Conservation biologists have long been concerned that species extinction could have significant consequences for the stability of entire ecosystems-groups of interacting organisms and the physical environment that they inhabit.An ecosystem could survive the loss of some species, but if enough species were lost, the ecosystem would be severely degraded.ln fact, it is possible that the loss of a single important species could start a cascade of extinctions that might dramatically change an entire ecosystem.A good illustration of this occurred after sea otters were eliminated from some Pacific kelp(seaweed) bed ecosystems: the kelp beds were practically obliterated too because in the absence of sea otter predation, sea urchin populations exploded and consumed most of the kelp and other macroalgae.
lt is usually claimed that species-rich ecosystems tend to be more stable than species-poor ecosystems.Three mechanisms by which higher diversity increases ecosystem stability have been proposed.First, if there are more species in an ecosystem, then its food web will be more complex, with greater redundancy among species in terms of their nutritional roles.ln other words, in a rich system if a species is lost, there is a good chance that other species will take over its function as prey, predator, producer, decomposer, or whatever role it played.Second, diverse ecosystems may be less likely to be invaded by new species,,notably exotics(foreign species living outside their native range), that would disrupt the ecosystem's structure and function.Third, in a species-rich ecosystem, diseases may spread more slowly because most species will be relatively less abundant, thus increasing the average distance between individuals of the same species and hampering disease transmission among individuals.
Scientific evidence to illuminate these ideas has been slow in coming, and many shadows remain.0ne of the first studies to provide data supporting a relationship between diversity and stability examined how grassland plants responded to a drought.Researchers D.Ti and JA.Downing used the ratio of above- ground biomass in1988 (after two years of drought) to that in1986 (predrought) in207 plots in a grassland field in the Cedar Creek Natural History Area in Minnesota as an index of ecosystem response to disruption by drought.ln an experiment that began in1982, they compared these values with the number of plant species in each plot and discovered that the plots with a greater number of plant species experienced a less dramatic reduction in biomass.Plots with more than ten species had about half as much biomass in1988 as in 1986, whereas those with fewer than five species only produced roughly one-eighth as much biomass after the two-year drought.Apparently, species-rich plots were likely to contain some drought-resistant plant species that grew better in drought years,,compensating for the poor growth of less- tolerant species.
To put this result in more general terms, a species-rich ecosystem may be more stable because it is more likely to have species with a wide array of responses to variable conditions such as droughts.Furthermore, a species-rich ecosystem is more likely to have species with similar ecological functions, so that if a species is lost from an ecosystem, another species,,probably a competitor, is likely to flourish and occupy its functional role.Both of these,,variability in responses and functional redundancy, could be thought of as insurance against disturbances.
The Minnesota grassland research has been widely accepted as strong evidence for the diversity- stability theory; however, its findings have been questioned, and similar studies on other ecosystems have not always found a positive relationship between diversity and stability.Clearly, this is a complex issue that requires further field research with a broad spectrum of ecosystems and species: grassland plants and computer models will only take us so far.ln the end, despite insightful attempts to detect some general patterns, we may find it very difficult to reduce this topic to a simple, universal truth.
保护生物学家长期关注的是, 物种灭绝可能会对整个生态系统的稳定性有很大的影响一和 油相互作用的物种以及所居住的物理环境 。一个生态系统可以在某些物种消失后生存下来，但是如果有足够的物种丧失,生态系统就会受到严重退化。事实上, 一个重要的物种的喪失会启动一个级联的灭绝,可能会极大地改变整个生态系统。 一个很好的例子就发生在海独n从太平洋海帯 (海草床生态系统)被消除之后) :实际上,海帯床也被除去了,因为在缺乏海独n捕食的情况下,海胆数量暴增,吃光了大部分的海帯和其他海藻。
据说物种丰富的生态系统往往比物种實乏的生态系统更稳定。更丰富的多样性增加生态系统的稳定性的三种机制已经被提出。首先,如果有更多的物种在一个生态系统,那么它的食物网将更复杂, 从营养作用方面来看, 物种间有更多的剩余部分。換句话说,在一个丰富的系统,如果一个物种失去了,那么其他物种可能有一个很好的机会去取代它的功能,作为猎物、捕食者、生产者、分解者、或任何它所起到的作用。第二,多样的生态系统不太可能遭到新物种的入侵,特别是外来的(本地范围内以外生存的物种) ,这样会影响生态系统的结构和功能。第三,在一个物种丰富的生态系统中,疾病可能会传播得比较慢,因为大多数物种的数量相对较少,从而增加了同一物种个体间的平均距离, 从而阻碍了个体间的疾病传播。
说明这些想法的科学证据进展缓慢,许多的不确定性仍然存在 。 为多样性和稳定性之间的关系提供数据支持的最早的研究之一 , 研究了草原植物如何应对干旱。研究人员D。 Tilman和JA唐宁在用1988年(经过两年的干旱)和1986年(干a 前) 明尼苏达地区雪松溪的207块草地场地上物量的比値,作为生态系统应对干旱的指数。在一个始于1 982年的实验中, 他们将这些値与每一块地的植物物种的数量作比较,发现拥有较大数量植物种类的地块在生物量上減少的最少。拥有十多个品种的地块,在1986大约只有1988年一半的生物量,而那些不到五种生物种类的地块,在两年干旱后,只产生了大约八分之一的生物量。显然,物种丰富的地块很可能包含一些抗旱植物品种,在干旱年生长地更好,补偿那些不耐旱的物种生长。
从大体上来看这个结果,一个物种丰富的生态系统可能更稳定, 因为它更可能拥有一些物种, a 些物种针对不同情况有着各种反应, 如干旱 。此外，物种丰富的生态系统更可能有生态功能相似的物种, 因此, 如果生态系统失去了一个物种, 另一个物种,可能是竟争对手,有可能蓬勃发展,并占据其职能作用。这两个(特点) ,反应的多样性和功能剩余,可以被认为是作为是针对干扰的预防措施。