Diversity loss with persistent human disturbance increases vulnerability to ecosystem collapse

摘要

在北美西北太平洋沿岸地区长有橡树的成片稀rn树草原上,自19世纪中期以来一直都采取防火rn措施.受这些措施的影响,那里的草地失去了rn植物物种,但每年却能稳定高产,就像个高rn产的农业系统样,并且抵抗入侵物种和气候rn波动的能力仍比较强.现在,在人为选择的一rn些周期性被烧掉、再让其恢复的草地上所进行rn的一项长期研究,显示了当生态系统经历扰动rn时生物多样性的关键价值.只有在本地植物多rn样性相对较高的地方,草地才能从烧荒中恢rn复.这项研究工作显示了持续的人类活动何以rn能够均化个生态系统的结构和功能、同时弱rn化补偿突然扰动所需的与多样性相关的机制.rn今天,有很多陆地系统已被持续的人类活动所rn均化,它们在突然的环境变化面前可能同样脆rn弱,而这种变化只有在系统崩溃发生之后才会rn显而易见.%Long-term and persistent human disturbances have simultaneously altered the stability and diversity of ecological systems, with disturbances directly reducing functional attributes such as invasion resistance, while eliminating the buffering effects of high species diversity. Theory predicts that this combination of environmental change and diversity loss increases the risk of abrupt and potentially irreversible ecosystem collapse, but long-term empirical evidence from natural systems is lacking. Here we demonstrate this relationship in a degraded but species-rich pyrogenic grassland in which the combined effects of fire suppression, invasion and trophic collapse have created a species-poor grassland that is highly productive, resilient to yearly climatic fluctuations, and resistant to invasion, but vulnerable to rapid collapse after the re-introduction of fire. We initially show how human disturbance has created a negative relationship between diversity and function, contrary to theoretical predictions. Fire prevention since the mid-nineteenth century is associated with the loss of plant species but it has stabilized high-yield annual production and invasion resistance, comparable to a managed high-yield low-diversity agricultural system. In managing for fire suppression, however, a hidden vulnerability to sudden environmental change emerges that is explained by the elimination of the buffering effects of high species diversity. With the re-introduction of fire, grasslands only persist in areas with remnant concentrations of native species, in which a range of rare and mostly functionally redundant plants proliferate after burning and prevent extensive invasion including a rapid conversion towards woodland. This research shows how biodiversity can be crucial for ecosystem stability despite appearing functionally insignificant beforehand, a relationship probably applicable to many ecosystems given the globally prevalent combination of intensive long-term land management and species loss.