Department of Ecology, Evolution, and Behavior, University of MN, St. Paul, MN, 55108, USA.
Glob Chang Biol. 2013 Dec;19(12):3677-87. doi: 10.1111/gcb.12370. Epub 2013 Oct 16.
Invasions have increased the size of regional species pools, but are typically assumed to reduce native diversity. However, global-scale tests of this assumption have been elusive because of the focus on exotic species richness, rather than relative abundance. This is problematic because low invader richness can indicate invasion resistance by the native community or, alternatively, dominance by a single exotic species. Here, we used a globally replicated study to quantify relationships between exotic richness and abundance in grass-dominated ecosystems in 13 countries on six continents, ranging from salt marshes to alpine tundra. We tested effects of human land use, native community diversity, herbivore pressure, and nutrient limitation on exotic plant dominance. Despite its widespread use, exotic richness was a poor proxy for exotic dominance at low exotic richness, because sites that contained few exotic species ranged from relatively pristine (low exotic richness and cover) to almost completely exotic-dominated ones (low exotic richness but high exotic cover). Both exotic cover and richness were predicted by native plant diversity (native grass richness) and land use (distance to cultivation). Although climate was important for predicting both exotic cover and richness, climatic factors predicting cover (precipitation variability) differed from those predicting richness (maximum temperature and mean temperature in the wettest quarter). Herbivory and nutrient limitation did not predict exotic richness or cover. Exotic dominance was greatest in areas with low native grass richness at the site- or regional-scale. Although this could reflect native grass displacement, a lack of biotic resistance is a more likely explanation, given that grasses comprise the most aggressive invaders. These findings underscore the need to move beyond richness as a surrogate for the extent of invasion, because this metric confounds monodominance with invasion resistance. Monitoring species' relative abundance will more rapidly advance our understanding of invasions.
入侵增加了区域物种库的规模，但通常被认为会降低本地多样性。然而，由于关注外来物种丰富度而不是相对丰度，因此很难对这一假设进行全球范围的测试。这是有问题的，因为低入侵丰富度可能表明本地群落具有抗入侵能力，或者相反，由单一外来物种主导。在这里，我们使用全球复制的研究来量化在六大洲 13 个国家的草主导生态系统中，外来物种丰富度与丰度之间的关系，范围从盐沼到高山冻原。我们测试了人类土地利用、本地群落多样性、食草动物压力和养分限制对外来植物优势的影响。尽管广泛使用，但外来丰富度在低外来丰富度时对外来优势的代理效果较差，因为包含少数外来物种的地点范围从相对原始（低外来丰富度和覆盖度）到几乎完全由外来物种主导（低外来丰富度但高外来覆盖度）。外来物种覆盖度和丰富度均受本地植物多样性（本地草丰富度）和土地利用（与耕作的距离）的预测。虽然气候对外来物种覆盖度和丰富度的预测都很重要，但预测覆盖度的气候因素（降水变率）与预测丰富度的气候因素（最暖季度的最高和平均温度）不同。食草动物和养分限制不能预测外来物种的丰富度或覆盖度。在站点或区域尺度上，本地草丰富度低的地区，外来优势最大。尽管这可能反映了本地草的取代，但更可能的解释是缺乏生物抗性，因为草是最具侵略性的入侵物种。这些发现强调了需要超越丰富度作为入侵程度的替代指标，因为这一指标将单优势与抗入侵混淆。监测物种的相对丰度将更快速地推进我们对入侵的理解。
