As pressures on biodiversity increase, a better understanding of how assemblages are responding is needed. Because rare species, defined here as those that have locally low abundances, make up a high proportion of assemblage species lists, understanding how the number of rare species within assemblages is changing will help elucidate patterns of recent biodiversity change. Here, we show that the number of rare species within assemblages is increasing, on average, across systems. This increase could arise in two ways: species already present in the assemblage decreasing in abundance but with no increase in extinctions, or additional species entering the assemblage in low numbers associated with an increase in immigration. The positive relationship between change in rarity and change in species richness provides evidence for the second explanation, i.e. higher net immigration than extinction among the rare species. These measurable changes in the structure of assemblages in the recent past underline the need to use multiple biodiversity metrics to understand biodiversity change.

随着生物多样性压力的增加，需要更好地了解生物群落如何应对。由于稀有物种（此处定义为局部丰度较低的物种）在组合物种列表中所占比例很高，因此了解组合内稀有物种的数量如何变化将有助于阐明近期生物多样性变化的模式。在这里，我们表明，平均而言，整个系统中组合内稀有物种的数量正在增加。这种增加可能以两种方式出现：种群中已经存在的物种数量减少，但灭绝数量没有增加，或者由于移民增加而进入种群的其他物种数量较少。稀有度变化与物种丰富度变化之间的正相关关系为第二种解释提供了证据，即稀有物种的净移民量高于灭绝量。最近组合结构的这些可测量的变化强调需要使用多种生物多样性指标来了解生物多样性变化。