Data from: Large niche differences emerge at the recruitment stage to stabilize grassland coexistence

Niche differences and average fitness differences jointly determine coexistence. However, little empirical information about the magnitude of these two mechanisms is available. Using multispecies population models fit to long-term demographic data for common, co-occurring species in five grassland and shrubland plant communities in western North America, we estimated the strength of stabilizing niche differences and average fitness differences. In all five communities, both pairwise and full community comparisons showed evidence for strong stabilizing mechanisms and relatively small average fitness differences. For a total of 17 species pairs, a measure of niche differences based on simulations of invasion growth rates ranged from 0.59 to 0.93 with a mean of 0.81, where 0 indicates complete niche overlap and 1 indicates zero niche overlap. A corresponding measure of average fitness differences ranged from 1.02 to 2.54 with a mean of 1.53, where 1 indicates identical fitness and a value of 2 indicates a four-fold difference in sensitivity to competition. Comparisons of full communities displayed similar patterns: niche differences ranged from 0.58 to 0.69 with a mean of 0.64, and the average fitness differences ranged from 1.42 to 1.63 with a mean of 1.47. In almost every case, the stabilizing mechanisms were much stronger than minimally necessary to prevent competitive exclusion. Considering that all but one of the species we studied are perennial grasses, which are often grouped in the same functional type, the magnitude of these niche differences is surprising. In all five communities, differences between intra- and interspecific effects at the recruitment stage contributed far more to stabilization than interactions affecting growth and survival. Our results indicate that for these abundant, co-occurring species 1) dynamics are far from neutral, with strong niche differences and weak fitness differences combining to stabilize coexistence, and 2) processes operating at early life stages account for a large proportion of the stabilizing effect. Given the limitations of our inductive approach, both these findings represent hypotheses in need of experimental testing.生态位分化（niche differences）与平均适合度差异（average fitness differences）共同决定物种共存格局。然而，目前关于这两种共存机制的强度，尚缺乏充足的实证数据。本研究针对北美西部5个草原与灌丛植物群落中的常见共生物种，采用基于长期种群统计数据拟合的多物种种群模型，估算了稳定化生态位分化（stabilizing niche differences）与平均适合度差异的强度。在全部5个群落中，无论是物种种对水平还是全群落水平的比较，均检测到强烈的稳定化共存机制信号，且平均适合度差异相对较小。针对总计17个物种种对，基于入侵生长率模拟得到的生态位分化度量值介于0.59至0.93之间，均值为0.81；其中0代表完全生态位重叠，1代表完全无生态位重叠。对应的平均适合度差异度量值则介于1.02至2.54之间，均值为1.53；其中1代表适合度完全一致，数值2代表对竞争敏感性存在4倍差异。全群落水平的比较也呈现出相似规律：生态位分化值介于0.58至0.69之间，均值为0.64；平均适合度差异值介于1.42至1.63之间，均值为1.47。几乎在所有情形中，稳定化机制的强度均远高于阻止竞争排斥所需的最低阈值。鉴于本研究涉及的物种除1种外均为多年生草本植物，且这类植物常被归为同一功能群，上述生态位分化的强度着实令人意外。在全部5个群落中，补充阶段的种内与种间作用差异对稳定化的贡献，远高于影响植株生长与存活的种间互作。本研究结果表明，针对这些常见共生物种：其一，群落动态远非中性，强烈的生态位分化与微弱的适合度差异共同维持了物种共存的稳定性；其二，发生在生命早期阶段的过程，贡献了稳定化效应的绝大部分。考虑到本研究采用归纳式研究方法存在的局限性，上述两项发现均为有待实验验证的假说。