Data from: Environmental filtering and competitive exclusion drive biodiversity-invasibility relationships in shallow lake plant communities

1. Understanding the processes that influence the diversity of ecological communities and their susceptibility to invasion by exotic species remains a challenge in ecology. In many systems, a positive relationship between the richness of native species and exotic species has been observed at larger spatial (e.g., regional) scales, while a negative pattern has been observed at local (e.g., plot) scales. These patterns are widely attributed to (1) biotic interactions, particularly biotic resistance, limiting invasions in high-diversity locations, producing negative local-scale relationships, and (2) native and exotic richness covarying at larger spatial scales as a function of environmental conditions and heterogeneity, producing positive large-scale relationships. However, alternative processes can produce similar patterns and need to be critically evaluated to make sound inferences about underlying mechanisms.
2. We aggregated a large dataset of aquatic vegetation surveys from 1,102 Minnesota shallow lakes collected over 13 years to quantify spatial and temporal patterns of community composition. Using those data and additional information on environmental conditions we evaluated evidence for four distinct mechanisms that could drive patterns of native and exotic species richness: biotic resistance, competitive exclusion, environmental filtering, and environmental heterogeneity.
3. We found the classic pattern of a negative native-exotic richness relationship at local scales and a positive relationship at lake scales. However, we found no evidence for local-scale biotic resistance; instead, competitive exclusion by invasive species appeared to reduce native richness after locations became invaded. Evaluating the influence of environmental filtering and heterogeneity, we found that native and exotic species occupied somewhat different niches. Invaders were less sensitive to environmental gradients and more tolerant of a wider range of conditions. This segregation of habitat preferences alone could produce a negative local native-exotic richness relationship and a positive regional pattern without the involvement of biotic interactions.
4. Synthesis: Our findings somewhat conflict with established expectations, which come from research predominantly conducted in terrestrial ecosystems. This illustrates the importance of explicitly evaluating underlying mechanisms in diversity-invasibility research and for comparisons across different types of ecosystems. Identification of different drivers of diversity also has direct implications for decisions about management of freshwater plant communities.

1. 解析影响生态群落多样性及其对外来物种入侵敏感性的过程，仍是生态学领域的核心难题之一。在多数生态系统中，大空间尺度（如区域尺度）下可观测到本地物种丰富度与外来物种丰富度呈正相关关系，而在局地尺度（如样地尺度）下则呈负相关格局。这类格局普遍被归因于两类机制：其一为生物相互作用，尤其是生物抵抗（biotic resistance），在高多样性生境中抑制入侵过程，从而形成局地尺度的负相关关系；其二为大空间尺度下，本地与外来物种丰富度随环境条件与环境异质性发生协变，进而催生大尺度的正相关关系。然而，其他过程同样可产生类似格局，因此需对其进行严谨评估，以准确推断背后的核心机制。
2. 本研究整合了13年间针对1102个明尼苏达州浅水湖泊的水生植被调查数据集，以量化群落组成的空间与时间格局。依托该数据集及补充的环境条件信息，我们对可调控本地与外来物种丰富度格局的四类明确机制展开了验证：生物抵抗、竞争排斥（competitive exclusion）、环境过滤（environmental filtering）以及环境异质性。
3. 研究结果显示，局地尺度下本地-外来物种丰富度呈负相关的经典格局，而湖泊尺度下则呈正相关关系。但我们并未发现局地尺度下存在生物抵抗的证据；相反，入侵物种引发的竞争排斥似乎在生境被入侵后降低了本地物种的丰富度。通过评估环境过滤与异质性的影响，我们发现本地与外来物种占据了存在一定差异的生态位（niche）：入侵物种对环境梯度的敏感性更低，且可耐受更广范围的环境条件。仅依托生境偏好的分化，即可在无生物相互作用参与的情况下，形成局地尺度的负相关关系与区域尺度的正相关格局。
4. 综合分析：本研究结果与既有学术认知存在一定冲突——既有认知多来自陆地生态系统的相关研究。这一结果凸显了在多样性-可入侵性（diversity-invasibility）研究中，明确评估核心机制的重要性，同时也为不同类型生态系统间的对比研究提供了参考。明确多样性的不同驱动因子，对淡水植物群落的管理决策亦具有直接的指导意义。