位素比率质谱仪联用的NC2500元素分析仪；原始同位素数据通过使用认证同位素参考物质（USGS-40、USGS-41、IAEA-N1、IAEA-N2、IAEA-CH3与IAEA-CH7）的多点校准进行归一化处理。

培育的白蚁伞属真菌的物种身份（未命名的“A”“B”或“C”物种），基于从菌圃表面生长的真菌结节中提取的核糖体ITS1–5.8S–ITS2区域的DNA条形码（DNA barcoding）鉴定（Vesala等，2017、2019）。冠层覆盖度通过2014年3月飞行测绘任务中获取的激光雷达（LiDAR, Light Detection and Ranging）点云数据进行评估。利用Lastools处理软件（https://rapidlasso.com/lastools/）对激光雷达点云进行分类，并分别转换为代表地面与植被冠层的地形模型与表面模型。从地形模型与表面模型中导出空间分辨率为1米的冠层高度模型，并使用QGIS 3.6计算冠层覆盖百分比。每个研究蚁丘周边的冠层覆盖度，分别针对三个不同的圆形缓冲区（半径35米、50米与100米）内的三个树木高度等级（>1米、>3米与>6米）进行计算，最终得到九种不同的冠层覆盖估算值。