Data from: Accurate predictions of coexistence in natural systems require the inclusion of facilitative interactions and environmental dependency

1. Coexistence between plant species is well known to depend on the
outcomes of species interactions within an environmental context. The
incorporation of environmental variation into empirical studies of
coexistence are rare, however, due to the complex experiments needed to do
so and the lack of feasible modelling approaches for determining how
environmental factors alter specific coexistence mechanisms. 2. In this
paper, we present a simple modelling framework for assessing how variation
in species interactions across environmental gradients impact on niche
overlap and fitness differences, two core determinants of coexistence. We
use a novel formulation of an annual plant population dynamics model that
allows for competitive and facilitative species interactions, and for
variation in the strength and direction of these interactions across
environmental gradients. Using this framework, we examine outcomes of
plant-plant interactions between four commonly co-occurring annual plant
species from Western Australian woodlands. We then determine how niche
overlap and fitness differences between these species vary across three
environmental gradients previously identified as important for structuring
diversity patterns in this system: soil phosphorus, shade and water. 3. We
found facilitation to be a wide-spread phenomenon and that interactions
between most species pairs shift between competitive and facilitative
across multiple environmental gradients. Environmental conditions also
altered the strength, direction and relative variation of both niche
overlap and fitness differences in non-linear and unpredictable ways.
Synthesis We provide a simple framework for incorporating environmental
heterogeneity into explorations of coexistence mechanisms. Our findings
highlight the importance of the environment in determining the outcome of
species interactions and the potential for pairwise coexistence between
species. The prevalence of facilitation in our system indicates a need to
improve current theoretical frameworks of coexistence to include
non-competitive interactions, and ways of translating these effects into
explicit predictions of coexistence. Our study also suggests a need for
further research into determining which factors result in consistent
responses of niche overlap and fitness differences to environmental
variation. Such information will improve our ability to predict outcomes
of coexistence, invasion events and responses of whole communities to
future environmental change.

1. 物种共存现象广为人知，其发生依赖于环境背景下物种互作的结果。然而，将环境变异纳入共存实证研究的工作却相对匮乏，这是因为相关实验复杂度极高，且目前缺乏可行的建模方法以解析环境因子如何改变特定的共存机制。
2. 本文提出一套简洁的建模框架，用以评估环境梯度上的物种互作变异如何对生态位重叠（niche overlap）与适合度差异（fitness differences）这两大共存核心决定因素产生影响。我们采用了一种全新的一年生植物种群动态模型（annual plant population dynamics model）的表述形式，该模型可同时涵盖竞争与促进两类物种互作，并允许这些互作的强度与方向随环境梯度发生变化。依托该框架，我们针对澳大利亚西部林地中4种常见伴生的一年生植物开展了种间互作结果分析。随后，我们进一步探究了这4个物种间的生态位重叠与适合度差异，如何沿此前在该系统中被证实对塑造群落多样性格局至关重要的三类环境梯度（土壤磷含量、遮阴程度与水分条件）发生变化。
3. 研究结果显示，促进作用是一种广泛存在的现象，且多数物种对间的互作会在多个环境梯度上于竞争与促进模式之间发生转变。环境条件还会以非线性且难以预测的方式，改变生态位重叠与适合度差异的强度、方向及其相对变异程度。综合分析：我们提出了一套可将环境异质性纳入共存机制探索的简洁框架。本研究结果凸显了环境在决定物种互作结果以及物种间两两共存潜力方面的重要性。本系统中促进作用的普遍存在，意味着现有共存理论框架亟需完善，以纳入非竞争性互作，并探索如何将这些效应转化为明确的共存预测模型。此外，本研究还提示，未来需进一步开展相关研究，以明确哪些因素可使生态位重叠与适合度差异对环境变异产生一致性响应。此类信息将有助于提升我们预测物种共存结果、生物入侵事件以及整个群落对未来环境变化响应的能力。