Empirical evidence for the ecological significance of interaction network indices within a mutualistic network

Community resilience and stability have often been considered outcomes of interaction network structure. These assumptions however have faced strong criticism given that the ecological outcomes of the interactions are often ignored, leading to the overinterpretation of network structural properties. Evaluating the link between network structure and components of reproductive success across species can thus help provide a more ecologically relevant understanding of the relationship between network structure and function (i.e., stability). Specifically, within pollination networks, there is a need to integrate knowledge of interaction network structure with pollination outcomes that reflect the full complexity of the pollination process. Here, we obtained and integrated data on network structural properties resulting from the interactions between 20 plant and 110 floral-visiting species, with multiple estimates of male and female reproductive success for all plant species. Network properties differentially affected male and female reproductive functions. An increase in plant species partner diversity and contribution to nestedness positively associated with the proportion of flowers receiving conspecific pollen but had no effect on pollen dispersal or tube formation. Specialization was the only network property that maximized both functions, suggesting this component play a larger role in long-term stability of plant communities. These results emphasize the need to incorporate empirical knowledge of interaction outcomes into our understanding of the functional consequences of network structure. This knowledge is key to overcoming fundamental limitations in network ecology providing meaningful insights into the resilience and stability of ecological systems.

Methods
This data set was used to evaluate the relationship between interaction network properties at species (strength, specialization and nestedness) and components of pollination successData collection was conducted at a serpentine seep meta-community in the McLaughlin Natural Reserve (38.8582º N, 122.4093º W) in Northern California, United States. To obtain network indices, we built a visitation-based plant-pollinator network containing 20 plants and 110 flower-visiting insect species, and calculated species strength, specialization (d') and contribution to nestedness for each plant species (n = 20). All flower-visiting insects contacting plant reproductive structures (i.e., anthers and stigmas) were collected by 2-3 people that simultaneously walked established trails between 09h00 and 15h00 using entomological nets from May 9th through June 1st, 2021 (˃ 120 hr of total observation). The following components of the pollination succes were quantified for each plant species during the same period: 1) proportion of flowers with conspecific pollen (CP), 2) mean pollen dispersal on pollinators, 3) proportion of pollen loss to heterospecific stigmas, 4) heterospecific pollen (HP) receipt, and 5) proportion of pollen tubes formed. For 1, 3, 4 and 5, we sampled 1319 flowers (styles), with a mean of 65.95 (± 32.62) styles per plant species. After appropriate processing, softening and staining, the total number of conspecific and heterospecific pollen grains on the stigmas were identified and counted. We also recorded the number of pollen tubes that reached the base of the style under a fluorescent microscope. For 2, we sampled the pollen loads carried by all 744 insects collected to estimate pollen dispersal success by counting and identifying pollen grains.  Additionally, we estimated flower abundance in the flowering season using 40 plots (1x2-meter each) to understand their influence on the considered male and female components.

群落恢复力与生态系统稳定性常被视作物种互作网络结构的产物。然而这类假设遭到了强烈批判——因为互作过程的生态结果常被忽略，导致对网络结构属性的过度解读。评估跨物种的网络结构与繁殖成功组分之间的关联，有助于更贴合生态学语境地理解网络结构与功能（即稳定性）之间的关系。具体而言，在传粉网络研究中，需将互作网络结构的认知与反映传粉过程全部复杂性的传粉结果相结合。本研究获取并整合了20种植物与110种访花物种互作所产生的网络结构属性数据，同时为所有植物物种提供了雄性和雌性繁殖成功的多项评估指标。

网络属性对雄性和雌性生殖功能存在差异化影响：植物物种的互作伙伴多样性提升及其对嵌套性（nestedness）的贡献，与接收同种花粉（conspecific pollen）的花朵比例呈正相关，但对花粉扩散或花粉管形成无显著影响。特化度（specialization）是唯一能同时最大化这两类生殖功能的网络属性，表明该组分在植物群落的长期稳定性中发挥更为关键的作用。本研究结果强调，需将互作结果的实证知识纳入对网络结构功能后果的认知框架中。这类知识是克服网络生态学基础局限的核心，可为生态系统的恢复力与稳定性提供具有价值的科学见解。

## 方法
本数据集用于评估物种水平的互作网络属性（物种强度、特化度（specialization）与嵌套性（nestedness））与传粉成功组分之间的关联。数据采集于美国北加利福尼亚州麦克劳克林自然保护区（McLaughlin Natural Reserve，坐标38.8582°N，122.4093°W）的蛇纹岩渗流复合群落。为获取网络指数，我们构建了基于访花记录的植物-传粉者网络，包含20种植物与110种访花昆虫物种，并计算了每种植物的物种强度、特化度（d'）及其对嵌套性的贡献（样本量n=20）。

2021年5月9日至6月1日期间，由2-3名研究人员于每日9:00至15:00沿固定样线同时行进，使用昆虫网采集所有接触植物生殖结构（即花药与柱头）的访花昆虫，总观测时长超过120小时。同期针对每种植物量化了以下传粉成功组分：1）携带同种花粉（conspecific pollen，CP）的花朵比例；2）传粉者携带的平均花粉扩散量；3）异交花粉（heterospecific pollen）柱头落置导致的花粉损失比例；4）异交花粉接收比例；5）花粉管形成比例。

针对指标1、3、4、5，我们共采样1319朵花（花柱），每种植物平均采样65.95±32.62个花柱。经适当处理、软化与染色后，对柱头上的同种与异交花粉粒进行鉴定与计数，并在荧光显微镜下记录到达花柱基部的花粉管数量。针对指标2，我们对采集到的全部744只昆虫携带的花粉负载进行采样，通过计数与鉴定花粉粒来评估花粉扩散成功率。此外，我们在开花季利用40个样方（每个1×2米）估算花的丰度，以解析其对所关注的雄性与雌性生殖组分的影响。