Mapping trait versus species turnover reveals spatiotemporal variation in functional redundancy and network robustness in a plant‐pollinator community

1. Functional overlap among species (redundancy) is considered important
in shaping competitive and mutualistic interactions that determine how
communities respond to environmental change. Most studies view functional
redundancy as static, yet traits within species – which ultimately shape
functional redundancy – can vary over seasonal or spatial gradients. We
therefore have limited understanding of how trait turnover within and
between species could lead to changes in functional redundancy or how loss
of traits could differentially impact mutualistic interactions depending
on where and when the interactions occur in space and time. 2. Using an
Arctic bumblebee community as a case study, and 1,277 individual measures
from 14 species over three annual seasons, we quantified how inter- and
intraspecific body-size turnover compared to species turnover with
elevation and over the season. Coupling every individual and their trait
with a plant visitation, we investigated how grouping individuals by a
morphological trait or by species identity altered our assessment of
network structure and how this differed in space and time. Finally, we
tested how the sensitivity of the network in space and time differed when
simulating extinction of nodes representing either morphological trait
similarity or traditional species groups. This allowed us to explore the
degree to which trait-based groups increase or decrease interaction
redundancy relative to species-based nodes. 3. We found that i) groups of
taxonomically and morphologically similar bees turn over in space and time
independently from each other, with trait turnover being larger over the
season; ii) networks composed of nodes representing species versus
morphologically similar bees were structured differently; and iii)
simulated loss of bee trait groups caused faster coextinction of bumblebee
species and flowering plants than when bee taxonomic groups were lost.
Crucially, the magnitude of these effects varied in space and time,
highlighting the importance of considering spatiotemporal context when
studying the relative importance of taxonomic and trait contributions to
interaction network architecture. 4. Our finding that functional
redundancy varies spatiotemporally demonstrates how considering the traits
of individuals within networks is needed to understand the impacts of
environmental variation and extinction on ecosystem functioning and
resilience.

1. 物种间的功能重叠（又称冗余）在塑造竞争与互利互作中至关重要，而这类互作决定了群落如何响应环境变化。现有多数研究将功能冗余（functional redundancy）视为静态属性，但物种内部的功能性状——最终决定功能冗余的核心因素——会随季节或空间梯度发生变化。因此，我们对物种内外的性状周转如何引发功能冗余的改变，以及性状丢失如何根据互作发生的时空位置，差异化影响互利互作的机制仍知之甚少。
2. 本研究以北极熊蜂群落为研究案例，基于3个年度季节中14个物种的1277个个体水平测量数据，量化了种间与种内的体型性状周转相较于沿海拔梯度与季节变化的物种周转的差异情况。通过将每只熊蜂个体及其性状与植物访花记录进行关联，本研究分析了基于形态性状或物种身份对个体进行分组时，会如何改变我们对互作网络结构的评估，且这类差异在时空尺度上存在变化。最后，我们模拟了代表形态性状相似类群或传统物种类群的节点的灭绝事件，以此测试网络在时空维度上的敏感性差异，进而探究相较于基于物种类群的节点，基于性状类群的分组会在多大程度上提升或降低互作冗余（interaction redundancy）。
3. 本研究结果显示：① 分类学与形态学相似的熊蜂类群在时空尺度上的周转彼此独立，且季节尺度上的性状周转幅度更大；② 以物种为节点的网络与以形态相似熊蜂类群为节点的网络，其结构存在显著差异；③ 相较于移除熊蜂分类类群的模拟实验，移除熊蜂性状类群会更快引发熊蜂物种与显花植物的协同灭绝。尤为关键的是，这些效应的强度随时空尺度发生变化，这凸显了在研究分类学与性状对互作网络架构的相对重要性时，考虑时空背景的必要性。
4. 我们发现功能冗余具有时空异质性，这一结论表明，若要理解环境变化与物种灭绝对生态系统功能与恢复力的影响，必须将网络中个体的性状纳入考量范畴。