大自然可不那么简单
2004年2月25日 星期三

大卫·铃木

人们都喜欢简单的答案。由此及彼。

因果关系。很简单。

但是大自然并非如此。正当我们自以为把事情摸透了的时候，另一种思想出现了，将我们原本的设想全盘推翻。尽管科学取得了很大的进步，但是我们对生态系统运行机制的了解还刚刚起步。

想想入侵物种。任意一个生态系统中，各种植物和动物都扮演着特定的角色。每一个物种的数量都受到其他力量的限制，比如气候条件、掠食者的存在和食物的丰富程度。如果我们让一个物种离开原产地，运到别处去，那么在新环境中，这个物种可能会遭遇到新的威胁而相继死亡，也可能发现几乎没有任何一种可以制约自己的力量。

当后一种情况发生时，这个物种就可能变成“入侵”物种。也就是说，这个物种可能会泛滥成灾，影响人类生活，颠覆当地原产物种。实际上，入侵物种被认为是威胁世界上生态多样化的一个主要原因。生物学家早就知道，是否缺少昆虫天敌是决定一个植物物种是否会变成入侵物种的一个重要因素，但是，现在我们发现，情况比这要复杂得多。植物物种是否构成入侵，还受到一个意料之外的力量的制约——土壤中的微生物。

我们大多数人往往都会认为土壤是没有活性的，但实际上，土壤生机勃勃，充满着各种微小的生物，对植物的生长状况起着促进或者阻碍的作用。在《自然》杂志上发表的一项近来的研究发现，斑点矢车菊一世纪以前随着苜蓿种子的进口进入北美洲，随后四处扩散的原因并不是缺少昆虫天敌，而是缺少微小的土壤敌人。在欧洲和亚洲，某种土壤微生物制约着这种植物，使它不具有“入侵性”，但是，北美洲的土壤里没有这种微生物。

没几个人会想到土壤微生物对植物物种有这样大的影响，同样，也没几个人会认为为了保护鱼类资源而减少鱼类捕捞居然会减少海鸟的数量。但是，欧洲的北海就发生了这样的事。
北海的掠食者大贼鸥在过去一直从渔业捕捞的剩余物中获得好处。这些年来，这种鸟类学会了以捕鱼船丢弃的鱼内脏和小鱼为食。它们吃得很好。结果，大贼鸥现在的数量比一个世纪前增加了200倍。

但是，北海的鱼类资源由于多年以来的过度捕捞而日渐稀少。欧盟的新条例计划要给鱼群慢慢恢复的空间。但是，随着捕捞的减少，贼鸥所能得到的丢弃物也减少了。

贼鸥为了不饿肚子，开始袭击其他的海鸟，比如海雀和三趾鸥。

这对于其他的海鸟造成了不小的影响。最近的一项研究发现，贼鸥的食谱中海鸟的部分增加5个百分点，就意味着每年多损失数千只海鸟。在某些地方，贼鸥的掠食已经让生态系统已经不堪承受。作者认为，这种情况“对某些海鸟族群是一个潜在的巨大威胁。”

我们要时刻牢记，大自然并不是按照简单的线性发展演变。自然界在过去的上百万年间，已经发展出一套复杂的制衡系统。人类现在有能力干涉这一系统，结果是好是坏，尚未可知。我们在考虑气候变化、转基因食品和其他新的科学问题时，必须牢记这点。我们必须谨慎。
而且，我们不能自以为可以预测事态的发展，因为大自然很少会给你简单的答案。
(朱轶凡译)

Nothing simple about nature

Wednesday, February 25, 2004
By David Suzuki

People generally like simple answers. This happened, so this happened.

Cause and effect. Simple.

But nature doesn't work that way. Just when we think we've got something figured out, another idea comes along that turns our preconceived notions upside down. In spite of all our scientific advances, we are only just beginning to understand how ecosystems work.

Consider invasive species. Plants and animals evolve to fill particular roles within a given ecosystem. The population of each of these species is usually held in check by other forces, such as climate, predators or food availability. When we take a species out of its native home and transport it elsewhere, it may face new challenges and die off, or it may find itself virtually unencumbered by constraining forces.

When the latter happens, the species can become "invasive." That is, it can flourish, become a pest to human beings and overwhelm native species. In fact, invasive species are believed to be a major cause of the loss of biodiversity worldwide. Biologists have long known that the lack of insect enemies is a key factor in determining if a plant species becomes invasive, but we are finding out that the situation is decidedly more complex. We now know that another, surprising agent may also be involved in the success of an invasive plant - soil organisms.

Most of us tend to think of soil as inert matter, but it is very much alive, replete with microscopic organisms that can either enhance a plant's capacity to grow and flourish, or hinder it. A recent study published in the journal Nature has found that spotted knapweed, which was introduced to North America through imported alfalfa seeds more than a century ago, has likely been able to spread so profusely, not because it lacked insect enemies, but because it lacked microscopic soil enemies. In Europe and Asia, specific soil organisms help keep the plant from becoming an invasive weed, but these microbes aren't found in North American soil.

Just as few people would have thought that soil microbes could have such a profound effect on the success of a plant species, few would likely think that reducing fishing to protect fish stocks would actually harm seabird populations. Yet that is exactly what has happened in Europe's North Sea.

One of the North Sea's top predators, the great skua, has greatly benefited from the leftovers of commercial fishing. Over the years, these birds have learned to scavenge fish guts and undersized fish tossed back by fishing boats. They've eaten well. As a result, the great skua population is now 200 times larger than it was a century ago.

However, fish stocks in the North Sea are in trouble from years of overfishing. New European Union policies have been designed to allow stocks to recover, but reduced fishing also means fewer discards for the skuas.

And rather than go hungry, the birds are preying on other seabirds like puffins and kittiwakes.

The impact on other seabird species is not insignificant. A recent study found that a five per cent increase of birds in the skua diet would result in an annual loss of thousands of other seabirds. In some areas, the level of predation by skuas is already unsustainable and the authors conclude that the situation "presents a potentially serious threat to some seabird communities."

We have to remember that nature does not behave in a simple, linear fashion. Our natural systems have, over millions of years, developed complex systems of checks and balances. Humanity is now powerful enough to meddle with those systems and the results are unpredictable. That's something to keep in mind when we consider climate change, genetically modified food and other
emerging scientific issues. We have to be cautious.

And we can't assume we know how things will turn out, because with nature, the answer is rarely simple.

Source: David Suzuki Foundation