Ecological corridors | 生态廊道到底是不是个好办法？

文摘文化 2024-12-21 07:00 法国

在分散的自然空间之间建立连接通道，是减缓生物多样性丧失的重要一步。这一观点长期以来受到生态学家推崇，被奉为自然保护工作的圭臬。但越来越多的科学家正在研究生态系统碎片化的优势。

门诺·施尔图伊曾（Menno Schilthuizen）

荷兰莱顿自然生物多样性中心进化生物学家和生态学家、莱顿大学生物进化论教师。

在连接海牙和鹿特丹的A4高速公路上，往来两地之间的通勤族可能会注意到，代尔夫特南部有一座立交桥的桥面格外宽阔，司机们驾车从这里飞速通过，并没有特别在意。不过，这座百米宽的大桥是荷兰境内最大的“生态产品”（ecoduct）。这项浩大的土木工程包含了沼泽地、芦苇丛、泥炭沟渠、牧场和一条完整的小溪，是一处教科书般典型的自然廊道。

自然廊道并非荷兰独有。尼泊尔和印度边境两侧有多处分散的自然保护区，将它们连接起来的廊道规模要大得多，现在已经形成了300公里长的提莱弧状地景观。北京建成了绿色走廊网络系统，在这座不断扩张的城市里，让分散各处的众多城市绿地“获得自由生长的空间”。人们为扭转自然栖息地的碎片化趋势进行了多次尝试，以上这些只是其中几个实例。

这是由于碎片化是一种危险的发展趋势，会导致基因和生态资源贫瘠。试想一下，一片自然栖息地被分割成多个不相邻的片区，而且彼此之间相隔很远，喜爱这片栖息地的独特物种就这样被困在残存的碎片里。这样的碎片可能有很多个，但由于“灭绝漩涡”的存在，这些独特物种的存亡变成了未知数。下面介绍一下灭绝漩涡的作用原理。

灭绝漩涡

想象一下，欧洲西北部有一小片草地，孤零零地生长在白垩质土壤上，星列弄蝶在草丛间飞舞。这里生活着成千上万只蝴蝶，但它们无法在四周的农田里生存。假设冬季潮湿多雨，致使许多毛虫死于霉菌；而这一年的春天很温暖，幸存下来的毛虫从冬眠中醒来时，供它们食用的植物还没有长出足够多的嫩叶，许多毛虫因此死于饥饿。

现在想象一下，夏季的一场野火进一步减少了蝴蝶种群的数量。为数不多的幸存者开始繁衍后代，可由于后代数量太少，种群内部不得不近亲交配。这种近亲繁殖会导致遗传病的流行，星列弄蝶种群可能因此陷入灭绝漩涡。

一个物种遭遇如此厄运的概率很低，但也确有其事。这是芬兰生态学家伊尔卡·汉斯基（Ilkka Hanski）的观点，他在20世纪80年代提出了“复合种群生态学”理论。汉斯基认为，局部地区的物种灭绝是不可避免的。每年都会有一些物种从自然栖息地的某一块地方消失。但与此同时，相同或不同的物种会从其他地方迁来，落户于此。因此，每个物种都以复合种群的形态存在于种群网络这样的景观中：种群网络内部不断重复着灭绝和迁移的过程，使得整个种群网络免于灭绝。

但有一个前提，当一个种群灭绝时，周边有足够数量的种群过来补充。如今碎片化栖息地的面积普遍太小，彼此又相距过远，外来种群的补充已经不可能了。星列弄蝶的飞行能力很强，但也无法轻易穿越恶劣的地形抵达另一片草地。

由此可见，许多物种灭绝是由于其种群“分布不均”，缺失部分得不到及时补充。一旦外来种群的大量繁殖无法抵消本地种群的灭绝，生物多样性就会受损。这就是开辟生态廊道的理论基础——降低局部灭绝的概率，增加种群迁移的机会。

“许多物种灭绝是由于其种群‘分布不均’，缺失部分得不到及时补充”

白足鼠案例

碎片重组已经成为自然保护工作中不可撼动的一部分，你可能会认为人人都赞同这种做法。但近年来，科学界掀起了一股反其道而行之的潮流，部分科学家认为碎片化有时或许不是坏事。

许多分散的种群的基因已经适应了当地环境。福坦莫大学的杰森·蒙希-索斯（Jason Munshi-South）发现，生活在纽约市内和城市周边几处封闭公园里的白足鼠身上出现了这种现象。在短短几百年间，每座公园里的白足鼠都进化出了能够更好地适应当地食物和疾病的典型特征。如将这些碎片化栖息地连接起来，产生基因流动，基因“污染”可能会让当地的物种进化成果前功尽弃。另一个问题是，生态廊道可能助长入侵物种或极具竞争力的物种扩散，导致稀有的孑遗物种走向灭绝。

在景观尺度上很难就这个问题开展实验研究，但在试管和培养皿组成的微观世界里是可行的。英国的一个研究小组建立了微型生态系统，可以在不影响整个生态系统规模的情况下，任意改变其中的碎片化程度和连通情况。这个生态系统里生活着八种单细胞原生动物，其中三种是捕食者，另五种是猎物。研究人员发现，生态系统的碎片化程度越高，灭绝的物种越少。主要原因是碎片化状态阻碍了更具竞争力的捕食者的散布。

有争议的定论

渥太华卡尔顿大学的莱诺·法赫里格（Lenore Fahrig）认为，将碎片重组的做法作为一种定论，还可能会助长错误观念，即分散存在的小片栖息地没有保存价值，除非将它们连接起来，扩大栖息地的面积。她指出：“这种情况时有发生。在安大略省，面积不足两公顷的湿地没有资格成为保护区，结果造成了小片湿地不断丧失。”

为消除这种误解，弄清碎片化的积极作用能否抵消其负面影响，法赫里格对相关研究进行了统合分析。分析结果出人意料。她研究了近400个案例，其中超过四分之三的案例均表明，碎片化产生了积极作用，而非负面影响。无论在热带还是温带气候下，在任意空间尺度上，就包括保护动物和濒危动物在内的所有生物种群而言，碎片化栖息地都呈现出更加丰富的生物多样性。

是时候捐弃前嫌了

这项分析遭到了许多研究人员的抗议。法赫里格与21位合著者发表了一篇措辞激烈的驳斥文章，回击批评意见。法赫里格说，冗长的学术论战无助于保护自然环境和制定环保政策。因此，她和同事们在2023年发文寻求和解，在文章中解释了冲突的起因和解决方案。

“冗长的学术论战无助于保护自然环境和制定环保政策”

这篇文章承认，碎片化确实会导致局部物种灭绝和近亲繁殖，从而降低生物多样性。不过，碎片化固然不利于单一碎片化栖息地的生物多样性，但对整个地区的生物多样性却是有所补益的。这取决于景观、碎片化栖息地的面积和相关生物种类。

文章作者呼吁生态学界捐弃前嫌，公开对话，争取为环境政策制定者提出一整套切实可行的建议。而在此之前，环保从业者们依然面临着尴尬的两难境地，他们必须在支持还是反对碎片化之间迅速做出抉择。但至少人们已经改变了长久以来的看法，盲目的碎片重组在环保界不再被视为万能应急方案。

Ecological corridors: not such a good idea after all?

Creating connections between natural spaces is crucial to slowing biodiversity loss. This position, advocated by scientific ecologists, has long been the absolute standard in nature conservation. However, a growing number of scientists are now exploring the benefits of fragmentation of ecosystems.

Menno Schilthuizen

Evolutionary biologist and ecologist at Naturalis Biodiversity Center in Leiden (Netherlands).

He also teaches evolution at Leiden University.

Commuters on the A4 motorway between The Hague and Rotterdam might notice that the flyover they zip past just south of Delft is unusually broad, but pay it no further notice. Yet, the 100-meter-wide structure is the largest “ecoduct” of the Netherlands. Swampland, fringes of reeds, a peaty ditch, pasture, and an entire brook are squeezed into this feat of civil engineering. It is a textbook example of a nature corridor.

These corridors are not exclusive to the Netherlands; those connecting isolated nature reserves on either side of the Nepal-India border are much larger. Together, they now constitute the 300-kilometre-long Terai Arc Landscape. And Beijing boasts a network of green corridors to “liberate” the many isolated urban green spaces of the sprawling city. These are a few examples of the many attempts to reverse the fragmentation of natural habitats.

For fragmentation is bad: it causes genetic and ecological impoverishment. Imagine a natural habitat that has become carved up into separate pieces that lie at considerable distances from one another. Locked into them are unique species that depend on those remaining snippets of their favourite habitat. While those snippets may be numerous, the lives of those unique species hang in the balance because of something called the extinction vortex. Such a whirlpool of demise may proceed as follows.

The extinction vortex

Imagine a butterfly like the dingy skipper, in a small, isolated meadow on chalky soil somewhere in north-western Europe. There, it lives by the thousands, but it cannot survive in the agricultural fields around. Now suppose there is a wet winter and many caterpillars die from mould; imagine that the same year enjoys a warm spring which makes the surviving caterpillars wake up from hibernation before their food plant has grown enough young leaves, and many caterpillars die from starvation.

And now imagine that the population is further reduced by a wildfire in summer. The few survivors reproduce, but their offspring are so few that they are forced to mate with siblings. Inbreeding causes an epidemic of a genetic disease that can make the dingy skipper population go down the extinction vortex.

Chances of a species experiencing so much bad luck are low, but it happens. Such was the realization of Ilkka Hanski, the Finnish ecologist who developed “metapopulation ecology” in the 1980s. Local extinction is unavoidable, Hanski realized. Each year, a few species disappear from a fragment of natural habitat. But at the same time, the same or different species arrive as colonists from elsewhere. Thus, each species exists in the landscape as a metapopulation: a network of populations that are constantly going extinct and being resettled without the entire network ever disappearing.

Provided, that is, that there are enough surrounding populations to resettle a population when it goes extinct. Today, many habitat fragments are so small and lie so far apart that resettling is no longer an option. The dingy skipper is a strong flyer, but it will not easily cross hostile terrain to another meadow.

Thus, many species go extinct because they are “patchy” and the patches cannot be replenished in time. When extinction and colonization can no longer balance each other, biodiversity erodes away. That’s the idea behind all those corridors: reduce the chances of local extinction and enhance those of resettlement.

“Many species go extinct because they are ‘patchy’ and the patches cannot be replenished in time”

The white-footed mice

Defragmenting has become so engrained in the practice of nature conservation that you might expect everyone to support it. Yet, over the past years, a countermovement has been stirring among scientists who feel that fragmentation may sometimes also be a good thing.

Many fragmented populations are genetically adapted to local conditions. Jason Munshi-South of Fordham University (United States) discovered this in the white-footed mice living in isolated parks in and around New York City. Characteristics that enable the animals to deal better with locally available food or local diseases had evolved within each park in just a few centuries. If one were to connect those fragments and make genes flow among them, genetic “pollution” could undo local evolution’s work. Another concern is that corridors could also help invasive or super-competitive species spread and drive rare relict species to extinction.

It is hard to study such situations experimentally on a landscape scale, but in the mini world of test tubes and petri dishes, it is possible. A team of United Kingdom-based researchers built a system of micro-ecosystems in which they could manipulate the degree of fragmentation and connectivity without changing the size of the entire ecosystem itself. The system was inhabited by eight species of single-celled protozoa: three predators, and five prey. The researchers discovered that the more fragmented the system was, the fewer species went extinct. This was mainly because fragmentation hampered the spread of one of the more competitive predators.

Dogma disputed

Another risk of turning defragmentation into a dogma, says Lenore Fahrig at Carleton University in Ottawa, is that it feeds the mistaken idea that small isolated fragments are not worth preserving unless they are connected to bigger ones. “This happens all the time,” she says. “In Ontario, wetlands of less than two hectares are not eligible for protected status. The consequence is that constantly small swamps are lost.”

To fight that misconception and to find out if the positive effects of fragmentation can sometimes override the negative ones, Fahrig did a meta-analysis of studies. Her results were surprising. In over three-quarters of the almost 400 cases she studied, the effects of fragmentation were positive, rather than negative. Fragmented habitats supported a greater biodiversity, also in protected and threatened species, at tropical as well as temperate climes, in all groups of organisms, and all spatial scales.

Time to bury the hatchet

This analysis has sparked a wave of protest from many researchers. But in a passionate rebuttal, Lenore Fahrig and 21 co-authors defuse the criticisms. Nature and conservation policies do not benefit from lengthy academic trench warfare, Lenard Fahrig says. Therefore, she and her colleagues published, in 2023, a reconciliatory article, in which they explain what causes the conflict and where its resolutions lie.

“Nature and conservation policies do not benefit from lengthy academic trench warfare”

Indeed, they say, fragmentation causes local extinction and inbreeding, and therefore a reduction of biodiversity. But while fragmentation might be bad for biodiversity in each individual fragment, it can be beneficial for the biodiversity in the region as a whole. It all depends on the landscape, the size of the fragments and the types of organisms involved.

The authors therefore call upon the ecological world to bury the hatchet and embark on an open dialogue to work towards a set of recommendations that nature policy makers can work with. Until that time, the conservation practitioner who needs to make snap decisions for or against fragmentation continues to face frustrating dilemmas. But at least uncritical defragmentation is no longer seen as the universal conservation band-aid that it has been for such a long time.