物种迁移：一场无声的革命

全球变暖，除了冰川消退、北极海冰消融等明显表现以外，还引起了一些并不显著、鲜为人知的变化，悄然改变着物种的分布。这些微不可察的变化不但对生物多样性造成了影响，而且与我们的饮食和健康息息相关。

乔纳森·勒努瓦

法国国家科研中心（CNRS）的生态学研究员，该中心设在法国亚眠的皮卡第儒勒-凡尔纳大学。

我们所处星球的每个角落都在时时刻刻地发生变化。对此，我们往往无所察觉，但我们直接依赖的物种的分布范围却随之悄然改变。生物的这种重新分布是等温线（即人为设定的温度相同各点的连线）在全球变暖作用下发生无形运动——朝两极与山峰呈波浪形移动——的一种有形体现。

为了识别生物分布的这些变化，需要参考大量历史数据和最新数据。通过详细记录生物多样性随时间推移而产生的变动，科学家们可以绘制出过去和当前物种的分布图，从中窥出潜在迁移的端倪。

科学家发现，在陆地环境中，生物迁移主要呈现了沿纬度向两极发展和沿海拔朝山峰迈进的趋势。哺乳动物、鸟类、两栖动物、淡水鱼、昆虫，甚至是植物之类流动性差的生物，概莫能外。由于栖息地因人类活动而支离破碎，这些生物在低地的平均迁徙速度大大减缓。

山区的情况则有所不同，那里人类活动的影响较小，等温线比较接近。

1971年至1993年，法国的森林植物物种以每10年30米的平均速度进行迁移。由于温度不断上升，等温线不断上移，许多植物物种已经被迫侵殖欧洲数座山脉的顶峰。相关专家对欧洲302座山峰进行了观察，结果显示，侵殖山顶的植物物种数量不断增加——随着全球变暖的加剧，这一趋势正在变得愈发明显。平均而言，2007年至2016年侵殖山顶物种数量的增幅是1957年至1966年的5倍之多。在研究范围内，有87%的欧洲山峰存在物种侵殖现象。

新的物种共生形态

受上述现象影响，某些山峰特有物种（例如仅存在于东比利牛斯山脉的纤毛点地梅）与原本生长在海拔较低处，竞争力更强的物种（例如山金车花或高山草甸牧草）之间形成了一种新的混搭共生状态。欧洲山脉顶峰生物多样性的增加乍看似乎是个好消息，但从长远来看，入侵种和土著种之间的竞争恐会对后者造成危害，甚至会导致某些土著种彻底灭绝。这种灭绝现象在秘鲁山区已有实证——1985年，当地海拔1300米以上的山脊区域能够观察到16种鸟类，而到了2017年，其中8种已了无踪迹。

在海洋环境中，大多数生物对温度的升高更为敏感，因此，相比陆地物种，海洋物种的极向迁移速度更快、表现更明显，平均速度达到每10年60千米左右，比陆地生物快5到6倍。

一些研究还发现，某些海洋鱼类还出现了垂直迁移现象——由于表层海面温度升高，它们被迫逃向更深的海域。

以北海为例，1980年到2004年，海底（海床）区域的鱼类以平均每10年4米的速度向深处迁移。与陆地环境相比，海洋生物多样性的这种快速变化更为显著，因此更容易为人所觉察，这也让海洋成为监测全球变暖对生物再分布影响的最佳风向标。

人类的饮食、健康、福祉、生产活动、娱乐和文化繁荣，无一不依赖生物多样性，因此，与渔业资源相关的物种迁移尤为引人关注，直接影响着人类的营养状况。例如北大西洋鲭鱼群的北迁就曾于2010年引发了重大经济和地缘政治冲突，牵涉到在该海域捕捞的各个欧洲国家。

鱼类物种的迁移直接影响着人类的营养状况

国家间冲突的源头之一

生物多样性不分国界。由此可以预见，随着具有商业价值的物种从一个经济区迁向另一个经济区，相邻国家之间的冲突与紧张局势将会愈加频发。所有商品和服务，凡是以生物多样性为基础的，其地理分布将在全球范围内彻底洗牌。

同样，病原体载体乃至疾病也会进行再分布。随着全球变暖，蚊子、蜱虫等病媒开始入侵先前从未涉足的高纬度和高海拔地区，这已经是不争的事实。人们必须做好为此付出健康、社会和经济代价的准备。

气温升高导致蚊子向更高海拔地区扩散，引发了疟疾

在南美和东非，山区人口比过去更容易染上疟疾，这是因为气温升高导致携带致病寄生虫的蚊子向更高海拔地区扩散。在北欧，由于夏季变干，冬季变暖，啮齿动物等小型哺乳动物数量增多，分布范围也发生改变。啮齿动物是蓖麻硬蜱的主要宿主，而蜱正是莱姆病病原体，即伯氏疏螺旋体菌的传播媒介。

除了对人类健康造成直接影响之外，生物的重新分布还对全球变暖动态有着间接影响。例如，在北极，海冰消融、苔原灌木扩张、北方森林向北极推进等因素导致北极反照率（反射率）降低，继而进一步加剧了变暖。

全球变暖的应对之策

生物的这种重新分布不一定与全球变暖同步。一般而言，观察发现的向山峰移动的平均速度为每10年18米，是同期等温线上移速度（平均每10年40米）的一半。例如，森林植物物种只能以散播种子的方式进行繁殖，从而实现下一代的迁移。

只有遇到适宜的气候条件，散落四方的种子才会发芽，进而在物种最初分布范围之外壮大成群。因此，像树木这种生命周期长、成熟缓慢的物种对气候变化的反应要更慢一些。这就意味着，即便全球变暖在今天戛然而止，未来几十年内，我们依然会看到生物分布继续变化。

全球变暖引起的生物再分布都向人类提出了新的挑战。因此，当务之急是要加大研究支持力度，从而提高我们对这种现象产生的后果的认识，并在政治与经济决策时考虑到这一点。只有结合这一全球动态，妥善施以国际治理，我们才更有可能最大限度地减轻生物再分布对人类福祉潜在的负面影响。

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By Jonathan Lenoir

There are changes taking place all over the planet, at all latitudes. Most often, we are unaware of them. And yet they are altering the distribution ranges of the species on which we depend directly. This redistribution of living things is the tangible manifestation of the invisible movement of isotherms – imaginary lines of the same mean temperature that move towards the poles and mountain peaks like waves, driven by global warming. 

The identification of these changes in the distribution of living things requires large amounts of data, both historical and recent. By meticulously documenting biodiversity over time, scientists can map the distribution of past and present species in an attempt to detect potential migrations.

For terrestrial environments, scientists have identified migrations that are mainly oriented towards the poles in latitude, and towards mountain peaks in altitude. This includes mammals, birds, amphibians, freshwater fish, insects, and even organisms that are less mobile, such as plants. The average migration speed of these organisms in the lowlands is largely slowed down by the fragmentation of habitats linked to human activities.

This is not so much the case in mountain areas, where the impact of human activities is reduced, and the isotherms are closer together. 

In France, forest plant species migrated at an average speed of thirty metres per decade, between 1971 and 1993. Driven by rising temperatures and the shifting of isotherms, many plant species have already reached the summits of several European mountain ranges. Observations made on 302 mountain peaks in Europe show that the number of plant species that have colonized the summits is increasing over time – this trend is becoming more pronounced with the acceleration of global warming. 

On average, the gain in species – affecting eighty-seven per cent of the European summits studied – was five times greater between 2007 and 2016 compared to the period between 1957 and 1966.    

These phenomena have led to a new cohabitation between species endemic to certain peaks – such as the ciliated androsaceus, which is only present in the eastern Pyrenees – and more competitive species found at lower levels, such as mountain Arnica or Alpine meadow grass. The increase in biodiversity at the summits of European mountain ranges may at first seem like good news. However, in the long term, there is a risk that competition between colonizing and endemic species will be to the detriment of the latter, and lead to the total disappearance of some of them. 

This phenomenon of extinction has already been noticed in the Peruvian mountains – eight of the sixteen species of birds observed in 1985, and living on mountain ridges above an altitude of 1,300 metres, could no longer be found in 2017.

In the marine environment, most organisms are much more sensitive to a rise in temperatures. The poleward movement of species is therefore much more rapid and significant in the seas and the ocean than on land. The average poleward migration of marine organisms is around sixty kilometres per decade – five to six times faster than that of terrestrial organisms. 

Some studies have also shown vertical migrations of several species of marine fish to deeper waters, to escape the temperature increase in surface waters. In the North Sea, for example, fish in the demersal (seabed) zone migrated to the depths at an average speed of four metres per decade between 1980 and 2004. These rapid shifts in marine biodiversity are more spectacular, and therefore more easily perceptible than in terrestrial environments. They make the seas and the ocean the best monitors of the consequences of global warming on the redistribution of life.

Humans depend on biodiversity for food, health, well-being, production activities, recreation  and cultural enrichment. Therefore, a global redistribution of this biodiversity will have an impact on all these aspects. 

The movement of species that affects our halieutic (linked to fishing) resources are particularly striking, and has a direct effect on human nutrition. This is particularly the case in the northward migration of mackerel schools in the North Atlantic – which by 2010, had led to major economic and geopolitical conflicts between different European countries fishing in the area.

Since biodiversity knows no borders, we can expect an increase in the number of conflicts and tensions between neighbouring countries related to the movement of commercial species from one economic zone to another. The geographical distribution of all the goods and services provided by biodiversity will be completely reshuffled on a global scale. 

The same applies to the redistribution of vectors of pathogens, and therefore, diseases. With global warming, the emergence of new disease vectors (mosquitoes and ticks) at latitudes and altitudes previously free of these species is already a reality – with health, social and economic costs that must be anticipated.

In South America and East Africa, populations living in mountainous regions are more affected by malaria than in the past. Rising temperatures favour the altitudinal migration of mosquitoes carrying the parasites causing the disease. In northern Europe, drier summers and milder winters are modifying the abundance and distribution of small mammals such as rodents – the main hosts of the Ixodes ricinus tick, vector of the bacterium Borrelia burgdorferi, the Lyme disease pathogen. 

Beyond these direct impacts on human well-being, the redistribution of living organisms also has an indirect effect on the very dynamics of global warming. In the Arctic, for example, in the same way, melting sea ice, tundra shrub expansion and the advance of the boreal forest towards the North Pole are factors that reduce the albedo (reflectivity) of the North Pole and thus accentuate warming.

This redistribution of living things is not necessarily synchronous with global warming. In general, the average speed of movement observed towards mountain summits is eighteen metres per decade, half as fast as the speed at which isotherms moved upwards over the same period – forty metres per decade, on average. In forest plant species, for example, reproduction is the only way to displace the next generation – by dispersal, in the form of seeds. 

Only seeds that have been displaced under favourable climatic conditions germinate and allow the establishment of a new population beyond the initial distribution range. Species with a long and slow life cycle, such as trees, thus show even greater delays in response to climate forcing. These delays mean that even if warming were to stop today, we would continue to observe changes in the distribution of life for decades to come.

The redistribution of living organisms in response to global warming poses new challenges. There is, therefore, an urgent need to step up efforts to support research to improve our understanding of the consequences of this phenomenon – and to take it into account while making political and economic decisions. It is through appropriate international governance integrating this global dynamic, that we will increase the chances of minimizing the potential negative consequences that this redistribution of living things could have on our well-being.